Microhard Systems 99P2 User Manual MHX 900 Manual03

Microhard Systems Inc MHX 900 Manual03

MHX 900 User Manual

Operating Manual900 MHz WirelessSpread-Spectrum OEM ModemRevision 03 - May 12, 1999Microhard Systems Inc. has made every effort to assure that this document is accurate and complete. However, thecompany reserves the right to make changes or enhancements to the manual and/or the product described herein atany time and without notice. Furthermore, Microhard Systems Inc. assumes no liability resulting from any omissionsin this document, or out of the application or use of the device described herein. Microhard Systems Inc. Leaders in Wireless Telecom
MHX-900 Operating Manual: Contents iContents1. Introduction1.0 Product Overview ........................................................................................................................................................ 11.1 Features........................................................................................................................................................................ 11.2 About this Manual ....................................................................................................................................................... 22. Initial Setup and Configuration2.0 Unpacking and Inspection............................................................................................................................................ 32.1 Additional Requirements ............................................................................................................................................. 32.2 Connectors and Indicators ........................................................................................................................................... 32.3 Configuration............................................................................................................................................................... 52.4 Checking the Link......................................................................................................................................................... 63. Configuration Options3.0 Command Line Interface.............................................................................................................................................. 73.1 AT Commands............................................................................................................................................................. 8A - Answer.................................................................................................................................................................... 8D - Dial ........................................................................................................................................................................ 8E - Command Echo...................................................................................................................................................... 8I - Identification ........................................................................................................................................................... 8O - Online Mode.......................................................................................................................................................... 8Q - Quiet Mode............................................................................................................................................................ 9V - Result Codes Display............................................................................................................................................. 9W - Connection Result................................................................................................................................................. 9Z - Reset Modem and load stored configuration.......................................................................................................... 9&C - DCD (Data Carrier Detect) ................................................................................................................................. 9&D - DTR (Data Terminal Ready)............................................................................................................................... 9&F - Load Factory default configuration ..................................................................................................................... 9&K - Handshaking........................................................................................................................................................ 9&S - DSR (Data Set Ready)....................................................................................................................................... 10&V - View Configuration........................................................................................................................................... 10&W - Write Configuration to memory....................................................................................................................... 10Sxxx? - Read S register value .................................................................................................................................... 10Sxxx=yyy - Set S register value ................................................................................................................................. 103.2 Command Result Codes............................................................................................................................................. 103.3 S Registers ................................................................................................................................................................. 11S Register 2 - Escape Code......................................................................................................................................... 11S Register 3 - CR Control Code................................................................................................................................. 11S Register 4 - Linefeed Control Code........................................................................................................................ 11S Register 5 - Backspace Control Code ..................................................................................................................... 11S Register 101 - Operating Mode .............................................................................................................................. 12S Register 102 - Serial Baud Rate.............................................................................................................................. 13S Register 103 - Wireless Link Rate.......................................................................................................................... 14S Register 104 - Network Address............................................................................................................................. 14S Register 105 - Unit Address.................................................................................................................................... 14S Register 106 - Hopping Pattern .............................................................................................................................. 14S Register 107 - Encryption Key ............................................................................................................................... 14S Register 108 - Output Power Level ........................................................................................................................ 15S Register 109 - Hopping Interval ............................................................................................................................. 16S Registers 111 to 113 - Packet Parameters - Point to Point Mode ............................................................................ 16S Registers 111 to 115 - Packet Parameters - Point to Multipoint Mode................................................................... 17AppendicesA. Modem Command Summary ............................................................................................................................. 19B. Serial Interface................................................................................................................................................... 21C. Sample Schematic Diagram ............................................................................................................................... 23D. Modem Setup Overview .................................................................................................................................... 25E. Technical Specifications .................................................................................................................................... 27F. Glossary ............................................................................................................................................................. 29
MHX-900 Operating Manual: Chapter 1 Introduction. 11. Introduction1.0 Product OverviewCongratulations on choosing the MHX-900 wireless modem module! Yournew MHX-900 modem is a state-of-the-art, 900 MHz frequency-hoppingspread-spectrum communications transceiver module. When the MHX-900module is incorporated into the design of new or existing equipment, terminaldevices (DTEs) up to 30 km (or more)1 apart will be capable of establishinghigh-speed2 communications wirelessly.MHX-900 modules provide a practical and reliable alternative to usingtraditional analog phone-line modems or “permanent wire” serial cable(RS-232) connections for data communications between terminal equipment.Moreover, wireless data communications using the MHX-900 module meansyou will benefit from:ngreater flexibility and freedom to relocate terminal equipment,neliminated requirement for access to wire-based transfer mediasuch as telephone lines,nthe ability to communicate through walls, floors, and manyother obstacles.While the MHX-900 module is compact in its design, it delivers power andconvenience and offers quality and dependability. The MHX-900 module’sversatility makes it the ideal solution for applications ranging from office-productivity to industrial data control and acquisition.While a pair of MHX-900 modules can link two terminal devices (“point-to-point” operation), multiple MHX-900 modules can be used together to createa network of various topologies (“point-to-multipoint” operation). Multipleindependent networks can operate concurrently, so it is possible for unrelatedcommunications operations to take place in the same or a nearby area withoutsacrificing privacy, functionality, or reliability.1.1 FeaturesKey features of the MHX-900 module include:ntransmission within a public, license-exempt band of the radiospectrum3 – this means there are no conditions on usage of theMHX-900 module, and that it can be used without restrictionsor access fees (such as those incurred by cellular airtime);na serial I/O data port (TTL levels) with handshaking andhardware flow control, allowing the MHX-900 module to inter-face directly to any microprocessor with an asynchronous serialinterface. 1Ideal conditions with clear line-of-sight communications, using high-gain antennas.2Up to 115,200 bits per second (bps).3902-928 MHz, which is license-free within North America; may need to be factory-configureddifferently for some countries.
2MHX-900 Operating Manual: Chapter 1 Introductionntwenty different user-selectable pseudo-random hoppingpatterns to offer the possibility of separately operating multiplenetworks while providing security, reliability and high toleranceto interference;nencryption key with 65536 user-selectable values to maximizesecurity and privacy of communications;nbuilt-in CRC-16 error detection and auto re-transmit to provide100% accuracy and reliability of data;nease of installation and use – the MHX-900 module uses asubset of standard AT style commands making it compatiblewith most communication packages, such as HyperTerminal.While the typical application for the MHX-900 is to provide a mid- to long-range wireless communications link between DTEs, it can be adapted toalmost any situation where an asynchronous serial interface is used and dataintercommunication is required.1.2 About this ManualThis manual has been provided as a guide and reference for installing andusing MHX-900 wireless modem modules. The manual contains instructions,suggestions, and information which will help you set up and achieve optimalperformance from your equipment using the MHX-900 module.It is assumed that users of the MHX-900 module have either systemintegration or system design experience. Chapter 2 details the requirementsand connections of the MHX-900 module. Chapter 3 describes the ATcommand register setup and configuration. The Appendices, including theGlossary of Terms, are provided as informational references which you mayfind useful throughout the use of this manual as well as during the operationof the wireless modem.Throughout the manual, you will encounter not only illustrations that furtherelaborate on the accompanying text, but also several symbols which youshould be attentive to:Caution or Warning: Usually advises against some action which couldresult in undesired or detrimental consequences.Point to Remember: Highlights a key feature, point, or step which is worthnoting, Keeping these in mind will make using the MHX-900 more usefulor easier to use.Tip: An idea or suggestion is provided to improve efficiency or to makesomething more useful.With that in mind, enjoy extending the boundaries of your communicationswith the MHX-900 module.
MHX-900 Operating Manual: Chapter 2 Initial Setup and Configuration 32. Initial Setup and Configuration2.0 Unpacking and InspectionThe following items should be found in the shipping carton. Inspect thecontents for any shipping damage. Report damages or shortages to thedistributor from which the unit was purchased. Keep all packing materials inthe event that transportation is required in the future.Package contents (normal distribution):1MHX-900 Wireless Modem module 12Operating Manual (this document) 12.1 Additional RequirementsSince the MHX-900 module is a unique product in a class of its own, it willcommunicate only with another MHX-900 module which has beencompatibly configured. Thus, at least two MHX-900 modules will berequired to establish a wireless communications link.Additionally, the following requirements should be taken into considerationwhen preparing to incorporate the MHX-900 module in new or existingdesigns.nRelatively small footprint for the MHX-900 module (e.g., 2.1”W x 3.5” L x 0.8” H) on the OEM PCB.nSerial port and control signals from the host microprocessor.See sample application schematics in Appendix C.n+5 Vdc supply from the host OEM electronics (+ 10%).nAn external antenna (customer supplied).2.2 Connectors and IndicatorsThe MHX-900 module connects to the host equipment by a dual row headeron the underside of the unit. This enables the MHX-900 module to bedirectly soldered onto the host equipment manufacturer’s PCB.The MHX-900 radio deck is attached via a 40 pin header soldered directly tothe PCB. The radio deck also has an antenna connector. A suitable antennamust be used to ensure adequate performance of the MHX-900 module.The MHX-900 module is equipped with status LEDs which show the currentoperating mode for the unit. The LED signals also are also brought out toexternal connectors to be monitored by the OEM microprocessor.The output drive capability of any output signal pin is approx. 5 mA.The pin description and location of the MHX-900 is shown on the followingpage.
4MHX-900 Operating Manual: Chapter 2 Initial Setup and ConfigurationFor maximum power, useseparate power supplies forthe radio and the digitalelectronics.For OEM design simplicity,connect both the radio andlogic VCC connectionstogether. This saves aregulated power supply.Caution: Using any other powersupply which does not provide theproper voltage or current coulddamage the MHX-900 module.120Top ViewAntenna Connector2140Vcc RadioVcc RadioVcc LogicN/C/CONFIG/RESETGNDGNDGNDN/CTx Mode LEDRSSI LED 3RSSI LED 2RSSI LED 1CTSRTSDSRN/CDTRTxDRxDDCDVcc LogicVcc LogicVcc LogicVcc LogicGNDGNDGNDGNDN/CN/CN/CN/CN/CN/CN/CN/CN/CRx Mode LEDThe interface connectors and indicator lights are summarized below:Vcc Radio - These connections supply power for the MHX-900 module’s radiodeck electronics. For 1 watt maximum output, the required supply ratingshould be 5.25 VDC +- 5%. Limiting this to 5.0 Vdc will limit the outputpower slightly. Required current depends on transmit duration and outputpower. Maximum current at full transmit power is 600 mA DC.Vcc Logic - These connections supply power for the MHX-900 module’s digitalelectronics. To simplify the OEM design, the Radio VCC can beconnected to the Logic VCC. Maximum current while transmitting isapprox. 100 mA DC.GND - These are the Ground supply connections for the digital and radioelectronics./RESET - This momentary active low input signal (100 ms typical) resets thedigital electronics./CONFIG - Leave this pin unconnected. Do NOT ground.Receive Signal Strength Indicator (RSSI) LEDs - These outputs show thequality/strength of the received signal. As the signal strength increases,the number of active outputs increase incrementally.Data Port (DCE) - Pins 21 through 28 inclusive. This port is used to interfacethe MHX-900 module to a DTE device and operates at 2400 to 115,200bps. The same port is used to configure the modem by interfacing to thehost microprocessor. See Appendix B for details on the serial interfacesignals.RX Mode LED - This active low output indicates when the modem receives dataover the wireless link, as well as during internal carrier search andsynchronization operations.TX Mode LED - This active low output indicates that the modem is transmittingdata wirelessly. This light flashes during initial startup and carriersynchronization.Antenna Connector - The antenna is attached to this connection.N/C - These pins are reserved for future use. Do not connect to this pins.
MHX-900 Operating Manual: Chapter 2 Initial Setup and Configuration 52.3 ConfigurationPrior to establishing a wireless link, each MHX-900 module that willparticipate in the link must be correctly configured for compatibility and forthe desired mode of operation.The two most common types of networks used are:nPoint-to-point: A Master station communicates with a singleSlave station.nPoint-to-multipoint: A Master station communications withtwo or more Slave stations.Within any network, the Master will communicate only with Slave(s)assigned to the same network. Similarly, a Slave will only communicate withthe Master of the network to which it is assigned.2.3.1 Quick Start ApproachThe minimum configuration requirements for point-to-point and point-to-multipoint are summarized below. These requirements will get you startedand only ensure that a link can be established, but do not necessarily providethe best performance; optimization of the communications link is discussedin later sections.A. Point-to-PointTo establish a point-to-point communications link, the following requisitesmust be satisfied:nThe Operating Mode for one modem must be configured as a“Master - Point-to-Point”, and the other as a “ Slave - Point-to-Point”.nThe Wireless Link Rate for both modems must be the same.nThe Network Address assigned for both modems must be thesame.nThe Unit Address assigned for both modems must be the same.nThe same Hopping Pattern must be selected for both modems.nThe same Hopping Interval must be selected for both modems.nBoth modems must use the same Encryption Key.
6MHX-900 Operating Manual: Chapter 2 Initial Setup and ConfigurationWarning: Using an antenna thatis inappropriate for use with theMHX-900 module could result inundesired performance, and maydamage the unit. It is the user’sresponsibility to ensure theantenna has adequate lightningprotection.B. Point-to-MultipointTo establish a point-to-multipoint network, the following requisites must besatisfied:nThe Operating Mode for one modem must be configured as aPoint-to-Multipoint Master, and the others as Point-to-Multipoint Slaves.nThe Wireless Link Rate for all modems must be the same.nThe Network Address assigned to all modems must be the same.nEach Slave must be assigned a unique Unit Address (Unit Address0 is not supported).nThe same Hopping Pattern must be selected for all modems.nThe Hopping Interval is determined by the Master modem, and istherefore configured only at the Master.nAll modems must use the same Encryption Key.Each of the parameters above are defined by specific S registers. Settings arenot immediately stored in non-volatile memory. The command &W writesthe current configuration into non-volatile memory, and are therefore retainedeven after powering down. Each item and the settable parameters within themodem configuration are described in detail in Chapter 3: ConfigurationOptions.2.4 Checking the LinkOnce configured properly, a pair or set of communicating modems can be testedto ensure that a link can be successfully established. Connect an appropriateantenna to each MHX-900 module. The modems should indicate the status ofthe wireless link via the RSSI outputs on each unit:If the link is good, up to three RSSI outputs should be active; and if the linkis absent (due to a fault at one end or another, such as misconfiguration), theoutputs will be inactive.It is recommended that if MHX-900 modules will be deployed in a fieldwhere large distances separate the units, the modems should be configuredand tested in close proximity (e.g., in the same room) first to ensure a goodlink can be established and settings are correct. This will facilitatetroubleshooting, should problems arise. In a multipoint system, only theSlave modems give valid RSSI readings.Microhard Systems Inc. provides a development system which supplies allsystem power and RS-232 level shifting circuits required to connect a MHX-900 module to a PC. This can serve as an invaluable aid in OEM systemdesign.
MHX-900 Operating Manual: Chapter 3 Configuration Options 73. Configuration OptionsRefer to Appendix A (page19) for a summary of themodem commandsConfiguration options arenot stored in non-volatilememory until the WRITEcommand (&W) is executed3.0 Command Line InterfaceThe MHX-900 modem module is easily configured to meet a wide range ofneeds and applications. You can have your modem perform a variety offunctions by sending it instructions (in the form of commands). Sending acommand overrides the modem’s operating characteristics.Your modem must be in Command Mode for it to execute a command. Ifyou send characters when the modem is in Data Mode, the modem sends thecharacters across the Wireless Link.When the modem is initially powered up, the interface will operate incommand mode.You can place the modem into Data Mode either by1. Dialing a remote modem2. Issuing the answer command3. Issuing the online commandYou can place the modem into Command Mode either by1. Sending the escape sequence2. Toggling the DTR line,Note: When the connection is lost to the remote modem, the modem willswitch to command mode.To enter a command line and have your modem execute it, use the followingprocedure:1. With your modem in the Command Mode, type AT. These characters,known as the attention characters, must be typed at the beginning oreach command line.2. Type the command(s) in upper-case letters. Include any parametersrequired by the command. To make the command line more readable,you can insert as many spaces as desired. The command line holds upto 40 characters, not including the AT prefix.3. Press the ENTER key. Your modem executes the command line andsends you a word response (usually OK).For example, to set the Operating Mode S101 register to Master point topoint mode, enter the following command line; AT S101 = 1 <return>.To display what a register is currently set to, enter the following commandline; AT Sxxx? <return>. The modem will output the value of the register.
8MHX-900 Operating Manual: Chapter 3 Configuration OptionsNote: If you want to send more than one command line, wait for a responsebefore entering the AT prefix at the start of the next command line.To re-execute the previous command, enter A/. The modem will executethe previous command line.For the AT command protocol, an escape sequence consists of threeconsecutive escape codes preceded and followed by at least 1 second ofinactivity. Typically, the ‘+’ character is used as the escape code.+++ preceded and followed by 1 second of inactivity3.1 AT CommandsSeveral AT Commands are supported by the MHX-900 module. Thefollowing is a short description of all available commands. ‘*’ denotesstandard factory settings.AAnswerThe A command causes the modem to attempt to connect with anotherremote modem.DDialUsing the D command, the modem will set itself for master point to pointmode, and then try to synchronize with the modem address specified in thecommand (Dxxxx). Register S105 is overwritten with value xxxx. Note -DT or DP are also value (tone dial or pulse dial).ECommand EchoYour modem is preset to return (or echo) commands to the hostmicroprocessor when in Command Mode.E0 No Command Echo*E1 Command EchoIIdentificationThe I command returns various modem information settings.I0 Product Code (MHX-900)I1 3 Digit computed checksumI2 Issue ROM Check (OK or ERROR)I3 Product Identification (Firmware Version)I4 Not SupportedI5 Firmware CopyrightOOn-line ModeThe O command attempts to communicate with a remote modem.
MHX-900 Operating Manual: Chapter 3 Configuration Options 9QQuiet ModeYour modem is preset to send responses when it executes commands, andthere after to keep the host informed of its status.*Q0 Enable modem responsesQ1 Disable modem responsesVResult Codes displayYour modem can either display result codes as words or numbers.V0 Display Result Codes as numbers*V1 Display Result Codes as wordsWConnection ResultThe W command returns your modem to the Data Mode from the CommandMode*W0 Reports computer (DTE) rate as CONNECT xxxxW1 Reports computer (DTE) rate and wireless rate between modems as CARRIER xxxx.W2 Reports modem (DCE) rate as CONNECT xxxxZReset and load stored configurationThe Z command resets the modem and loads the stored configuration.&C DCD (Data Carrier Detect)The &C command controls the modems DCD output signal to the hostmicroprocessor. This command determines when the DCD is active.&C0 DCD is always ON*&C1 DCD on when modems are synchronized&D DTR (Data Terminal Ready)The &D command controls what action the modem performs when the DTRinput line is toggled. The DTR input is controlled by the hostmicroprocessor.&D0 DTR line is ignored&D1 Not Supported*&D2 DTR disconnects and switches to Command Mode&D3 DTR disconnects and resets modem&F Load Factory Default ConfigurationThe &F command resets the modem and loads the default factoryconfiguration.&K HandshakingThe &K command controls the handshaking between the modem and hostmicroprocessor.&K0 Disable handshaking*&K3 Enable hardware handshaking (RTS/CTS)
10 MHX-900 Operating Manual: Chapter 3 Configuration Options&S DSR (Data Set Ready)The &S command controls the DSR line for the modem, and determineswhen it is active&S0 DSR is always ON*&S1 DSR is ON in Data Mode, OFF in Command Mode&V View ConfigurationThe &V command displays the current (DTE) baud rate, and all setablemodem parameters including S register values.&W Write Configuration to memoryThe &W command stores the active configuration into the modem’s non-volatile memory.Sxxx? Read S register valueThis command causes the modem to display the current setting of S registerxxx.Sxxx=yyy Set S register value (see section on S-Registers)This command sets the specified S register to a value specified by yyy.3.2 Command Result CodesThe MHX-900 module can either display the results of a command as eithertext strings or numerical data. The following chart shows resulting textstring and corresponding numeric result.0OK1CONNECT2RING3NO CARRIER4ERROR8NO ANSWER10 CONNECT 240011 CONNECT 480012 CONNECT 960014 CONNECT 1920017 CONNECT 3840018 CONNECT 5760033 CONNECT 115200
MHX-900 Operating Manual: Chapter 3 Configuration Options 113.3 S RegistersS Register 0 - Number of Rings - Auto AnswerIf this register is set to zero, the modem will power up in command mode. Ifthis register is non-zero, the modem will power up in data mode.S Register 2 - Escape CodeThis register contains the ASCII value of the escape character.The default value (decimal 43) is equivalent to the ASCII character ‘+’.Values greater than 127 disable the escape feature and prevent you fromreturning to the Command Mode.Default is ‘+’ (decimal 43).S Register 3 - CR Control CodeThis register contains the ASCII value of the carriage return character.This is the character that is used to end the command line and is also thecharacter that appears after the modem sends a response.Default is ‘CR’ (decimal 13).S Register 4 - Linefeed Control CodeRegister S4 sets the ASCII value of the linefeed character.The modem sends the linefeed character after sending a carriage returncharacter when sending text responses.Default is ‘LF’ (decimal 10).S Register 5 - Backspace Control CodeRegister S5 sets the ASCII value of the backspace character.This character is both the character created by entering BACKSPACE andthe character echoed to move the cursor to the left.Default is ‘BS’ (decimal 8).
12 MHX-900 Operating Manual: Chapter 3 Configuration OptionsEach unit must be either aMaster or Slave.Only one Master can existfor each network.S Register 101 - Operating ModeThe Operating Mode (register S101) partly defines the “personality” of theMHX-900 module. Allowable settings for this register are 1 through 4.Default is 2 (Slave point to point).Each unit should be configured as a Master or Slave. The user shoulddecide, prior to deployment, whether a point-to-point or point-to-multipointnetwork is appropriate for the application, and how each unit will beassigned to terminals.1)Master - Point to Point One unit in a point-to-point linkshould be set as the Master, obligating the other modem to be used as aSlave. During operation, it makes little difference which end is which,especially if data is generally transmitted unidirectionally at any givenmoment. When both terminals have data to send, more bandwidth isdynamically allocated to the Master; otherwise, the transmitting end,whichever it is at the time, is given the full bandwidth.* 2)Slave - Point to Point The terminal in a point-to-pointnetwork which is not set as the Master, is obligated to be the Slave. TheSlave will communicate with the Master, whose network address, unitaddress, hopping pattern and hopping interval matches its own.In point-to-point operation, the Slave modem acknowledges all packets ofdata sent by the Master, and vice versa. These acknowledgements, alongwith CRC error checking ensure that data is passed through exactly once,and that the data is not corrupted.3)Master - Point to Multipoint In a point-to-multipointnetwork, one unit should be set as the Master, obligating all other modemsto be Slaves. The Master designation is generally given to the station whichfunctions as a hub for all remote nodes, and it is the Master station’sresponsibility to control communications with Slaves.Point-to-multipoint operation differs from point-to-point operation in manyways. One key difference is that the Slaves cannot acknowledge packets ofdata sent by the Master. Clearly this would cause conflicts when there aremultiple Slaves. The Master does, however, send acknowledgements to allmessages it receives from Slaves. The Master initiates communications bysending a broadcast message to all Slaves. All Slaves are free to respond ina “Slotted ALOHA” fashion, meaning that each Slave can choose one ofseveral windows in which to transmit. If there happens to be two Slavesattempting to talk at the same time, then the Master would not receive thedata, and the Slaves therefore would not get an acknowledgement. At thispoint, the Slaves would attempt to get the information through at randomtime intervals, thus attempting to avoid any more conflicts. Specialparameters related to Point-to-Multipoint operation are set in the PacketParameters menu, and are described in a later section.4) Slave - Point to Multipoint Several Slaves exist in apoint-to-multipoint network, all of which communicate with the commonMaster. Slaves cannot directly communicate with other Slaves.
MHX-900 Operating Manual: Chapter 3 Configuration Options 13All units in a network mustbe set to the same wirelesslink rate.S Register 102 - Serial Baud RateThe Serial Baud Rate is the current speed that the modem is using tocommunicate with the DTE. When the AT command prefix is issued, themodem performs an ‘autobaud’ operation and determines what the currentDTE baud rate is set to. The S register value returns the current setting ofthe DTE baud rate.The possible values are:1 1152002 576003 384004 288005 192006 14400*7 96008 72009 480010 360011 2400It is generally advisable to choose the highest rate that your terminalequipment will handle to maximize performance, unless a limitation on theavailable bandwidth is desired. If the DTE is a personal computer, the portcan usually be used reliably at 115200.S Register 103 - Wireless Link RateThe Wireless Link Rate is the speed and optimization method for whichmodems will communicate over the RF link.The allowable settings are:* 1 Turbo2Fast3NormalDepending on the application requirements, each mode will providedifferent throughputs and performance optimizations as follows:Mode Expected Performance(Maximum Throughput) Primary OptimizationTurbo 96 kbps Optimized for SpeedFast 60 kbps Optimized for Distance and SpeedNormal 30 kbps Optimized for DistanceGenerally, the lowest rate which provides sufficient bandwidth should beselected. For example, if DTEs are set to communicate at 19.2 kbps, thenthe wireless rate can be set to Normal (with the added advantage of slightlygreater range). If DTEs require nearly 115.2 kbps of sustained bandwidth,then the wireless rate should be set to Turbo (setting it lower would“bottleneck” data transmissions in the wireless link). Note that there is acompromise between speed, performance and range.
14 MHX-900 Operating Manual: Chapter 3 Configuration OptionsSelect a Network Addressand assign it to all unitswhich will be included in thenetwork.Use the same Unit Addresson both units for point-to-point mode. In multipointmode, set all Slaves to aunique Unit Address.In Point-to-Point, valid UnitAddresses are 0 to 65535. InPoint-to-Multipoint, validSlave Unit Addresses are 1to 65535.Ensure that all units within anetwork use the samehopping pattern, and thatmultiple concurrent networkseach have different hoppingpatterns.All units within a networkmust use the sameencryption key.S Register 104 - Network AddressThe Network Address defines the membership to which individual units canbe a part of. By establishing a network under a common Network Address,the network can be isolated from any other concurrently operating networkusing the same hardware. As well, the Network Address provides a measureof privacy and security. Only those units which are members of the networkwill participate in the communications interchange. Valid values for theNetwork Address range from 0 to 65535, inclusive.To enhance privacy and reliability of communications where multiplenetworks may operate concurrently in close proximity, it is suggested that anatypical value be chosen – perhaps something meaningful yet not easilyselected by chance or coincidence.Default is 0.S Register 105 - Unit AddressIn point-to-point operation, the Unit Address on both the Master and Slaveunits must be the same. In point-to-point operation, valid Unit Addressesrange from 0 to 65535. In a multipoint system, the Unit Address uniquelyidentifies each Slave from the others.. In a multipoint system, a Slave cantake on any Unit Address between 1 and 65535.Remember that each Slave should have a Unit Address which is unique fromany other Slave in the network.Default is 1.S Register 106 - Hopping PatternSince the MHX-900 is a frequency-hopping modem, the carrier frequencychanges periodically according to one of twenty pseudo-random patterns,selected by defining the Hopping Pattern.A value from 1 to 20, inclusive, can be used to select the pattern. It isimportant that all units which will participate in a network use the samehopping pattern, or the communication link will fail.Default is 1.S Register 107 - Encryption KeyThe Encryption Key provides a measure of security and privacy ofcommunications by rendering the transmitted data useless without thecorrect key on the receiver. Valid Encryption Keys range from 0 to 65535.Keep in mind that all units within the network must use the same key forcommunications to succeed.
MHX-900 Operating Manual: Chapter 3 Configuration Options 15Test the communicationslink using a low power leveland work upward. Avoidusing a higher power thannecessary since performancemay actually degrade.S Register 108 - Output Power LevelThe Output Power Level determines at what power the MHX-900 transmits.The super-sensitive MHX-900 can operate with very low power levels, so itis recommended that the lowest power necessary is used; using excessivepower contributes to unnecessary “RF pollution”.The allowable settings are:* 1 1 mW210 mW3100 mW41000 mWIdeally, you should test the communications performance between unitsstarting from a low power level and working upward until the RSSI issufficiently high and a reliable link is established. Although the conditionswill vary widely between applications, typical uses for each setting aredescribed below:Power Use1 mW For in-building use, typically provides a link up to 300 feet on thesame floor or up/down a level. Outdoors, distances of 10 km can beachieved if high-gain (directional) antennas are placed high aboveground level and are in direct line-of-sight.10 mW 200-500 ft indoors, 8-15 km outdoors.100 mW 400-800 ft indoors, 15-25 km outdoors.1000 mW(1 W)Typically provides communications up to a distance of 1000 feet ormore in-building on the same floor or up/down a few levels,depending on building construction (wood, concrete, steel, etc.). Inideal line-of-sight conditions, up to 30 km or more can be achieved.Note that only an antenna with a gain of no more 6 dBi may be used.Any higher is a violation of FCC rules. See IMPORTANT warningbelow.IMPORTANT:FCC and Industry Canada Regulations allow up to 36 dBi effectiveradiated power (ERP). Therefore, the sum of the transmitted power (indBm), the cabling loss and the antenna gain cannot exceed 36 dBi.1 mW = 0 dBm10 mW = 10 dBm100 mW = 20 dBm1000 mW = 30 dBmFor example, when transmitting 1 Watt (30 dBm), with cabling losses of2 dB, the antenna gain cannot exceed 36 - 30 + 2 = 8 dBi. If an antennawith a gain higher than 8 dBi were to be used, the power setting mustbe adjusted appropriately.Violation of FCC or IC regulations can result in severe fines. It isthe responsibility of the user to understand and ensure compliancewith these regulations.
16 MHX-900 Operating Manual: Chapter 3 Configuration OptionsIn a point-to-point network,the master and slave must beset to the same hoppinginterval.In a point-to-multipointnetwork, the hoppinginterval is controlled by themaster. The slave units willuse the hopping intervalsetting from the master.S Register 109 - Hopping IntervalThis option determines the frequency at which the modems change channel.Note that all modems must have the same hopping interval setting.The allowable settings are:125 msec250 msec3100 msec4150 msec*5 200 msec6250 msec7300 msec8350 msecIn Slave - Point-to-Multipoint operation, this parameter is determined by theMaster, and cannot be altered.Packet ParametersPacket Parameters define the characteristics of the internal packets or frameswhich are transmitted. The settings should be set only by an expert sinceadequate care must be taken to maintain reliability and optimumperformance. Packet Parameters will vary depending on the modem’sOperating Mode.S Registers 111 to 113 - Packet Parameters -Point to PointModeIn Point-to-Point (both Master and Slave), the following settings areavailable:S Register 111 - Minimum SizeThis setting has a range of 0 to 255, and defines the number of bytes toaccumulate from the DTE before transmitting a packet. Setting this value to0 ensures that all characters sent by the DTE are immediately transmitted.The value may not exceed the maximum packet size.S Register 112 - Maximum SizeThis setting has a range of 1 to 255, and defines the maximum number ofbytes from the DTE which should be encapsulated in a packet. This valueshould be greater than the minimum packet size, but not smaller than isnecessary for reliable communications. If the wireless link is consistentlygood and solid, a maximum size of 255 will yield the best throughput(depending on the higher level protocols of the connected equipment).However, if the link is poor (e.g., experiencing excessive interference) anddata is frequently retransmitted, the maximum packet size should bereduced. This decreases the probability of errors within packets, andreduces the amount of traffic in the event that retransmissions are required.Since a smaller packet size results in a proportionally higher overhead andlower overall throughput on a good connection, this should only be reducedif many errors are being detected by CRC (see Radio Statistics option ofconfiguration).
MHX-900 Operating Manual: Chapter 3 Configuration Options 17S Register 113 - Retry LimitThis setting has a range of 0 to 255, and determines the number of attemptsthat will be made to retransmit data which failed the CRC checksum. Oncethe limit is reached, the modem will give up and discard the data. If thewireless connection is poor and data often needs to be retransmitted, amodem could be “tied up” trying to retransmit, thus holding up pendingdata.S Registers 111 to 115 - Packet Parameters - Point-to-Multipoint ModeIf the Operating Mode is Slave - Point-to-Multipoint, the default values forthe S registers are:S111 Minimum Size (not used)S112 Maximum Size (not used)S113 Retry Limit 4S115 Repeat Interval 8In Master - Point-to-Multipoint mode, the default values for the S registersare: S111 Minimum Size 0S112 Maximum Size 255S113 Retransmissions 255S114 Address Header DisabledS Register 111 - Minimum SizeThe Minimum Size parameter prevents the Slave from sending data until ithas buffered a minimum number of bytes. The Master determines thisparameter and passes the information to all Slaves in a Multipoint system.S Register 112 - Maximum SizePacket sizes in excess of the minimum size are limited by the MaximumSize parameter. If the Slave is sending more than the minimum, the unit willwait until it has buffered the maximum packet size or until a packet timeoutof two characters occurs on the serial port. The Master unit tells all Slaveswhich maximum and minimum packet values to use. In addition, the Mastertells all Slaves which Hopping Interval to use (see Section 3.11).S Register 113 - Retry Limit (Slave Point toMultipoint)Similar to the Retry Limit in Point-to-Point, the Slaves in a multipointsystem will reattempt to send a packet of data until it receives anacknowledge or until it reaches the Retry Limit.
18 MHX-900 Operating Manual: Chapter 3 Configuration OptionsS Register 113 - Retransmissions (Master Point toMultipoint)In a multipoint system, the Master does not receive acknowledgments fromSlaves, and therefore has a slightly modified parameter calledRetransmissions. The Master will retransmit each data packet exactly thenumber of times defined by the Retransmissions parameter. The Masterretransmits once per hopping interval until the limit is reached.S Register 114 - Address HeaderA packet parameter unique to Master - Point-to-Multipoint is the AddressHeader parameter. The allowable settings are:*1 Disabled2Binary3TextWhen the Master receives a packet from a Slave, it has the option ofattaching an address header at the beginning of each packet.Option 2 will attach two bytes of binary data that represent the Slave UnitAddress. The first byte is MSB.Option 3 will attach five bytes of data that represent the text version of theSlave Unit Address. The Maximum Packet Size includes the AddressHeader. For example, if the Maximum Packet Size is set to 128 and theText Address Header option is selected, then the Master will tell the Slaveto send a maximum of 123 bytes.No address header is attached when Option 1) is used. In this case, it is upto the equipment connected to the Slave to attach an address header of somesort so that the Master equipment can determine the source of the datatransmission.S Register 115 - Packet Repeat IntervalA parameter that is specific to the Slave in multipoint operation is theRepeat Interval.The allowable settings are:1 1...12 1...33 1...74 1...155 1...316 1...637 1...127*8 1...255This parameter defines a range of random numbers that the Slave will use asthe next slot in which it will attempt to send the packet. For example, ifOption 3 (1...7) is selected, and the random number generator picks 5, thenthe Slave will transmit after five time slots. A Slave will transmit amaximum of once per hopping interval. The Slave will transmit morefrequently when a Repeat Interval with a smaller range is selected. Choose1) for the most frequent repeats and choose 8) for the least frequent repeats.
MHX-900 Operating Manual: Appendix A Modem Command Summary 19A. Modem Command SummaryThe following provides a command summary for the MHX-900 module. Factory settings are denoted with a ‘*’.AT CommandsAAnswerDDialECommand EchoE0 No Echo* E1 Command EchoIIdentificationI0 Product CodeI1 ChecksumI2 ROM ChecksumI3 Product IDI4 Not SupportedI5 Firmware VersionOOn-line ModeQQuiet Mode* Q0 Enables Result CodesQ1 Disables Result CodesVResult Codes DisplayV0 Display as Numbers* V1 Display as WordsWConnection Result* W0 Reports DTE as CONNECT xxxxW1 Reports computer (DTE) rate and wirelessrate between modems as CARRIER xxxx.W2 Reports DCE as CONNECT xxxxZReset and load stored configuration&C DCD (Data Carrier Detect)&C0 DCD is always on* &C1 DCD is on when modems aresynchronized&D DTR (Data Terminal Ready)&D0 DTR ignored* &D2 DTR disconnects and switches tocommand&D3 DTR disconnects and resets modem&F Load Factory Default&K Handshaking&K0 Disable Handshaking* &K3 Enable Handshaking&S DSR (Data Set Ready)&S0 DSR is always on* &S1 DSR on in data, off in command mode&V View Configuration&W Write configuration to memorySxx? Read S register valueSxx=yy Set S register valueResult Codes0OK1CONNECT2RING3NO CARRIER4ERROR8NO ANSWER10 CONNECT 240011 CONNECT 480012 CONNECT 960014 CONNECT 1920017 CONNECT 3840018 CONNECT 5760033 CONNECT 115200S RegistersS0 Number of Rings - Auto Answer [0...255]S2 Escape code [0...255] default ‘+’S3 CR character [0...255] default <cr>S4 Line Feed [0...255] default <lf>S5 Backspace [0...255] default <bs>S101 Operating Mode1 - Master point to point* 2 - Slave point to point3 - Master point to multipoint4 - Slave point to multipointS102 Serial Baud Rate1 = 115200, 2 = 57600, 3 = 384004 = 28800, 5 = 19200, 6 = 14400* 7 = 9600, 8 = 7200, 9 = 4800,10 = 3600, 11 = 2400S103 Wireless Link Rate*1 = Turbo, 2 = Fast, 3 = NormalS104 Network Address [0...65535]S105 Unit Address [0...65535]S106 Hopping Pattern [1...20]S107 Encryption Key [0...65535]S108 Output Power Level* 0 = 1 mW, 1 = 10 mW, 2 = 100 mW3 = 1000 mWS109 Hopping Interval1 = 25 msec, 2 = 50 msec, 3 = 100 msec,4 = 150 msec, * 5 = 200 msec, 6 = 250 msec,7 = 300 msec, 8 = 350 msecS111 Packet Minimum Size [0...Maximum Size]S112 Packet Maximum Size [1...255]S113 Packet Retry Transmissions [0...255]S114 Address Header* 0 = Disabled, 1 = Binary, 2 = TextS115 Packet Repeat Interval1 = 1...1, 2 = 1...3, 3 = 1...7, 4 = 1...155 = 1...31, 6 = 1...63, 7 = 1...127, * 8 = 1...255
20 MHX-900 Operating Manual: Appendix A Modem Command Summary
MHX-900 Operating Manual: Appendix B Serial Interface 21B. Serial InterfaceModem(DCE) SignalHostMicroprocessor(DTE)21  DCD →IN22  RX →IN23 ← TX  OUT24 ← DTR  OUT17  SG 26  DSR →IN27 ← RTS  OUT28  CTS →INArrows denote the direction thatsignals are asserted (e.g., DCDoriginates at the DCE and tells theDTE that a carrier is present).The MHX-900 module uses 8 pins on the 40 pin connector for asynchronousserial I/O. The interface conforms to TTL level RS-232 signals (ie. withoutlevel shifting), so direct connection to a host microprocessor is possible.The signals in the asynchronous serial interface are described below:DCD Data Carrier Detect - Output from Modem - When asserted (TTL low),DCD informs the DTE that a communications link has been established withanother MHX-900.RX Receive Data - Output from Modem - Signals transferred from the MHX-900 are received by the DTE via RX.TX Transmit Data - Input to Modem - Signals are transmitted from the DTE viaTX to the MHX-900.DTR Data Terminal Ready - Input to Modem - Asserted (TTL low) by the DTE toinform the modem that it is alive and ready for communications.SG Signal Ground - Provides a ground reference for all signals transmitted byboth DTE and DCE.DSR Data Set Ready - Output from Modem - Asserted (TTL low) by the DCE toinform the DTE that it is alive and ready for communications. DSR is themodem’s equivalent of the DTR signal.RTS Request to Send - Input to Modem - A “handshaking” signal which isasserted by the DTE (TTL low) when it is ready. When hardwarehandshaking is used, the RTS signal indicates to the DCE that the host canreceive data.CTS Clear to Send - Output from Modem - A “handshaking” signal which isasserted by the DCE (TTL low) when it has enabled communications andtransmission from the DTE can commence. When hardware handshaking isused, the CTS signal indicates to the host that the DCE can receive data.Notes: It is typical to refer to RX and TX from the perspective of the DTE. This should bekept in mind when looking at signals relative to the modem (DCE); the modemtransmits data on the RX line, and receives on TX.“DCE” and “modem” are often synonymous since a modem is typically a DCE device.“DTE” is, in most applications, a device such as a host microprocessor.
22 MHX-900 Operating Manual: Appendix B Serial Interface
MHX-900 Operating Manual: Appendix C Sample Schematic Diagram 23C. Sample Schematic DiagramThe following is a sample microprocessor implementation with a MICROCHIP PIC 16C74 and the MHX-900. The MHX-900performs no level shifting on the serial port, so direct connection to the host microprocessor is possible.DO NOT CONNECT THE MHX-900 TO RS 232 DRIVER OUTPUTS. DAMAGE TO THE UNIT MAY RESULT.On this implementation, the onboard SCI of the PIC 16C74 is directly connected pins 22 and 23 of the MHX-900. The bi-directional Port D is used for asserting or monitoring control signals from the MHX-900.The RESET signal is a momentary active low signal asserted by the host microprocessor.RESET initializes the MHX-900 and places the system in a known state. This signal should be set high after the hostmicroprocessor has been reset.RXDTXDDCDDTRDSRRTSCTSRESETPIC16C74MHX-9002223212426272810Power Connections are not shownRC7RC6RD0RD1RD2RD3RD4RD6
24 MHX-900 Operating Manual: Appendix C Sample Schematic Diagram
MHX-900 Operating Manual: Appendix D Modem Setup Overview 25D. Modem Setup OverviewThe required commands required to setup an MHX-900 module for point-to-point and point-to-multipoint aresummarized below. These requirements will get you started and only ensure that a link can be established, but donot necessarily provide the best performance.Point-to-PointTo establish a point-to-point communications link, the following commands must be issued by the host EOMmicroprocessor:nThe Operating Mode S Register (S101) for one modem must be configured as a “Master - Point-to-Point”(1), and the other as a “ Slave - Point-to-Point” (2).nThe Wireless Link Rate S Register (S103) value for both modems must be the same.nThe Network Address S Register (S104) value assigned for both modems must be the same.nThe Unit Address S Register (S105) value assigned for both modems must be the same.nThe same Hopping Pattern S Register (S106) value must be selected for both modems.nThe same Hopping Interval S Register (S109) value must be selected for both modems.nThe same Encryption Key S Register (S107) value must be selected for both modems.The master can now attempt to synchronize with the slave using the D<slave address> command.Point-to-MultipointTo establish a point-to-point communications link, the following commands must be issued by the host OEMmicroprocessor:nThe Operating Mode S Register (S101) for one modem must be configured as a “Point-to-MultipointMaster ” (3), and the other as a “Point-to-Multipoint Slave” (4).nThe Wireless Link Rate S Register (S103) value for both modems must be the same.nThe Network Address S Register (S104) value assigned for both modems must be the same.nEach Slave must be assigned an unique Unit Address S Register (S105) value (Unit Address 0 is notsupported).nThe same Hopping Pattern S Register (S106) value must be selected for both modems.nThe same Hopping Interval S Register (S109) value must be selected for both modems.nThe same Encryption Key S Register (S107) value must be selected for both modems.The master can now attempt to synchronize with the slave using the D<slave address> command.
26 MHX-900 Operating Manual: Appendix D Modem Setup Overview
MHX-900 Operating Manual: Appendix E Technical Specifications 27E. Technical SpecificationsElectrical/PhysicalData Interface Asynchronous Serial Port, TTL LevelsRS-232 Signals Sig. Gnd, TX, RX, DCD, DSR, DTR, RTS, CTSOther Signals Reset, RxMode, TxMode, RSSI1, RSSI2, RSSI3(All except Reset are status signals)User Interface AT Command line interfaceBandwidth / Data Rate 2,400 - 115,200 bps, uncompressed half-duplex,Approx. 100 kbps sustained in intelligent asymmetrical full-duplextransmission modeCommunications Range Up to 30 kilometres (19 miles) line of sightPower Requirements 4.75 to 5.5 VDCPower Consumption 5500 mA at 1 W transmit, 220 mA receiveOperating Frequency 902 - 928 MHzSystem Gain 135 dBSensitivity -105 dBmOutput Power 1mW, 10mW, 100mW, 1W (user-selectable)Spreading Code Frequency HoppingHopping Patterns 20 pseudo-random, user-selectableError Detection CRC-16 with auto re-transmitDimensions (WxDxH) Encl: 2.1” x 3.5” x 0.75” (53 mm x 89 mm x 19 mm)Weight 75 gramsOperating Environment Temperature: -40 to +65°CHumidity: 5 to 95%, non-condensing
28 MHX-900 Operating Manual: Appendix E Technical Specifications
MHX-900 Operating Manual: Appendix F Glossary 29F. GlossaryTerminology Used in the MHX-900 Operating ManualAsynchronous communications A method oftelecommunications in which units of single bytesof data are sent separately and at an arbitrary time(not periodically or referenced to a clock). Bytesare “padded” with start and stop bits to distinguisheach as a unit for the receiving end, which neednot be synchronized with the sending terminal.Attenuation The loss of signal power throughequipment, lines/cables, or other transmissiondevices. Measured in decibels (dB).Bandwidth The information-carrying capacity of adata transmission medium or device, usuallyexpressed in bits/second (bps).Baud Unit of signaling speed equivalent to thenumber of discrete conditions or events persecond. If each signal event represents only onebit condition, then baud rate equals bits persecond (bps) – this is generally true of the serialdata port, so baud and bps have been usedinterchangeably in this manual when referring tothe serial port; this is not always the case duringthe DCE-to-DCE communications, where anumber of modulation techniques are used toincrease the bps rate over the baud rate.Bit The smallest unit of information in a binarysystem, represented by either a 1 or 0.Abbreviated “b”.Bits per second (b/s or bps) A measure of datatransmission rate in serial communications. Alsosee baud.Byte A group of bits, generally 8 bits in length. Abyte typically represents a character of data.Abbreviated “B”.Characters per second (cps) A measure of datatransmission rate for common exchanges of data.A character is usually represented by 10 bits: an 8-bit byte plus two additional bits for marking thestart and stop. Thus, in most cases (but notalways), cps is related to bits per second (bps) bya 1:10 ratio.CRC (Cyclic Redundancy Check) An error-detectionscheme for transmitted data. Performed by usinga polynomial algorithm on data, and appending achecksum to the end of the packet. At thereceiving end, a similar algorithm is performedand checked against the transmitted checksum.Crossover cable (Also known as rollover, null-modem, or modem-eliminator cable) A cablewhich allows direct DTE-to-DTE connectionwithout intermediate DCEs typically used tobridge the two communicating devices. Can alsobe used to make cabled DCE-to-DCE connections.The name is derived from “crossing” or “rolling”several lines, including the TX and RX lines sothat transmitted data from one DTE is received onthe RX pin of the other DTE and vice-versa.Data Communications Equipment (DCE, alsoreferred to as Data Circuit-TerminatingEquipment, Data Set) A device which facilitates acommunications connection between DataTerminal Equipment (DTEs). Often, two or morecompatible DCE devices are used to “bridge”DTEs which need to exchange data. A DCEperforms signal encoding, decoding, andconversion of data sent/received by the DTE, andtransmits/receives data with another DCE.Common example is a modem.Data Terminal Equipment (DTE) An end-device which sends/receives data to/from a DCE,often providing a user-interface for informationexchange. Common examples are computers,terminals, and printers.dBm Stands for “Decibels referenced to onemilliwatt (1 mW)”. A standard unit of powerlevel commonly used in RF and communicationswork. n dBm is equal to 10(n/10) milliwatt, so0dBm = 1mW, -10dBm = 0.1mW, -20dBm =0.01mW, etc.DCE See Data Communications Equipment.DTE See Data Terminal Equipment.Flow Control A method of moderating thetransmission of data so that all devices within thecommunications link (DTEs and DCEs) transmitand receive only as much data as they can handleat once. This prevents devices from sending datawhich cannot be received at the other end due toconditions such as a full buffer or hardware not ina ready state. This is ideally handled by hardwareusing flow-control and handshaking signals, but
30 MHX-900 Operating Manual: Appendix F Glossarycan be controlled also by software using X-ON/X-OFF (transmitter on/off) commands.Frequency-hopping A type of spread spectrumcommunication whereby the carrier frequencyused between transmitter and receiver changesrepeatedly in a synchronized fashion according toa specified algorithm or table. This minimizesunauthorized jamming (interference) andinterception of telecommunications.Full-duplex Where data can be transmitted,simultaneously and independently, bi-directionally.Half duplex Exists when the communicationsmedium supports bi-directional transmission, butdata can only travel in one direction at the sametime.Handshaking A flow-control procedure forestablishing data communications wherebydevices indicate that data is to be sent and awaitappropriate signals that allow them to proceed.Line-of-sight Condition in which a transmittedsignal can reach its destination by travelling astraight path, without being absorbed and/orbounced by objects in its path.Master The station which controls and/or polls oneor more Slave stations in a point-to-point or point-to-multipoint network. Often functions as a serveror hub for the network.Non-volatile memory Memory which retainsinformation which is written to it.Null modem cable See Crossover cable.Point-to-point A simple communications networkin which only two DTEs are participants.Point-to-multipoint A communications networkin which a Master DTE communicates with two ormore Slave DTEs.Repeater A device which automatically amplifiesor restores signals to compensate for distortionand/or attenuation prior to retransmission. Arepeater is typically used to extend the distancefor which data can be reliably transmitted using aparticular medium or communications device.RS-232 (Recommended Standard 232; moreaccurately, RS-232C or EIA/TIA-232E) Definedby the EIA, a widely known standard electricaland physical interface for linking DCEs and DTEsfor serial data communications. Traditionallyspecifies a 25-pin D-sub connector, althoughmany newer devices use a compact 9-pinconnector with only the essential signaling linesused in asynchronous serial communications.Lines have two possible states: “high” (on, active,asserted, carrying +3 to +25 V) or “low” (off,inactive, disasserted, carrying -3 to -25 V).RTU (Remote Terminal Unit) A common termdescribing a DTE device which is part of a wide-area network. Often a RTU performs data I/O andtransmits the data to a centralized station.Serial communications A common mode ofdata transmission whereby character bits are sentsequentially, one at a time, using the samesignaling line. Contrast with parallelcommunications where all bits of a byte aretransmitted at once, usually requiring a signal linefor each bit.Shielded cable Interface medium which isinternally shrouded by a protective sheath tominimize external electromagnetic interference(“noise”).Slave A station which is controlled and/or polled bythe Master station for communications. Typicallyrepresents one end of a point-to-point connection,or one of the terminal nodes in a point-to-multipoint network. Often a RTU is linked by aSlave DCE.Spread spectrum A method of transmitting asignal over a wider bandwidth (using severalfrequencies) than the minimum necessary for theoriginally narrowband signal. A number oftechniques are used to achieve spread spectrumtelecommunications, including frequency hopping.Spread spectrum provides the possibility ofsharing the same band amongst many users whileincreasing the tolerance to interference and noise,and enhancing privacy of communications.Throughput A measure of the rate of data trans-mission passing through a data communicationsystem, often expressed as bits or characters persecond (bps or cps).

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