TM8000 Internal Modem TECHNOTE/TM8000/TN 855a AN Application Note Version TN

User Manual: Pdf TECHNOTE/TM8000/TN-855a-AN - TM8000 Internal Modem Application Note version

Open the PDF directly: View PDF .
Page Count: 12

Download
Open PDF In Browser	View PDF

TM8000 mobile radios

TN-855-AN
TM8000
Internal Modem

Application Note version 1.2
23 July 2004

This application note provides an introduction to the data facilities that Tait
provides with the TM8000 series of mobiles. It covers the internal software
modems provided with the TM8000 range and the protocols used to drive
them.
This application note is intended for System Integrators.

Overview
As a standard feature, the TM8105 and TM8115 mobiles come with an
internal DSP (Digital Signal Processor) 1200-baud modem.
As an option, the radio can be upgraded to provide a high-speed 12000bps
or 19200bps modem. This high-speed option is known as THSD (Tait highspeed data).
The 19200bps (19k2bps) option can only be used on wideband channels
(25kHz) and is not available in some countries due to type approval
restrictions.
These modems have a number of configurable parameters, which are
configured using the TM8000 radio programming software.
The 1200 baud standard option uses the rugged FFSK (Fast Frequency Shift
Keying) modulation scheme; this works well at low signal levels and through
most repeaters.
The THSD option uses a 4 level frequency shift keying modulation scheme,
which occupies most of the available audio bandwidth and requires
synchronisation.
There are a number of features that have been added to the THSD option
to make it as rugged as possible. However, as with most high-speed data
modems, it will not work as well as FFSK at lower signal levels and may not
successfully pass through many repeater systems.
The choice of modem ultimately comes down to the amount of data to be
transferred and the RF path between the radios.

Page 2 of 12

Protocols
Two modes are used in conjunction with the modems; command mode and
transparent mode.

Command Mode
In this mode, a Tait Proprietary protocol, CCDI (Computer Controlled
Data Interface), can be used to change channels, send SDMs (short data
messages), and various other functions.
A CCDI protocol manual is available which gives a full list of commands.
The format of the commands is in ASCII text: a one letter identifier, eg “s”
for send SDM; the number of characters to follow, eg “13”; the data; and
finally a two digit checksum.
Command mode allows for a CCDI command to change to transparent
mode.

Transparent Mode
In transparent mode, any data sent to the serial port will be picked up,
processed, and sent over the air. At the receiving radio the data will be
demodulated, processed, and then sent to the serial port.
This provides a simple interface to an end user; any data sent to the serial
port of the radio will appear at the other end, and no special commands are
needed to “drive” the modem.
From transparent mode, an escape sequence (if enabled) can be used to
change back into command mode.
Transparent mode can use either 1200 baud FFSK or the optional THSD
modem.

DTE Baud Rates
Different baud rates can be used between the DTE and the radio modem;
from 1200 to 28800bps.
However, over-the-air data is always transmitted at 1200bps for the FFSK
modem, and at 12kbps or 19k2bps for the THSD modem.
If the baud rate is programmed higher than the over-the-air rate, then flow
control should be used to prevent buffer over-run.

Page 3 of 12

Flow Control
The TM8000 provides two forms of flow control:
■

Hardware RTS/CTS

■

Software Xon / Xoff.

These are configurable in the programming software.
The hardware flow control lines are assigned to programmable I/O pins.
Levels for these pins are 3v3 CMOS.
If using the microphone socket, only one GPIO is available, so only RTS or
CTS software flow control can be assigned; Xon/Xoff software flow control
may be more suitable.

Buffer
The radio provides a 128-byte receive and transmit buffer for both the
THSD and FFSK modems.

Error Checking and Correction
The 1200-baud FFSK modem does not provide any true form of error
checking or correction. This is left up to the third party application. There
is however, a TM8000 radio programming option called “Check Packet
Length”, that provides a limited kind of error checking for FFSK data.
Often error checking or correction is not necessary using FFSK, as this will
work well at low signal levels without the FEC enabled.
The THSD modem provides an FEC (Forward Error Correction) option.
This uses a Reed-Solomon algorithm and enables the modem to work at
lower signal levels.
It is important to note that the effective throughput of the modem will drop
when FEC is added. This is due to the information added to the data packet
to detect and correct errors.

Retries
Neither modem will perform retries of missed or incorrect packets; this is
instead left up to the third party application.

Page 4 of 12

Entering and Exiting Data Transfer Mode
Either command or transparent mode can be enabled upon start-up. When
the radio is powered on, it can automatically start accepting CCDI
commands or start transferring data in transparent mode.
Transparent mode can be entered via a CCDI command from command
mode, also, an external GPIO line can be assigned to switch the radio in and
out of THSD mode from command mode.
When in THSD mode, other audible (Selcall, DTMF, FFSK, etc) or subaudible (CTCSS, DCS) signalling schemes are disabled, as the modem
requires the full audio bandwidth.
If the radio is required for voice and data communications, the external
modem should change the radio into THSD mode (using a CCDI
command or an input line) when it has data to send, and then change the
radio back to command mode when it has finished sending data.
To activate the modem the following CCDI commands can be used:
■

“t02+0CF” - Enter 1200 baud FFSK transparent mode, using “+” as an
escape sequence.

■

“t02+HB7” - Enter THSD transparent mode (setting as programmed
into the radio), using “+” as an escape sequence.

If the radio accepts the command it will issue a prompt (‘.’), otherwise a
CCDI error message is returned.
To return to command mode from transparent mode, the escape sequence
is used (in the above examples this is the character “+”).
If the radio has been programmed to start up in transparent mode, the
default escape character is “+”.
To send the escape sequence, three escape sequence characters are sent in a
row (eg “+++”), and then a period of no serial communications for 2
seconds must be maintained.
The radio will then change to command mode.
It is important that the escape sequence does appear in the user data, sending
the modem back to command mode.
It is possible to program the radio to ignore the escape sequence.

Page 5 of 12

Serial Ports and Physical Connections
The TM8000 range supports three high-speed serial ports. One at the front
of the radio, one in the internal options connector, and one at the rear of
the radio.
One of these serial ports at a time can be assigned to accept CCDI
commands or to transfer data.
These serial ports are at 3v3 CMOS levels.
The front serial port is the microphone socket on the TM8115. This can be
converted to RS232 levels by using the T2000-A19 programming lead and
the TMAA20-04 RJ12 to RJ45 adaptor.
If using a TM8105 blank control head radio, then the front serial connection
is available as a 9-way D-range. The Rx and Tx data levels are 3v3 CMOS.
The internal options connector also provides a serial port at 3v3 CMOS
levels; this is available as an 18-way micromatch connector. This is typically
used to connect to internally mounted options boards.
The third serial port is available at the rear of the radio; this is a low-density
15-way connector. Levels are also 3v3 CMOS levels.
The radio programming software is used to configure the serial ports, baud
rates, and flow control.

Modulation Schemes
The modulation scheme for the 1200-baud modem is FFSK. Data is
represented by 1 cycle of 1200 Hz (logic 1) or 1.5 cycles of 1800 Hz (logic
0), and is transmitted at rates of 1200 bps.
It is important to note that while the FFSK format is widely used, it is
unlikely that a third party FFSK modem will be compatible with the Tait
1200 Baud FFSK implementation.
This is mainly due to the proprietary way in which the data is packed and
sent over the air.
The THSD modem uses a CP-4GFSK modulation scheme.
This modulation scheme is outlined in the ETSI DIIS specification.
This modulation scheme was chosen mainly for its ability to achieve high
data rates while maintaining compliance with ETS300-113, the European
data standard.12

Page 6 of 12

Over-air Packing and Processing
How the data is packed and sent over the air is often irrelevant to the end
user, as the radio takes care of the packing and formatting so that the external
controlling equipment does not have to present its data in any special format.
A brief description of over-air packing and processing is given here for
interest.

1200 baud Modem
The 1200-baud modem sends data over the air in the following format:
Lead in delay
(programmable)

Data block

nth data block

Lead out delay
(programmable)

A data block consists of:
Preamble

Sync

Length

Data

Dummy CRC

THSD Modem
The THSD modem sends data over the air in the following format:
Lead in delay
30 symbol 588 symbols 30 symbol . . . . . . . . 588 symbols 30 symbol Lead out delay
(programmable) start
data
start
data
start
(programmable)
sequence
sequence
sequence

Transmission of Start and Stop Bits
Data presented to the DCE should be no parity, eight data bits, one start bit
and one stop bit.
The start and stop bits will be stripped from the data before being sent over
the air and re-inserted at the receiving radio.

Scrambling
In The THSD implementation, data is scrambled before it is packed and sent
over the air. This is to prevent the occurrence of long periods of 1's or 0's in
the data, which have the potential for a DC offset to occur, possibly causing
the data transmission to fail.

Page 7 of 12

Synchronisation
Synchronisation (sync) sequences are added into the data stream. The sync
sequence that occurs every 588 symbols is the same as the start sync
sequence.
Synchronisation allows the receiving radio to synchronise its symbol clock
with the incoming data. This reduces the possibility of data errors when the
transmitting and receiving radios are not exactly on the same frequency due
to frequency drift or ageing.
The sync sequence is inverted if the modem is in FEC mode; this tells the
receiving radio to expect data packets with FEC data added.

Layer Two Protocol (THSD)
A simplified layer two protocol is available as a programmable option.
If layer two is not enabled, then the THSD modem will send data in 147byte 588 symbol blocks.
If there is not enough user data to fill 147 bytes, then the remaining bytes
will be padded with the “\” character.
The padding characters will be sent out through the serial port, with the
user data.
Enabling layer two provides a simple Start-Of-Text/End-Of-Text type
protocol.
Data is packed and transmitted as it is available in the radio, no padding
occurs.
The radio sending the data will stop transmitting as soon as the end of text
is detected.
With layer two enabled, baud rates to the DCE can be lower than the overair data rate.
Layer two is useful in situations where the controlling equipment is
expecting the original data sent from the transmitting equipment, and no
extra characters.
Layer two is also useful in situations where less than 147 bytes are sent and
timing is critical. As the radio will stop transmitting when the end of text is
detected, it will take less time to send a smaller data packet (see
“Throughput” on page 9).

Page 8 of 12

Throughput
The following table shows an approximate indication of throughput for the
THSD and FFSK modems, at specific user data values (20 bytes and 80
bytes).
This is intended to give the user an approximate measure to go by. As can
be seen from the table, the more data sent, the greater the effective
throughput will be.
The numbers in the table show that, when FEC is enabled, the effective
throughput will be reduced for the same amount of user data.

Test

LID
(ms)

FEC

Tx
Time

Lag

bps
(system

bps (OTA)

1200bps

N/A

166.2

136

429

334.4

503

293

342.7 318.0914513 546.0750853

12kbps

10.3

11.0

69.3

64.9

75.3

58.3

1200bps

N/A

665.4

135.8

989

334.4

1109.4

853.2

342.7 576.8884081 750.1172058

12kbps

41.5

10.5

102.4

61.7

113

91.9

70 5663.716814 6964.091404

12kbps

Yes

41.4

10.6

159

54.8

158.7

148.4

63.1 4032.766226 4312.668464

73.2

2124.8

2744.4

LID

Lead in delay (lead out delay set to 0)

FEC

Forward error correction (applicable to THSD modem only)

Number of characters sent to the transmitter

Time that the last bit of data was received by the transmitter

Time transmitter keyed up

Time transmitter keyed down

Time that first bit of data was output from the receiver’s serial port

Time that last bit of data was output from the receiver’s serial port

Tx Time

Is the time from Tx key up to Tx key down

Lag

Is the time from the start of the first bit being sent to the radio to the first byte
being output from the receiver’s serial port

bps (system) Is the effective bits per second (bps) from the first bit being sent to the last bit
being received
bps (OTA)

Note

Is the effective bits per second (bps) over the air from key up to key down

DTE to DCE baud rate was 1200 baud for 1200bps test and
19k2bps for the 12kbps test.

Page 9 of 12

BER (Bit Error Rate)
The following table gives a comparison of Bit error rates in four different
situations; the 1200 baud FFSK modem on a wide and narrow channel
spacing, and the high speed data modem (THSD) with and without error
correction (FEC).
The BERs were measured according to the approval specification of
Maximum Usable Sensitivity. This is the RF signal level at which the
modems achieve a BER of 1 x 10-2.
It can be seen that the FFSK modem produces a more satisfactory BER than
the high speed modem and that the BER for the high speed modem is
improved using FEC.

Modem

Sensitivity

1200bps FFSK modem, Wide Band

-122.0dBm

1200bps FFSK modem, Narrow Band -122.6dBm
Narrow Band THSD no FEC

-115.7dBm

Narrow Band THSD with FEC

-118.1dBm

These measurements were performed on radio that has an Rx sensitivity of
-121.99dBm for 12dB SINAD.
Sometimes a specification of sensitivity for a BER of 1 x 10-6 is quoted.
For the same radio above, a BER of 1 x 10-6 was measured at –110dBm with
the THSD modem with FEC.

Page 10 of 12

Publication Information
Compliance Issues
None

CSO Instruction
CSOs please inform registered Systems Integrators that this document is
available.

Issuing Authority
Issued by:

John Crossland
Technical Publications Manager

Confidentiality
Confidential - This message or document contains proprietary information
intended only for the person(s) or organisation(s) to whom it is addressed.
All Recipients are legally obliged to not disclose Tait technological or
business information to any persons or organisations without the written
permission of Tait.

Distribution Level
System Integrators and Tait Only

Document History
Version

Publication Date

Author

1.0

5 April 2004

J Campbell, SJ Glubb

1.1

9 June 2004

SJ Glubb

1.2

23 July 2004

SJ Glubb

Page 11 of 12

Tait Contact Information

Corporate Head
Office
New Zealand

Tait Electronics Ltd, P.O. Box 1645, Christchurch, New Zealand
E-mail (Marketing): taitnet@taitworld.com
E-mail (Sales): sales@taitworld.com

Technical Support

Technical Support Manager
Tait Electronics Ltd, P.O. Box 1645, Christchurch, New Zealand
E-mail: support@taitworld.com

Internet

http://www.taitworld.com

Page 12 of 12

Source Exif Data:

File Type                       : PDF
File Type Extension             : pdf
MIME Type                       : application/pdf
PDF Version                     : 1.2
Linearized                      : Yes
Producer                        : Acrobat Distiller 4.05 for Windows
Author                          : glubbs
Title                           : TM8000 Internal Modem.fm
Modify Date                     : 2004:07:23 12:53:16+13:00
Create Date                     : 2002:06:18 14:04:34
Creator                         : FrameMaker 6.0
Page Count                      : 12
Page Mode                       : UseOutlines

EXIF Metadata provided by EXIF.tools

TM8000 Internal Modem TECHNOTE/TM8000/TN 855a AN Application Note Version TN

Navigation menu

Versions of this User Manual:

Views

Navigation