18224 TEROS 12 Integrators Guide

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18224-00
3.1.2018

TEROS 12 INTEGRATOR GUIDE
SENSOR DESCRIPTION
The TEROS 12 Soil Moisture + Electrical Conductivity (EC) + Temperature Sensor is an accurate tool for
monitoring volumetric water content (VWC), electrical conductivity, and temperature in soil and soilless
substrates. The TEROS 12 determines VWC using capacitance/frequency-domain technology. The sensor uses
a 70 MHz frequency, which minimizes textural and salinity effects, making the TEROS 12 accurate in most
mineral soils. The TEROS 12 uses a thermistor in the center needle to measure temperature and electrical
conductivity using a stainless-steel electrode array.
For a more detailed description of how this sensor makes measurements, refer to the TEROS 12 User Manual.

APPLICATIONS
• Volumetric water content (VWC) measurement
• Soil/substrate water balance
• Irrigation management
• Soil electrical conductivity (EC) measurement
• Soil/substrate temperature measurement
• Solute/fertilizer movement

ADVANTAGES

Ferrite
core

• Three-wire sensor interface: power, ground, and data
• Digital sensor communicates multiple measurements over a
serial interface
• Robust thermistor for accurate temperature measurements
• Low-input voltage requirements
• Low-power design supports battery-operated data loggers
• Robust epoxy encapsulation resists corrosive environments
• Supports SDI-12 or DDI serial communications protocols

TEROS 12
sensor

• Modern design optimized for low-cost sensing

PURPOSE OF THIS GUIDE
METER provides the information in this integrator guide to
help TEROS 12 Soil Moisture + Electrical Conductivity (EC)
Figure 1 TEROS 12 sensor
+ Temperature Sensor customers establish communication
between these sensors and their data acquisition equipment or field data loggers. Customers using data
loggers that support SDI-12 sensor communications should consult the data logger user manual. METER
sensors are fully integrated into the METER system of plug-and-play sensors, cellular-enabled data loggers,
and data analysis software.

COMPATIBLE FIRMWARE VERSIONS
This guide is compatible with firmware versions 1.00 or newer.

METER Group, Inc. USA
2365 NE Hopkins Court, Pullman, WA 99163
T +1.509.332.2756 F +1.509.332.5158
E info@metergroup.com W metergroup.com

SPECIFICATIONS

MEASUREMENT SPECIFICATIONS
Volumetric Water Content (VWC)

Dielectric Measurement Frequency

Range:

70 MHz

Mineral soil
calibration:

0.00–0.62 m3/m3

Soilless
media
calibration:

0.0–1.0 m3/m3

Apparent
dielectric
permittivity (εa):

1 (air) to 80 (water)

Temperature

Resolution:

Accuracy:
Generic
calibration:

±0.03 m /m typical
in mineral soils that
have solution electrical
conductivity < 8 dS/m
±0.01–0.02 m3/m3 in any
porous medium

Apparent
dielectric
permittivity (εa):

1–40 (soil range),
±1 εa (unitless)

0.1 °C

Accuracy:

±1 °C

Range:

0–20 dS/m (bulk)

Resolution:

0.001 dS/m

Accuracy:

±3% of measurement

DDI serial or
SDI-12 communication protocol

Medium specific
calibration:

Resolution:

Output

0.001 m /m
3

−40 to 60 °C

Electrical Conductivity (ECb )

NOTE: The VWC range is dependent on the media the sensor
is calibrated to. A custom calibration will accommodate the
necessary ranges for most substrates.
3

Range:

Data Logger Compatibility
Any data acquisition system capable of
4.0- to 15-VDC power and serial or SDI-12
communication

40–80, 15% of
measurement

PHYSICAL SPECIFICATIONS
Cable Length

Dimensions
Length:

9.4 cm (3.70 in)

Width:

2.4 cm (0.95 in)

Height:

7.5 cm (2.95 in)

5 m (standard)
75 m (maximum custom cable length)
NOTE: Contact Customer Support if a nonstandard cable
length is needed.

Prong Length

Connector Types

5.5 cm (2.17 in)

3.5-mm stereo plug connector or stripped and
tinned wires

ELECTRICAL AND TIMING CHARACTERISTICS
Supply Voltage (VCC) to GND
Minimum
Typical
Maximum

Digital Input Voltage (logic low)

4.0 VDC

Minimum

NA
15.0 VDC

Digital Input Voltage (logic high)

–0.3 V

Typical

0.0 V

Maximum

0.8 V

Power Line Slew Rate

Minimum

2.8 V

Minimum

Typical

3.6 V

Typical

Maximum

3.9 V

Maximum

1.0 V/ms

Current Drain (during 25-ms measurement)

Power Up Time (SDI-12)

Minimum

3.0 mA

Minimum

Typical

3.6 mA

Typical

Maximum

16.0 mA

Minimum

Minimum
Typical

0.03 mA

Maximum

Measurement Duration

Typical

245 ms

Maximum

Current Drain (while asleep)

Maximum

25 ms
50 ms

Operating Temperature Range
Minimum

COMPLIANCE

–40 °C

Typical

Manufactured under ISO 9001:2015

Maximum

EM ISO/IEC 17050:2010 (CE Mark)

60 °C

2014/30/EU and 2011/65/EU

NOTE: Sensors may be used at higher temperatures under
certain conditions; contact Customer Support for assistance.

EN61326-1:2013 and EN55022/CISPR 22

Power Up Time (DDI serial)
Minimum

80 ms

Typical

Maximum

100 ms

EQUIVALENT CIRCUIT AND CONNECTION TYPES
Refer to Figure 2 and Figure 3 to connect the TEROS 12 to a data logger. Figure 2 provides a low-impedance
variant of the recommended SDI-12 specification.

GND

DATA

PIGTAIL CABLE
Power (brown)
Ground (bare)
Data (orange)

STEREO CABLE

220PF

510
R2

10UH

R1
100K

Ground
Data
Power

GND
Figure 2 Equivalent circuit diagram

Figure 3 Connection types

SAFETY PRECAUTIONS
METER sensors are built to the highest standards, but misuse, improper protection, or improper installation
may damage the sensor and possibly void the warranty. Before integrating szensors into a sensor network,
follow the recommended installation instructions and implement safeguards to protect the sensor from
damaging interference.

SURGE CONDITIONS
Sensors have built-in circuitry that protects them against common surge conditions. Installations in
lightning-prone areas, however, require special precautions, especially when sensors are connected to a
well-grounded third-party logger.
Visit metergroup.com for articles containing more information.

POWER AND GROUNDING
Ensure there is sufficient power to simultaneously support the maximum sensor current drain for all the
sensors on the bus. The sensor protection circuitry may be insufficient if the data logger is improperly
powered or grounded. Refer to the data logger’s installation instructions. Improper grounding may affect the
sensor output as well as sensor performance.
Visit metergroup.com for articles containing more information.

CABLES
Improperly protected cables can lead to severed cables or disconnected sensors. Cabling issues can be
caused by many factors, including rodent damage, driving over sensor cables, tripping over the cable, not
leaving enough cable slack during installation, or poor sensor wiring connections. To relieve strain on the
connections and prevent loose cabling from being inadvertently snagged, gather and secure the cable
travelling between the TEROS 12 and the data acquisition device to the mounting mast in one or more places.
Install cables in conduit or plastic cladding when near the ground to avoid rodent damage. Tie excess cable to
the data logger mast to ensure cable weight does not cause sensor to unplug.

SENSOR COMMUNICATIONS
METER digital sensors feature a serial interface with shared receive and transmit signals for communicating
sensor measurements on the data wire (Figure 3). The sensor supports two different protocols: SDI-12 and DDI
serial. Each protocol has implementation advantages and challenges. Please contact Customer Support if the
protocol choice for the desired application is not obvious.

SDI-12 INTRODUCTION
SDI-12 is a standards-based protocol for interfacing sensors to data loggers and data acquisition equipment.
Multiple sensors with unique addresses can share a common 3-wire bus (power, ground, and data). Two-way
communication between the sensor and logger is possible by sharing the data line for transmit and receive
as defined by the standard. Sensor measurements are triggered by protocol command. The SDI-12 protocol
requires a unique alphanumeric sensor address for each sensor on the bus so that a data logger can send
commands to and receive readings from specific sensors.
Download the SDI-12 Specification v1.3 to learn more about the SDI-12 protocol.

DDI SERIAL INTRODUCTION
The DDI serial protocol is the method used by the METER data loggers for collecting data from the sensor. This
protocol uses the data line configured to transmit data from the sensor to the receiver only (simplex). Typically,
the receive side is a microprocessor UART or a general-purpose I/O pin using a bitbang method to receive data.
Sensor measurements are triggered by applying power to the sensor.

INTERFACING THE SENSOR TO A COMPUTER
The serial signals and protocols supported by the sensor require some type of interface hardware to be
compatible with the serial port found on most computers (or USB-to-serial adapters). There are several
SDI-12 interface adapters available in the marketplace; however, METER has not tested any of these
interfaces and cannot make a recommendation as to which adapters work with METER sensors. METER
data loggers and the ProCheck hand-held device can operate as a computer-to-sensor interface for
making on-demand sensor measurements. For more information, please contact Customer Support.

METER SDI-12 IMPLEMENTATION
METER sensors use a low-impedance variant of the SDI-12 standard sensor circuit (Figure 2). During the
power-up time, sensors output some sensor diagnostic information and should not be communicated with
until the power-up time has passed. After the power up time, the sensors are compatible with all commands
listed in the SDI-12 Specification v1.3 except for the continuous measurement commands (aR0–aR9 and
aRC0–aRC9) and the concurrent measurement commands (aC–aC9 and aCC0–aCC9). M, R, and C command
implementations are found on pages 7–8. The aR3 and aR4 commands are used by METER systems and
as a result use space delimination, instead of the sign delimination required by the SDI-12 standard.
Out of the factory, all METER sensors start with SDI-12 address 0 and print out the DDI serial startup string
during the power-up time. This can be interpreted by non-METER SDI-12 sensors as a pseudo-break condition
followed by a random series of bits.

The TEROS 12 will omit the DDI serial startup string (sensor identification) when the SDI-12 address is nonzero.
Changing the address to a nonzero address is recommended for this reason.

SENSOR BUS CONSIDERATIONS
SDI-12 sensor buses require regular checking, sensor upkeep, and sensor troubleshooting. If one sensor goes
down, that may take down the whole bus even if the remaining sensors are functioning normally. Power cycling
the SDI-12 bus when a sensor is failing is acceptable, but METER does not recommend scheduling power cycling
events on an SDI-12 bus more than once or twice per day. Many factors influence the effectiveness of the bus
configuration. Visit metergroup.com for articles and virtual seminars containing more information.

SDI-12 CONFIGURATION
Table 1 lists the SDI-12 communication configuration.
Table 1

SDI-12 communication configuration

Baud Rate

1200

Start Bits

Data Bits

7 (LSB first)

Parity Bits

1 (even)

Stop Bits

Logic

Inverted (active low)

SDI-12 TIMING
All SDI-12 commands and responses must adhere to the format in Figure 4 on the data line. Both the
command and response are preceded by an address and terminated by a carriage return and line feed
combination () and follow the timing shown in Figure 5.

START

STOP

Figure 4 Example SDI-12 transmission of the character 1 (0x31)

DATA LOGGER
Break
(at least 12 ms)

SENSOR
Command

Response

Marking
(at least 8.33 ms)

Marking
(at least 8.33 ms)
Sensor must respond
within 15 ms

Maximum time*

*Maximum time is dependent upon the amount of data returned for the command sent.

Figure 5 Example data logger and sensor communication

COMMON SDI-12 COMMANDS
This section includes tables of common SDI-12 commands that are often used in an SDI-12 system and the
corresponding responses from METER sensors.

IDENTIFICATION COMMAND (aI!)
The Identification command can be used to obtain a variety of detailed information about the connected
sensor. An example of the command and response is shown in Example 1, where the command is in bold and
the response follows the command.
Example 1

1I!113METER␣ ␣ ␣TER12␣100631800001

Parameter

Fixed
Character
Length

Description

1I!

Data logger command.
Request to the sensor for information from sensor address 1.

Sensor address.
Prepended on all responses, this indicates which sensor on the bus is returning
the following information.

Indicates that the target sensor supports SDI-12 Specification v1.3.

METER␣ ␣ ␣

Vendor identification string.
(METER and three spaces ␣ ␣ ␣ for all METER sensors)

TER12␣

Sensor model string.
This string is specific to the sensor type.
For the TEROS 12, the string is TER12.

100

Sensor version.
This number divided by 100 is the METER sensor version
(e.g., 100 is version 1.00).

≤13,
variable

631800001

Sensor serial number.
This is a variable length field. It may be omitted for older sensors.

CHANGE ADDRESS COMMAND (aAB!)
The Change Address command is used to change the sensor address to a new address. All other commands
support the wildcard character as the target sensor address except for this command. All METER sensors
have a default address of 0 (zero) out of the factory. Supported addresses are alphanumeric (i.e., a–z, A–Z, and
0–9). An example output from a METER sensor is shown in Example 2, where the command is in bold and the
response follows the command.
Example 2

1A0!0

Parameter

Fixed
Character
Length

Description

1A0!

Data logger command.
Request to the sensor to change its address from 1 to a new address of 0.

New sensor address.
For all subsequent commands, this new address will be used by the
target sensor.

ADDRESS QUERY COMMAND (?!)
While disconnected from a bus, the Address Query command can be used to determine which sensors are
currently being communicated with. Sending this command over a bus will cause a bus contention where all
the sensors will respond simultaneously and corrupt the data line. This command is helpful when trying to
isolate a failed sensor. Example 3 shows an example of the command and response, where the command is in
bold and the response follows the command. The question mark (?) is a wildcard character that can be used in
place of the address with any command except the Change Address command.
Example 3

?!0

Parameter

Fixed
Character
Length
Description

Data logger command.
Request for a response from any sensor listening on the data line.

Sensor address.
Returns the sensor address to the currently connected sensor.

COMMAND IMPLEMENTATION
The following tables list the relevant Measurement (M), Continuous (R), and Concurrent (C) commands and
subsequent Data (D) commands when necessary.

MEASUREMENT COMMANDS IMPLEMENTATION
Measurement (M) commands are sent to a single sensor on the SDI-12 bus and require that subsequent Data
(D) commands are sent to that sensor to retrieve the sensor output data before initiating communication with
another sensor on the bus.
Please refer to Table 2 and for an explanation of the command sequence and see Table 7 for an explanation of
response parameters.
Table 2

aM! command sequence

Command

Response

This command reports instantaneous values.

aM!

atttn

aD0!

a+±+

NOTE: The measurement and corresponding data commands are intended to be used back to back. After a measurement command is
processed by the sensor, a service request a is sent from the sensor signaling the measurement is ready. Either wait until ttt
seconds have passed or wait until the service request is received before sending the data commands. See the SDI-12 Specifications v1.3
document for more information.

CONCURRENT MEASUREMENT COMMANDS IMPLEMENTATION
Concurrent (C) measurement commands are typically used with sensors connected to a bus. Concurrent (C)
commands for this sensor deviate from the standard C command implementation. First, send the C command,
wait the specified amount of time detailed in the C command response, and then use D commands to read its
response prior to communicating with another sensor.
Please refer to Table 3 for an explanation of the command sequence and see Table 7 for an explanation of
response parameters.
Table 3

aC! measurement command sequence

Command

Response

This command reports instantaneous values.

aC!

atttnn

aD0!

a+±+

NOTE: This command does not adhere to the SDI-12 concurrent command requirements. See METER SDI-12 Implementation for more
information.
NOTE: The measurement and corresponding data commands are intended to be used back to back. After a measurement command is
processed by the sensor, a service request a is sent from the sensor signaling the measurement is ready. Either wait until ttt
seconds have passed or wait until the service request is received before sending the data commands. See the SDI-12 Specifications v1.3
document for more information.

CONTINUOUS MEASUREMENT COMMANDS IMPLEMENTATION
Continuous (R) measurement commands trigger a sensor measurement and return the data automatically
after the readings are completed without needing to send a D command.
Please refer to Table 4 through Table 6 for an explanation of the command sequence and see Table 7 for an
explanation of response parameters.
Table 4

aR0! measurement command sequence

Command

Response

This command reports instantaneous values.

aR0!

a+±+

NOTE: This command does not adhere to the SDI-12 response timing. See METER SDI-12 Implementation for more information.

Table 5

aR3! measurement command sequence

Command

Response

This command reports instantaneous values.

aR3!

NOTE: This command does not adhere to the SDI-12 response format or timing. See METER SDI-12 Implementation for more information.

Table 6

aR4! measurement command sequence

Command

Response

This command reports instantaneous values.

aR4!

NOTE: This command does not adhere to the SDI-12 response format or timing. See METER SDI-12 Implementation for more information.

PARAMETERS
Table 7 lists the parameters, unit measurement, and a description of the parameters returned in command
responses for TEROS 12.
Table 7

Parameter Descriptions

Parameter

Unit

Description

—

Positive or negative sign denoting sign of the next value

—

SDI-12 address

—

Number of measurements (fixed width of 1)

—

Number of measurements with leading zero if necessary (fixed width of 2)

ttt

Maximum time measurement will take (fixed width of 3)

—

Tab character

—

Carriage return character

—

Line feed character

—

Calibrated ADC counts for Volumetric Water Content

°C

Air temperature

dS/m

Bulk Electrical Conductivity normalized to 25 °C

—

ASCII character denoting the sensor type
For TEROS 12, the character is g

—

METER serial checksum

—

METER 6-bit CRC

DDI SERIAL COMMUNICATION
The DDI serial communications protocol is ideal for systems that have dedicated serial signaling lines for each
sensor or use a multiplexer to handle multiple sensors. The serial communications are compatible with many
TTL serial implementations that support active-high logic levels using 0 to 3.6 V signal levels. When the sensor
is first powered, it automatically makes measurements of the integrated transducers then outputs a response
over the data line. Systems using this protocol control the sensor excitation to initiate data transfers from
the sensor. This protocol is subject to change as METER improves and expands the line of digital sensors and
data loggers. TEROS 12 will omit the DDI serial startup string when the SDI-12 address is nonzero.
NOTE: Out of the factory, all METER sensors start with SDI-12 address 0 and print out the startup string when power cycled.

DDI SERIAL TIMING
Table 8 lists the DDI serial communication configuration.
Table 8

DDI serial communication configuration

Baud Rate

1200

Start Bits

Data Bits

8 (LSB first)

Parity Bits

0 (none)

Stop Bits

Logic

Standard (active high)

At power up, the sensor will pull the data line high within 100 ms to indicate that the sensor is taking a reading
(Figure 6). When the reading is complete, the sensor begins sending the serial signal out the data line adhering
to the format shown in Figure 7. Once the data is transmitted, the sensor goes into SDI-12 communication
mode. To get another serial signal, the sensor must be power cycled.
NOTE: Sometimes the signaling from the sensor can confuse typical microprocessor UARTs. The sensor holds the data line low while
taking measurements. The sensor raises the line high to signal the logger that it will send a measurement. Then the sensor may take some
additional measurements before starting to clock out the first data byte starting with a typical start bit (low). Once the first start bit is sent,
typical serial timing is valid; however, the signal transitions before this point are not serial signaling and may be misinterpreted by the UART.

Up to 100 ms

Measurement
duration

SDI-12 ready

DDI serial

Power applied

Figure 6 Data line DDI serial timing

START

STOP

Figure 7 Example DDI serial transmission of the character 9 (0x39)

DDI SERIAL RESPONSE
This section contains tables detailing the DDI serial response.
Table 9

DDI serial response

COMMAND

RESPONSE

NOTE: There is no actual command. The response is returned automatically upon power up.

DDI SERIAL CHECKSUM
These checksums are used in the continuous commands R3 and R4 as well as the DDI serial response. The
legacy checksum is computed from the start of the transmission to the sensor identification character,
excluding the sensor address.

Legacy checksum example input is 2749.0 23.8 660g and the resulting checksum output is 8.
uint8_t LegacyChecksum(const char * Response)
{
uint16_t length;
uint16_t i;
uint16_t sum = 0;
// Finding the length of the response string
length = strlen(response);
// Adding characters in the response together
for( i = 0; i < length; i++ )
{
sum += response[i];
if(response[i] == '\r')
{
// Found the beginning of the meta data section of the response
break;
}
}
// include the sensor type into the checksum
sum += response[++i];
// Convert checksum to a printable character
sum = sum % 64 + 32;
return sum;
}

The more robust CRC6, if available, utilizes the CRC-6-CDMA2000-A polynomial with the value 48 added to the
results to make this a printable character and is computed from the start of the transmission to the legacy
checksum character, excluding the sensor address.
CRC6 checksum example input is 2749.0 23.8 660g8 and the resulting checksum output is O
(uppercase o).
uint8_t CRC6_Offset(const char *buffer)
{
uint16_t byte;
uint16_t i;
uint16_t bytes;
uint8_t bit;
uint8_t crc = 0xfc; // Set upper 6 bits to 1’s
// Calculate total message length—updated once the meta data section is found
bytes = strien(buffer)
// Loop through all the bytes in the buffer
for(byte = 0; byte < bytes; byte++)
{
// Get the next byte in the buffer and XOR it with the crc
crc ^= buffer[byte];
// Loop through all the bits in the current byte
for(bit = 8; bit > 0; bit--)
{
// If the uppermost bit is a 1...
if(crc & 0x80)
{
// Shift to the next bit and XOR it with a polynomial
crc = (crc << 1) ^ 0x9c;
}
else
{
// Shift to the next bit
crc = crc << 1;
}
}
if(buffer[byte] == '\r')
{
// Found the beginning of the meta data section of the response
// both sensor type and legacy checksum are part of the crc6
// this requires only two more iterations of the loop so reset
"bytes"
// bytes is incremented at the beginning of the loop, so 3 is added
bytes = byte + 3;
}
}
// Shift upper 6 bits down for crc
crc = (crc >> 2);
// Add 48 to shift crc to printable character avoiding \r \n and !
return (crc + 48);
}

CUSTOMER SUPPORT
Customer service representatives are available for questions, problems, or feedback Monday through Friday,
7 am–5 pm Pacific time.
Email:

support.environment@metergroup.com
sales.environment@metergroup.com

Phone:

+1.509.332.5600

Fax:

+1.509.332.5158

Website: metergroup.com
If contacting METER by email, please include the following information:
Name
Address
Phone number

Email address
Instrument serial number
Description of problem

NOTE: For TEROS 12 sensors purchased through a distributor, please contact the distributor directly for assistance.

REVISION HISTORY
The following table lists document revisions.
Revision

Date

Compatible Firmware

Description

3.1.2018

1.00

Initial release

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