4-20 mA & Serial Data Output RTD Temperature Transmitter

Exceptional linearity & accuracy at high update rates. DIN Rail mounted, digitally programmable.

4-20 mA and Serial Data Output RTD Transmitter


  • 4-20 mA, 0-20 mA, 0-10V or -10V to +10V transmitter output, 16 bits, isolated
  • RS232 or RS485 serial data output, Modbus or TDM ASCII protocol, isolated
  • Dual 120 mA solid state relays for alarm or control, isolated
  • Factory calibrated for 100Ω platinum, 10Ω copper and 120Ω nickel RTDs
  • 2, 3 or 4-wire RTD connection with lead resistance compensation
  • User selectable input span from entire RTD range down to 15.0°
  • Analog output resolution 0.0015% of span (16 bits), accuracy ±0.02% of span
  • Universal 85-264 Vac / 90-300 Vdc or 10-48 Vdc / 12-32 Vac power
  • DIN rail mount housing only 22.5 mm wide, detachable screw-clamp connectors

The RTD temperature transmitter provides a linearized, highly accurate, stable and repeatable transmitter output for 100 ohm platinum, 10 ohm copper and 120 ohm nickel RTDs. Pt100 platinum RTDs can have a DIN alpha of 0.00385 or ANSI alpha of 0.00392. The RTD type and temperature range, specified in °C or °F, are user-selectable. The temperature range can be as wide as the entire span of the RTD type or as narrow as 150 counts (such as 15.0°), limited only by considerations of electrical noise and digital filtering time constants.

RTD signal conditioner board

Digital calibration of all RTD ranges is performed the factory, with calibration data stored in EEPROM on the signal conditioner board. This allows signal conditioner boards and ranges to be changed in the field with no need for recalibration. Typical accuracy for a Pt100 is better than ±0.04°C (±0.07°F) ±0.01% of reading. Open sensor indication is standard and may be set up to indicate either upscale or downscale. RTD excitation is provided by the transmitter. RTD connections can be of the 2-, 3- or 4-wire type. With 3- and 4-wire connections, the transmitter automatically compensates for changes in lead resistance to the sensor. Instrument Setup Software allows user calibration for RTDs whose resistance is different than nominal at 0°C.

Fast read rate at up to 50 or 60 conversions per second while integrating the signal over a full power line cycle is provided by Concurrent Slope (Pat 5,262,780) analog-to-digital conversion. High read rate is ideal for peak or valley capture and for real-time computer interface and control.

Standard features of LT transmitters include:

  • 4-20 mA, 0-20 mA, 0-10V or -10V to +10V analog transmitter output, isolated, jumper-selectable and user scalable. All selections provide 16-bit (0.0015%) resolution of output span and 0.02% output accuracy of a reading from -99,999 to +99,999 counts that is also transmitted digitally. Output isolation from signal and power grounds eliminates potential ground loops.
  • Serial communications output, RS232 or RS485 (half or full duplex), jumper selectable. Three protocols are user selectable: Modbus RTU, Modbus ASCII, or TDM ASCII. Modbus operation is fully compliant with Modbus Over Serial Line Specification V1.0 (2002). The TDM ASCII protocol is simpler than the Modbus protocol and is recommended when all devices are s.
  • Dual solid state relays, isolated. Available for local alarm or control. Rated 120 mA at 140 Vac or 180 Vdc.
  • Universal 85-264 Vac power. Low-voltage 10-48 Vdc or 12-32 Vac power is optional.

Easy Transmitter programming is via TDM's Instrument Setup Software, which runs on a PC under MS Windows. This software can be downloaded from this website at no charge. The required transmitter-to-PC interface cable is available from TDM (P/N CBL04).

RTD Metal Alpha R at 0°C R at top
of range
Range Max Error
0.003850 (DIN) 100Ω 390.48Ω
at 850°C
196 µA -202°C to +850°C
-331°F to +1562°F
±0.03°C ±0.01% of rdg
±0.05°F ±0.01% of rdg
0.003902 (ANSI) 100Ω 394.36Ω
at 850°C
196 µA -202°C to +850°C
-331°F to +1168°F
±0.04°C ±0.01% of rdg
±0.07°F ±0.01% of rdg
0.00672 120Ω 380.31Ω
at 260°C
196 µA -80°C to +260°C
-112°F to +500°F
±0.05°C ±0.01% of rdg
±0.09°F ±0.01% of rdg
0.00427 9.035Ω 19.116Ω
at 260°C
5.0 mA -100°C to +260°C
-148°F to +500°F
±0.05°C ±0.01% of rdg
±0.09°F ±0.01% of rdg
RTD Input
Calibration, Pt 100 DIN IEC 751 (IPTS-68)
Calibration, Pt 100 ANSI NIST Monograph 126
Configuration 2, 3 or 4-wire connection
Excitation current 0.2 mA
Max error at 25°C, Pt100 ±0.04°C (±0.07°F) ±0.01% of reading
Span tempco ±0.003% of reading/°C
Zero tempco ±0.03 deg/deg
Sensor lead resistance 2-wire: 10 mdeg/Ω/deg up to 10Ω;
tempco per conductor 3 & 4-wire: 10 μdeg/Ω/deg up to 100Ω
Over-voltage protection 125 Vac
Open sensor indication 0 mA or > 20 mA output, selectable
Provision for user calibration Multiplier of RTD resistance plus offset in degrees
Analog Output (standard)
Output Levels 4-20 mA, 0-20 mA, 0-10 Vdc, -10 to +10Vdc (user selectable)
Compliance, 4-20 mA 10V (0-500Ω load)
Compliance, 0-10V 2 mA (5 kΩ load)
Output Resolution 16 bits (65,536 steps)
Output Accuracy ±0.02% of output span
Output Isolation 250V rms working, 2.3 kV rms per 1 minute test
Step response time 50 ms
Serial Data Output (standard)
Signal Types RS232 or RS485 (half or full duplex), jumper selectable
Data Rates 300, 600, 1200, 2400, 4800, 9600, 19200 baud
Output Isolation 250V rms working, 2.3 kV rms per 1 min test
Serial Protocols Modbus RTU, Modbus ASCII, TDM ASCII
Modbus Compliance Modbus over Serial Line Specification V1.0 (2002)
Ethernet Connector RJ45 jack for 10/100Base-T Ethernet cable
RS232/485 Connector  Screw terminals for easy daisy chaining 
Digital Addresses 247 for Modbus, 31 for TDM ASCII
Dual Relay Output (standard)
Relay Type Two solid state relays, SPST, normally open, Form A
Load Rating 120 mA at 140 Vac or 180 Vdc
Power Input
Standard Power 85-264 Vac or 90-300 Vdc
Low Power Option 10-48 Vdc or 12-32 Vac
Power Frequency DC or 47-63 Hz
Power Isolation 250V rms working, 2.3 kV rms per 1 min test
Power Consumption 2W typical, 3W with max excitation output
Dimensions 129 x 104 x 22.5 mm case
Mounting 35 mm rail per DIN EN 50022
Electrical Connections Plug-in screw-clamp connectors
Operating Temperature 0°C to 55°C
Storage Temperature -40°C to 85°C
Relative Humidity 95% at 40°C, non-condensing
Cooling Required Mount transmitters with ventilation holes at top and bottom. Leave 6 mm (1/4") between transmitters, or force air with a fan.

Ethernet & 4-20 mA Output RTD Temperature Transmitter and Signal Conditioner

 LT transmitter pinout

RTD hookup can be via 2, 3 or 4 wires to the J5 connector. The transmitter applies an excitation current of 196 µA (Pt 100 and Ni 120) or 5 mA (Cu 10).

4-wire hookup of RTD signal conditioner board In 4-wire hookup, different pairs of leads are used to apply the excitation current and sense the voltage drop across the RTD, so that the IR drop across the excitation leads is not a factor.
3-wire hookup of RTD signal conditioner board In 3-wire hookup, the transmitter senses the combined voltage drop across the RTD plus two excitation leads. It also senses the voltage drop across one excitation lead, and then subtracts twice this voltage from the combined total. This technique effectively subtracts all lead resistance and compensates for ambient temperature changes if the two excitation leads are identical.
2-wire hookup of RTD signal conditioner board In 2-wire hookup, the transmitter senses the combined voltage drop across the RTD and both lead wires. The voltage drop across the lead wires can be measured by shorting out the RTD during transmitter setup, and this voltage is then automatically subtracted from the combined total. However, changing resistance of the lead wires due to ambient temperature changes will not be compensated.

Operation as a Fast ON/OFF Controller or Supervisory Monitor

Temperature controller operation of  temperature meters and transmitters With the optional dual solid state relay output option, which has a typical response time of only 17 ms, temperature meters and transmitters can serve as extremely fast and accurate ON/OFF controllers for closed-loop temperature control. They can also serve as supervisory process monitors and provide alarms or shutoffs when processes exceed normal limits.

Multiple setpoint operating modes are individually selectable for each relay, as explained in the dual-setpoint controller page. Relay duty cycles and chatter can be minimized with programmable hysteresis and time delays. A band deviation operating mode can be selected for each relay, where an alarm is generated whenever the reading is a selected number of counts above or below the setpoint. The relay modes have to be non-latching, since 4-20 mA temperature transmitters do not offer external reset.

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