1756-IR12 — ControlLogix 12-Channel RTD/Resistance Input Module

The 1756-IR12 reads RTD (Resistance Temperature Detector) and resistance signals across 12 independent channels. It converts resistance measurements to calibrated temperature values in engineering units — degrees Celsius or Fahrenheit — directly in the module, so the controller program receives ready-to-use temperature data rather than raw resistance counts. The module supports the major RTD standards (Pt100, Pt200, Pt500, Pt1000, Ni120, Cu10) along with direct resistance input for custom calibration curves.

RTDs are the preferred temperature sensor for applications requiring stable, repeatable measurements over long periods. Unlike thermocouples, which depend on a voltage generated at a junction, RTDs measure the change in electrical resistance of a pure metal element — a more linear relationship with far lower drift over time. The tradeoff is that RTDs cost more than thermocouples and cover a narrower temperature range. For process control in the −200°C to +850°C range where measurement stability over years matters, RTDs are often the right choice, and the 1756-IR12 is the ControlLogix module designed around them.

Specifications

Parameter	Value
Part Number	1756-IR12
Platform	ControlLogix 1756
Input Channels	12
RTD Types	Pt100, Pt200, Pt500, Pt1000 (385, 3916 curves); Ni120; Cu10
Resistance Range	0–1300 Ω (direct resistance mode)
Temperature Range (Pt100)	−200°C to +870°C
Resolution	0.1°C (in engineering units mode)
Accuracy	±1°C typical @ 25°C ambient
Wiring Modes	2-wire, 3-wire, 4-wire (per channel, selectable)
Excitation Current	~1 mA (module-sourced, constant current)
Channel Isolation	Not isolated (channels share common)
Backplane Current (5V)	125 mA
RTB	1756-TBNH or 1756-TBS6H (36-pin)
Operating Temperature	0°C to 60°C
Standards	UL 508, CE, IEC 751 (RTD standard)

2-Wire vs. 3-Wire vs. 4-Wire Wiring

Lead resistance is the fundamental measurement challenge with RTDs. The module measures resistance across its input terminals — if the cable connecting the RTD to the module has significant resistance, that cable resistance adds directly to the RTD's measured resistance and appears as a false temperature offset.

2-wire wiring makes no correction for lead resistance. Use this only when cable runs are very short (under a few meters) and the small lead resistance error is acceptable for the application.

3-wire wiring uses a third wire to measure and compensate for lead resistance. The module drives a current through the measurement circuit and uses the third wire to cancel the lead resistance component. This is the most common installation method for industrial RTD applications with moderate cable lengths.

4-wire (Kelvin) wiring completely eliminates lead resistance error by using two wires to source measurement current and two separate wires to measure voltage. The measurement is independent of the lead resistance. Use 4-wire where the highest accuracy is required — laboratory measurements, precision process control, or cable runs long enough that 3-wire compensation may not fully eliminate the error.

FAQ

Q: Can thermocouple sensors be used with this module?

No. The 1756-IR12 measures resistance, not the millivolt-level voltage that thermocouples generate. Connecting a thermocouple will produce incorrect readings. Use the 1756-IT6I or 1756-IT6I2 for thermocouple inputs.

Q: Does the module supply excitation current to the RTD, or does the RTD need its own supply?

The module provides a constant current (~1 mA) to excite the RTD. No external supply is required for the RTD itself. This excitation current flows through the RTD and the measurement circuitry, and the module measures the resulting voltage to determine resistance.

Q: What is the self-heating effect, and should it concern me?

The excitation current flowing through the RTD element dissipates a small amount of power as heat (P = I² × R). At 1 mA through a 100 Ω Pt100, this is 0.1 mW — negligible in most industrial applications. In precision laboratory measurements or still-air environments where the RTD cannot dissipate heat freely, the self-heating effect can add a small positive offset to readings. For standard process control, it is not a concern.

Q: Can channels be configured for different RTD types independently?

Yes. Each of the 12 channels is configured independently in Studio 5000 — channel 0 can be Pt100 (IEC 385) while channel 5 is Ni120. Mixing RTD types across channels on the same module is fully supported.

Q: What happens if an RTD lead breaks?

An open-circuit RTD causes the measured resistance to go to infinity (or the module's measurement ceiling). The module sets an over-range status bit for that channel, which the controller program can use to trigger an alarm. Configure the open-wire detection response in Studio 5000 to meet the application's requirements.

1756-IR12 New ControlLogix Non-Isolated RTD PLC Input 1756IR12