Allen-Bradley 1756-CJC ControlLogix Cold Junction Compensation Module

Configured for cold junction temperature reference correction in thermocouple input processing within ControlLogix systems, the Allen-Bradley 1756-CJC, also cataloged as the 1756-CJC CJC thermistor module, provides direct electrical temperature compensation for analog measurement accuracy in ControlLogix IRT8I and IT16 input configurations. The device supplies reference junction stabilization through backplane-powered sensing elements used by compatible thermocouple modules.

Hardware Specifications

Backplane Bus Communication and CJC Integration

Within ControlLogix architecture, the 1756-CJC participates as a passive reference element for thermocouple linearization circuits. Temperature compensation values are sampled via backplane-assisted acquisition paths of the 1756-IRT8I or 1756-IT16 modules. The compensation function is applied at the module signal conditioning stage prior to A/D conversion, ensuring junction voltage correction relative to chassis temperature drift. Backplane power distribution at 5 V DC supports thermistor excitation and reference stabilization.

Frequently Asked Questions

Q: Does the 1756-CJC perform signal conversion or only temperature reference sensing?
A: The module provides only cold junction reference sensing. Signal conversion is handled by the associated 1756-IRT8I or 1756-IT16 input modules.

Q: Can the 1756-CJC operate without backplane power?
A: No. The thermistor network requires 5 V DC backplane supply for reference excitation and temperature measurement stability.

Q: Is the module electrically isolated from thermocouple input channels?
A: Isolation is defined at the input module level. The 1756-CJC itself functions as a passive sensing element without channel isolation circuitry.

Field Installation Guidelines

Install the 1756-CJC in a ControlLogix chassis adjacent to compatible thermocouple input modules (1756-IRT8I or 1756-IT16). Ensure full engagement with the backplane connector to maintain reference continuity. Route thermocouple wiring separately from power conductors to minimize thermal and electrical interference. Maintain proper grounding of the chassis to stabilize reference junction potential. Do not apply external voltage to the CJC terminals; the element operates strictly under backplane-supplied excitation.