Channel value difference too high

What does 55 mean?

A significant discrepancy has been detected between redundant input channels, exceeding the configured tolerance window. This implies a potential sensor malfunction, wiring issue, or configuration error that could compromise the integrity of safety-critical inputs.

• One of the redundant sensors (e.g., sensor A or sensor B in a 1oo2 or 2oo2 configuration) is providing an incorrect measurement due to malfunction or damage.
• Differential impedance or resistance in the wiring of one channel (e.g., a high resistance connection on channel 1, terminal X1.1, compared to channel 2, terminal X1.3).
• Calibration drift in one of the redundant sensors, causing its output to deviate from the other's, or incorrect sensor replacement with an uncalibrated unit.
• Incorrectly configured tolerance window (discrepancy limit) for the redundant input channels in the safety controller's parameters (e.g., Parameter S2-05), set too narrow for normal process variations.
• Electromagnetic interference (EMI) affecting one channel's signal more severely than the other due to cable routing or shielding issues.

Click steps to track your progress.

1. 1

   Using calibrated test equipment (e.g., two multimeters), simultaneously measure the outputs of both redundant sensors at their respective I/O module terminals (e.g., Channel 1 at X1.1, Channel 2 at X1.3).

2. 2

   Compare the measured values to determine which sensor or channel is providing the deviant reading, and inspect the physical sensor for damage.

3. 3

   Perform a continuity and resistance check on the wiring for both redundant channels from the sensor to the I/O module terminals to identify any differences.

4. 4

   Recalibrate both redundant sensors according to their manufacturer's specifications to ensure their outputs are synchronized when measuring the same physical quantity.

5. 5

   Review the safety controller's configuration (e.g., Parameter S2-05 for discrepancy tolerance) and, if justified by process variation, cautiously adjust the tolerance window after a thorough risk assessment.