The Problem
When using LP4 X80 ART (Analog RTD/Thermocouple) cards, the IO DDT (Device Derived Type) values drop to 0 degrees under certain fault conditions — but the card does not report a transmission failure or open-loop fault. The PLC program sees a valid-looking temperature of 0°C/0°F and continues processing it as a real measurement.
Why This Is Dangerous
A reading of 0 degrees is plausible for many processes. If the PLC logic does not have independent validation, it will act on this false reading. For heating control, this could mean the system cranks up output trying to raise a temperature that is not actually zero — potential overheating. For cooling systems, it could mean unnecessary shutdowns or alarms.
What Should Happen
Normally, if a thermocouple or RTD sensor fails (open circuit, broken wire), the analog input card should:
- Set a fault/status bit in the IO DDT indicating the reading is invalid.
- Drive the reading to an out-of-range value (upscale or downscale burnout) to make it obvious the sensor has failed.
The LP4 X80 ART card is not doing this reliably — the fault bits are not being set, and the reading defaults to zero instead of an obvious out-of-range value.
Workaround
- Add application-level validation in the PLC program — check if the temperature is outside a reasonable process range and flag it independently of the card diagnostics.
- Implement rate-of-change checks — a sudden jump from a normal operating temperature to exactly 0 degrees in one scan is not a real process event.
- Cross-reference with other sensors if available — if one sensor reads 0 and adjacent sensors read normal operating temperature, the zero reading is suspect.
- This issue needs review with Schneider to determine if it is a known firmware bug or a configuration issue with the LP4 card variant.