I'm guessing your signal is a voltage signal (0-10VDC for example) or a current signal of 0-20ma. What variable are you measuring? What type of sensor? What signal does your field sensor provide? Can your sensor output be adjusted? For example, can you program the sensor for a 4-20ma output? If so, you would know the fuse is blown, or wire disconnected, because you would get less than 4ma. If voltage, can you program the sensor for 1-10VDC for example. Is the signal dependent of any other factors? For example, if a pressure sensor on a pump. If the pump is running AND the sensor output is zero, THEN there is a problem. Is this the type of logic you mean by "stale data logic"?

This is a 4-20ma input on a Click Plus with a 4-20ma pressure transducer connected. Measuring instrument air compressor pressure. The input can't measure below 4ma or above 20ma. What I'm doing is taking a sample of the pressure every 10 seconds and comparing that to the input value. If they are equal for more than 30 seconds then trigger an alarm. So if the input data is stale/frozen then I get an alarm, similar to a communication failure alarm. When the fuse is pulled the input drops to 0 and doesn't change, so this works. If the compressor is blown down to atmosphere I may have issues with false transmitter fail alarms and require a startup bypass etc. I typically use the raw ADC counts or the built-in status bits so I was just asking if this PLC supported either of those. From what I can tell it doesn't.

Thank you for the description of your system. Your module could be configured for 0 - 20ma. And scale accordingly, so you could program an alarm if the signal goes below 4ma, and this would indicate a blown fuse.

You need minimum instrument air pressure. Do you have a low-pressure alarm? If so, how do you "disable" this alarm during a blow down?

Your "stale data logic" is a good idea, assuming you are always using instrument air, so the pressure is either decreasing or increasing. This stale data logic alarm would indicate a closed valve (downstream of sensor or at sensor tap), for example. (of course you would have plant problems is a valve was closed!) But a minimum pressure permissive would prevent this alarm during blow down, when the pressure is sitting at "0" for longer than 10-sec.

I typically set the analog input module configuration to scale to milliamps and uncheck the range limit tick box for the inputs. I use a Math block to scale from these raw values to engineering units. This lets me put the scaling parameters on the HMI so the tech can use it to calibrate and scale the inputs. For alarming, in PLC code I set an alarm if it falls below 3.8 or above 20.1 for a set time.

This works for Click Plus option slot combo modules, but there are certain Click expansion modules that will not provide a value below 4.0 when the circuit is open, so you may not be able to reliably program a "sensor fail" alarm if the system actually uses the full range of the sensor.