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Current loop input fault can't be reset
Current loop input faults, locks up if 4-20mA signal is out of range, will not reset without power cycle, even if the input comes back into range. Can I reset this or disable it without having to cycle power?
Customer support service by UserEcho
This is intentional, the CIN switch off and stay off until the next power cycle, it is done to avoid burning the measurement resistor.
In some earlier versions there was a problem that the overload detection was too fast and reacted to sensor overshoot, from 5.04 the detection was made more robust.
Understood, there is no way to reset this without a power cycle? A very brief (less than a second) pressure surge that causes the input to go slightly above 20mA now renders the system completely inoperable with no way to fix it. Rockwell analog inputs will fault at 22mA, but this will reset immediately once the input is within range again. This is not a good solution since some processes cannot just have their power cycled. At what point does the CIN turn off? Exactly 20mA or more?
On the MC4, the overload detection for CIN comes when it exceeds 25 mA for 20 ms.
Sorry I'm late to respond. I just now saw this. Tony, I think you can add a Current Regulating Diode (CRD) to suppress the surge current. I'm an EE, so am familiar with these parts, but know you don't see them used very often. I used some of these in another design I did 25 years ago, which is why I'm familiar with them. Think of them like a hydraulic orifice. These will limit the maximum amount of Flow (current in this case) that can get through, and are very fast acting.
You can read more about these at https://en.wikipedia.org/wiki/Constant-current_diode
In looking at the datasheets that are linked in the wikipedia article, I am thinking the Semitec 22 mA part would work great, the S223T: Semitec-CRD-Constant-Current-Diode_S223T.pdf
The downside for this particular part is that it is surface mount, and a leaded part would work better if one wants to integrate it into a harness. This datasheet has info on paralleling different parts to achieve whatever current regulation value is desired. I picked a value of 22 mA as it is under the 25 mA threshold that will trip IQAN's overcurrent protection and is above the expected range maximum which is 20 mA.
I've modified the drawing from the IQAN MC4x/XC4x manual to show where I would put the CRD.
Hope this helps, if it's not to late.