Limit Switches and 4-20mA Feedback in same device

Curious basic question about multi-signal transmitters, if I have a device that gives a 4-20mA feedback signal (say position or level) I can use the 4-20mA signal as a "limit switch" in a PLC program by designating certain mA values as end positions. In that case, having additional limit switches to mark the endpoints of said device doesn't seem to be very useful except maybe in cases where only open/closed is required, no controller is used, or as a fault sanity check for the signal (using both to check each other, i.e. if limit switch on and feedback in middle range = limit switch fault, for example). Are there scenarios where it is useful to have both, or is one or the other usually sufficient (depending on the application)?

In some cases the switches are useful as backup hi/lo alarms.

You can buy products that can pass additional digital information over a traditional 4-20mA current loop, known as HART Protocol (Highway Addressable Remote Transducer)- it might be worth seeing if your device support this.

The standardized HART data structure normally allows the passing of 4 or 5 numerical values from memory, however there are systems based on HART that can pass other data, however this has to be supported by the other hardware at the receiving end.

Do you have vendor part numbers?


Regards

Daniel

I have worked on several system where the limit switch were tied into the E-stop circuit as a safety shutdown of the system and a 4-20 mA through the PLC logic would not have been excepted.

In the good ole days. you could have the limits start/stop a pump, and the 4-20 went to a chart recorder for historical purposes.

I have also seen discrete limits on valves that did not have position feedback. Used to verify that a valve was fully closed or fully opened before some process event could take place.

Ken Moore said:

I have also seen discrete limits on valves that did not have position feedback. Used to verify that a valve was fully closed or fully opened before some process event could take place.

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This is the most common place I've seen limit switches, both physical and induction prox switches. It's nice to not have to do any scaling or GEQ/LEQ in the PLC when one needs to be replaced unexpectedly.