a quick question for you: does this arrangement function reliably in an actual working system – or is this a "textbook" type setup? ... This is from a working system.
my biggest question is where are you getting the actual Tieback value from? ... Tieback is the manual output from a PID faceplate at the HMI. When in manual mode, the operator has control over the output; when in auto, it is overwritten (bumpless transfer)
and I've never seen the Tieback being constantly moved into the "final destination" of the control loop ... (I'm assuming that's the function of your address N9:196 – which seems to be an I/P transducer if I'm reading the documentation correctly) ... Yes, N9:196 is the address of the point on the AO card
and then on another topic ...
I'm also very interested in what happens to the field output device if either the EQU or the LIM instruction on your PID rung ever got evaluated as a FALSE condition ... The EQU was used to load share multiple PIDs processing on the processor at 1 sec intervals... probably not of any consequence but thought it a good idea at the time, I guess. The LIM is much more significant and is intended to keep the last PID output on a bad signal while in automatic(this I/O has no open-circuit-detect, as I remember). I do this for regulatory PID control. You'd want to be careful using this if one end of your transmitter range corresponds closely to a normal operating point.
I'm betting that you must have some additional logic (on other rungs that you're not showing) to take care of that particular situation ... are you simply moving another value from some other control scheme out to the field device? ... No, an alarm will sound. In this example, the operator has the responsibility for controlling the mode of the PID loop.
