Changing an analogue output value

[Daryl 1]

Hi folks, not sure how much information I need to give, but my situation is that I have a Micrologix 1400 PLC that is used to measure oxygen in the atmosphere of a room, take that input, use a PID function and then control the output of a control valve. I'm at the testing stage of this and need to be able to alter the output value to the valve as testing goes on. Obviously the PID function gives me an output that is used to control the 4-20mA output, but I wish to intercept at the point of output so i can control the valve. the program has been written by contractors who are unavailable for testing at the moment and I'm having to step in to assist. the RSLogix500 program PID function gives an output to an integer N7 data file, which in turn, based on various conditions, passes a value to float F8 data file, which in turn is the input to an SCP instruction which gives the end output value to the physical output. The end instruction is attached. thanks

[phuz 2]

You will need to put the PID loop in manual and write a manual CV output. I am not sure where your F8:26 comes from, but it isn't directly from the PID instruction (they accept integer values for process variable and control variable). Edited 27 Feb 2014 by phuz

[Daryl 1]

Thanks! That works. Opened up the PID setup screen, changed the mode to manual and set the CV output percentage myself. Just as an FYI, the PID function gives an output to an N7 data file, which in turn is combined with other conditions to then move the value to one of three different scaled outputs (0 - 1 is an error, 1 - 99 is scaled to 0-50%, and 99 - 100 is an error). This then feeds F8:26 which is used for the shown end output. thanks