Control one analog device from two PLC's

Hello,
I am looking for suggestions for the best way to control the following example e.g. Logic, Relay etc. Please provide examples of the control strategy you suggest.

We have two AB PLC's that need to control one analog valve.

Filter #1 needs to be backwashed. The backwash is initiated and the PLC for that filter needs to control the valve. Once the backwash is complete the valve needs to return to the closed position 4 mA.

Filter #2 same as above.

The valve always needs the 4 mA signal to remain closed. So one of the PLC's need to be providing that signal.


Thank you for any assistance.

Two different PLCs controlling the same device? Instead of trying to work out a control strategy for that nightmare, spend the time redesigning the system. Put both filters under control of one PLC and use the other one for spare parts or a trainer system.

Have PLC2 send signals to PLC1 asking for those outputs.

I work on a wastewater treatment that has 4 panels networked together. The main decision maker (SLC5/03) has had another panel added (ML1400) for slightly different treatment. The second panel sets bits and values the SLC reads with messages, then the SLC opens and closes valves, turns pumps on and off and sets VFD speeds based on what the ML is asking for. Then the SLC messages the ML what is happening.

If the 2 PLC's are not networked a few digital and an analog signal between the 2 for conditions and value. If the values are set then PLC2 could send digital signals for Setting1, Setting3, etc.

When I do this I add pulsing signals from each PLC to the other so both can trip a Watchdog timer if the other one shuts down. If PLC1 is doing something for PLC2 then the signal is lost the command of PLC2 is no longer valid.

What interlocks does the valve have? Where are they wired into? Connect the valve into the PLC with most interlocks "tied" to that valve.

Add a digital input from the other PLC to request the valve to open. Wire two digital outputs to receive valve available and valve open (closed) back to the other PLC if there's no networking option.

"Once the backwash is complete the valve needs to return to the closed position 4 mA."

So there is no control of this valve when not flushing the filter??

We support systems like this. We use communications between the filter PLCs and the PLC that controls the backwash pump/valve(s). The filter requests control of the backwash valve, sends a desired flow rate, the backwash PLC controls the valve. If one filter is in backwash mode, no other filters using that same backwash infrastructure is allowed to enter that state (it must wait its turn, if quality is out of control, appropriate other states are entered ie filter to waste, or shutdown).

I dont know why you (really?) need 2 PLCs for controlling the same valve, I dont advise that, not directly


I would send values from PLC#2 to PLC#1 via ethernet or any other method and then wire the valve only to PLC#1

Sounds to me like someone is asking for help on a school assignment.

from a controls perspective, I would have 1 "master" PLC that has direct control over the analog device (so field wiring and safety).

I would have the "other" PLC taking information directly from the master PLC and making "requests" when required.

Thank you all for the replies. I appreciate all of the responses with information and suggestions that were relevant to my question.