PLC DCS interface signal


Thread Starter


We are facing a very typical problem in our plant control system.

A digital output from the DCS is wired to as a digital input to a PLC.
When the DCS output changes from 1 to 0, this is immediately reflected in the PLC input. However when DCS input changes back from 0 to 1, PLC input is still 0 for quite a long time (which is not fixed) and suddenly becomes 1.
No timer is defined in the PLC logic. Even if the timer is defined, the LED input indication of the Input card does not become ON when DCS output changes.

How should i go about trouble shooting this issue?

bob peterson

Several things come to mind that require additional information.


Distance the wiring runs.

Has it just started doing this? If so what has changed recently?

How long is "quite a long time", and how is it being measured.

What PLC is being used and what kind of input?

What kind of DCS output (triac, relay, transistor, etc.)?


William \(Bill\) L Mostia Jr PE

We really need some more information like:

1. Is this a new problem?

2. How many of these have the symptoms? Are they all on the same card or split with different input cards?

3. What type of signal is being transmitted to the PLC?

4. What is the typical time delay?

5. What have you tried so far?

In any case, I would propose the following.

1. Do you have any digital outputs that work correctly? If so, what is different between those and these?

2. The PLC input LED appears to track the PLC's internal signal representation. Does your DCS also have LED's? If so, check to see what they are doing?

3. If you don't have DCS LED's, I would disconnect the signal wires at the DCS FTA (this splits the system in half) and check the DCS signal at the FTA panel. If the problem is still there, it would appear to be internal to the DCS. If not, it is in the direction of the PLC. Reconnect the wires at the DCS and disconnect them at the PLC and check again. If the problem is still not there, the problem is with the PLC input card (configuration, how it is wired, etc.) or the signal interface between the DCS signal & the PLC input (review the signal spec's for each card).

4. If it is in the direction of DCS, do you have any other digital outputs that are working correctly? If so, do they use the same DO block in the HMI/DCS controller/MUX? If so, then compare the digital output block configurations or if different, use the other block. It is possible that this may be an DCS update time issue (how the system updates operator input) or a timer on the DCS side. If not, you may find that this just a feature of your DCS.

William (Bill) L. Mostia, Jr. PE
Sr. Consultant
SIS-TECH Solutions, LP

Any information is provided on Caveat Emptor basis.