How safe are multiple systems writing to the same PLC location?

General Discussion
1

Suppose you have 1 PLC, and 2 totally independent HMIs (and in physically separate locations). And you want to set a value in the PLC from either of the HMIs.

How bad would it be to simply have a button on each HMI that writes a value via OPC UA to the exact same location in the PLC (so last write wins)?

(Note that there is no way that there would be a person in front of both HMIs at the same time simultaneously mashing down on the buttons.)

My gut feeling says that this architecture would be problematic, but I don't know if that's true or not.

2

Lots of systems do this. In and of itself, it is not unsafe. It is most likely to be non-problematic if it is only user-initiated.

3

Thanks for the confirmation.

4

Let me clarify: HMIs using momentary buttons (aka jog buttons, press & release) will clash. My statement above assumes you use action buttons that just send on press.

If the HMIs must use momentary buttons, the two HMIs must use separate bits, and PLC must combine them appropriately in its logic.

5

This will be for action buttons. I agree that jog buttons would be Bad News™.