Advice for 2 control signal one field equipment

[BeepBob]

I am not sure why this is requested, but it was asked.

Currently I have one PLC , with one output to a relay, turning on a field equipment (just using one as an example)

We are upgrading the PLC with a more modern platform.

A request is to commission the new PLC system while having the current PLC system operational. Then once the new PLC system is successfully commissioned, the old PLC system can be taken offline.

Any suggestion on how to control one piece of equipment using two PLCs (same logic).

I guess what I am trying to do is the OR rung with one OTE, in electrical circuit.

 

[ErmaGersh]

No simple way to do this without complicated modifications to the existing control panel. Best approach to this scenario:

1. Create a detailed cutover plan for moving IO to the new PLC. Go over this plan exhaustively several times.
2. Schedule downtime on the equipment.
3. Execute your meticulous and well thought out plan.

 

[BeepBob]

Yeah we are doing the FDT to show them everything is kosher and stuff. Still working that angle where the system can be taken offline.

But for now , this is it.

For the inputs I can just parallel the signals to both PLC,

The output however

 

[TheWaterboy]

Heres a thought...

If you already map your PLC physical inputs and outputs to intermediate bits and tags you can do it by paralleling the old PLC IO with bits MSG'd from the new PLC (at a fast rate) . Use a single XIC/XIO pair on each IO point to determine which of the control points the old PLC uses to control the actual outputs

 

[danw]

Add a 2nd field relay, outputs in parallel driving whatever?
It’s still a logic OR.

 

[plvlce]

Add a 2nd field relay, outputs in parallel driving whatever?
It’s still a logic OR.

This is my initial thought. Of course it requires adding relays for every single output, not exactly what I'd consider an elegant solution.

 

[TheWaterboy]

Can you add the IO mapping to the PLC as I suggested?

 

[Steve Bailey]

not exactly what I'd consider an elegant solution.

I don't know if there is an elegant solution. If the I/O were all on a network you could first replace the PLC CPU with a newer model still compatible with the I/O and then replace the I/O points at scheduled downtime periods. During the commissioning of the replacement CPU you would have an easy fallback position.
You're going to wind up having to take some downtime to add parallel relays to the existing system. The customer will have to decide whether to swallow the additional time and expense to prepare the system for the migration or to bite the bullet and take a thoroughly planned shutdown to get it done.
For all of the PLC upgrade projects I've worked on, the strategy has been to do it during a shutdown.