DHRIO Card in Remote CLX Rack

I'm working on a project where a Control Logix PLC will be upgraded to become a redundant PLC.

The CPU Rack will be duplicated and has two ControlNet cards (one to supervisory and one to IO) as well as an Ethernet card.

The existing CPU rack also contained two DHRIO cards, these are being moved to a non redundant IO rack on the ControlNet IO network.

The problem I'm looking at is the fact that these DHRIO cards have SLC's connected as part of the DH+ networks.

The ControlLogix reads data using a message instruction, this is no problem to adjust the map route.

The problem is the SLC's read data from the ControlLogix (site standard PLC's only read). The SLC message instruction only points to the DH+ card. Reading up on the DH+ card, I see you can use RXLinx to configure the card to tell the DH+ card where the CPU is, but this only appears to be able to point to the CPU slot number, not the full path that I will now need.

This makes me think that the DH+ card, must be in the same rack as the CPU to enable the SLC to access the CPU tags with a message instruction.

I can't really try this out to see if this is the case until I get to0 site during the change-over, which obviously will be too late for me to do anything about it if I cannot enter a path.

Anyone ever done this and can you put a path?

I've done this sort of thing with DHRIO modules in the remote ControlNet chassis as RIO scanners only.

You're put your finger on the usual problem; when a 1756-DHRIO gets a Local DH+ message, it routes it to the "Default Slot" in its chassis. That's usually Slot 0 but can be any Slot number where there is a ControlLogix CPU.

In our case we had a DCS that could only do Local DH+ messaging through a knockoff KF2 style device, so we actually added a ControlLogix in Slot 0 of the rack with the DHRIO modules just to handle that data exchange.

In your case it's SLC-5/04 controllers sending the messages, so they should be able to use Remote DH+ Addressing, which specifies a Local Bridge Node and a Link Number and a Remote Node.

My thinking is that you *should* be able to set up a routing table in the 1756-DHRIO that will route a Remote DH+ Message to the Primary ControlLogix.

Do you have an SLC-5/04 some ControlLogix hardware to experiment with ?

No we don't unfortunately.

If we can't do it, two options come up, one they may not like which would be the easiest, to write to the SLC's from the ControLogix. This would be fairly simple but goes against their standard.

If they don't want option 1, then option two might be to use the original L61's in a similar method to what you described above. As part of the project we're upgrading the CPU to L71 and combining two PLC's. So we could slip one of these into the remote rack and set up messaging or produced tags etc between the processors.