Local network convergence

We currently have 3 individual machine centers using Controllogix 1756-L71 processors. Each machine center has its own network of 192.168.1.x devices. But we are wanting to combine all 3 machine centers onto one large local network with all devices from all 3 machine centers. Is that possible with 3 different controllers and what would be the easiest way to do that? Total devices number around 120 so I was hoping I would not have to re-address multiple devices. Is there a way to drop in a couple of 1756-ENBT modules in the racks and just have them talk back and forth and still be able to view everything?

If there're free slots you can add one 1756-ENBT on each chassis and then connect them to a switch. This way you don't need to modify the original configuration or re-address any device.

I'd use 1756-EN2T (more connections than ENBT) or EN2TR (you can create a ring).

+1 to nhatsen. You could also use NAT devices at each panel, which would probably be cheaper, but it's more work and the EN2T solution is (in my mind) much neater.

The solution depends on what you want to do.

If all you need is the controllers able to talk to each other, adding ENBT cards to each controller and configuring them to all be on the same network will do the job. This is a common approach when each 192.168.1.x network is entirely I/O devices for a single processor.

However, you said "But we are wanting to combine all 3 machine centers onto one large local network with all devices from all 3 machine centers." If that is truly what you need, then adding a NAT device to each machine to translate all the existing IP addresses on the 3 existing networks to a separate network where all the IP addresses are unique will meet that need.

Thinking this through tells me that having a 1756-ENBT within each separate PLC cabinet and then connecting them together through a switch still will not accomplish what I want. What of say.. my DC_SAW plc is using 192.168.1.10 on one of its ethernet devices and my RipLine PLC is also using 192.168.1.10 on one of its devices. If I connect a ENBT from the DC_Saw to a switch and then a ENBT from the Ripline PLC to the same switch.. wouldnt I get conflicting IP Addresses when I am trying to look at the 192.168.1.10 device through that switch?

No, because the two ethernet modules with the same address are isolated across the backplane.

Take the example of a MSG instruction. When you set it up, you tell it each step of the way where to go. Let's say you have PLC1 and PLC2. Both have the following:
- CPU in Slot 0
- ENBT in Slot 1 with IP address 192.168.1.10, connected to the machine's local network (I/O, drives, etc)
- ENBT in Slot 2, connected to your greater plant network, with IP address 10.10.10.nnn (unique address on the greater plant network)

If PLC1 wants to talk to PLC2, you set up a MSG instruction to go from:

PLC1 CPU in Slot 0 to
PLC1 Backplane to
PLC1 ENBT in Slot 2 to
Ethernet port on the front of ENBT in Slot 2 to
IP Address 10.10.10.nnn (Ethernet port on the front of ENBT in Slot 2 of PLC2) to
PLC2 Backplane to
PLC2 CPU in Slot 0

Notice that nowhere along the way does the ENBT in Slot 1 that has the "duplicate" IP address come into the picture, on either PLC. Your message whizzes right past it on the PLC backplane without ever knowing or caring that it's even there, let alone what it's IP address is and whether it's the same as another ENBT that it whizzed past and ignored a moment ago.

Using Produced/Consumed tags is different, but same principle. The message follows the same path, and the ENBT's in Slot 1 don't form part of the picture at all.

Does that clear things up at all?

Yes,


Thank you for that explanation.. I sincerely appreciate it!!!