1769 Chassis limits (five racks possible?)

We're converting a SLC 5/05 to 1769 CompactLogix. The existing PLC is spread among two cabinets, with five racks total (last two racks being read by a RIO scanner module) and 28 modules total. In order to simplify wiring and keep existing IO, we'd like to keep the same physical rack configuration.

We're going to be using a 1769-L36ERM for CPU, which has a 30 module limit. Can I setup a 1769 processor with five racks? IAB will only let me setup 3, and I'd have to go with an 1769-AENTR for the last two racks, which is fine, and what I'll probably do. I was just wondering if anyone has tried to make more than 3 racks while staying within the 30 module limit.

Without using IAB or Studio 5000 to confirm, I would probably use a rack extension cable for the first 3 racks (if possible) and then put the AENTR in the other cabinet with another rack extension cable. If that won't work, then use another AENTR for the 3rd rack in the first cabinet. I don't think the rack extension cable can be all that long.

joseph_e2 said:

Without using IAB or Studio 5000 to confirm, I would probably use a rack extension cable for the first 3 racks (if possible) and then put the AENTR in the other cabinet with another rack extension cable.

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This is exactly what I'm planning on doing. I was just wondering if anyone had tried having more than 3 racks. From the PLC point of view, it just numbers the modules sequentially, and as long as I stay under 30 it might be ok?

Reading a little more, Document 1769-in014 (for the expansion cables) says 'A maximum of two expansion cables may be used in a 1769 system.' which I suppose answers my question... They come in 1 foot, and 3 foot cables, which I guess is going to be the limiting factor anyways. The 3 foot cable won't reach between my two cabinets anyways. I guess I'm stuck with the AENTR.

The AENTR does add to the cost, but it shouldn't be difficult to deal with. I had a system that had 2 PLCs: an old L55 ControlLogix and an older CompactLogix. We upgraded the ControlLogix and replaced the CompactLogix PLC with an AENT module to turn its chassis into a remote rack. Except for running the CAT 5, the whole migration took about 20 minutes. Yours will be longer since you also have to transcribe the RSLogix program, but it should work out ok. Especially if the SLC chassis doesn't have anything too fancy in it. Are you planning to keep the SLC I/O or replace it with CompactLogix?