1771-IAD A, B, C, D Terminals

[Beau 1]

What are the A, B, C, and D terminals used for on a 1771-IAD Input module? All the literature I could find just says that they "are not used" but we have a system in our plant that has 120VAC wired to terminal A in each module.

[Mickey 71]

Are you sure it's not a 1771-OAD? As it does use those terminals.

 

http://literature.rockwellautomation.com/idc/groups/literature/documents/in/1771-in024_-en-p.pdf

[Joe E. 259]

Or that someone hasn't put an IAD in a slot that's wired for an OAD?

[pcmccartney1 141]

Seems he would have a fault on the module or the PLC5 were the wrong module be installed in the slot and differ from the program?

But as the others have said, those terminals are not used for the IAD.

[Mickey 71]

7 minutes ago, pcmccartney1 said:

Seems he would have a fault on the module or the PLC5 were the wrong module be installed in the slot and differ from the program?

But as the others have said, those terminals are not used for the IAD.

FYI, The PLC5's do not required a I/O configuration. It's documentation only if it's there at all.

I/O addressing would certainly be a problem.

[Joe E. 259]

If I remember right, there's an addressing scheme for those racks that alternates input and output modules with adjacent modules having the same word address. In other words, I:1/x and O:1/x would be valid addresses for adjacent input and output modules. If they were swapped, they wouldn't work right but the PLC processor itself would be perfectly happy. It would very likely fry some field components and maybe the modules, though, as the outputs applied voltage to the input devices and the input devices applied voltage to the outputs.

 

Edited to add:

By "not work right", I just mean the wiring would be backwards. If the wiring moved with the modules, they would function perfectly normally. In other words, you could put either the input or the output module first in the slot pair and have it work properly as long as the wiring was right.

Edited 29 Mar 2018 by Joe E.
Clarification

[Beau 1]

They are definitely 1771-IAD 120VAC Input Modules and they have been using them in a process that has been operational since early 2016. The line comes off the Tx secondary to an AB power distributions block, then to a fused terminal block before landing at the "A" terminal on the input modules.  There is also a second wire that is daisy-chained from the "A" input to terminal blocks in the MCC buckets that are associated with that input module.

If they are never used for any reason why not manufacture the modules without them? Since they are included, I have a hard time accepting that there is no situation where they would be used. This is mostly a curiosity for me, but its driving me nuts.

[Joe E. 259]

It makes sense for them to make a single wiring arm that works for all modules. They do that across their platforms, including the 1756 modules.

[pcmccartney1 141]

Beau,

Wondering if you go look at your electrical drawings (focused on the MCC buckets) that they used the A as an intermediate point to get the 120VAC (interrogation voltage) out to the bucket and thru the status contacts and then back to the input card?  They probably then "jumpered" the 120VAC to other buckets in the area that needed to send status back to the same input module.

[Beau 1]

@Joe E. thank you.  that makes sense to me.

@pcmccartney1 looking at the physical wiring, yes i believe you are correct.  The schematics do not make that immediately obvious but after your comment i get it. Now i'm curious why they used it as an intermediate point because it would have been easier ( and the wire run would have been shorter) to come straight off the terminal block and skip the input module   

Oh well.  Thanks for the replies everyone!