CompactLogix 8 Module limit?

I have 10 racks of CompactLogix all configured like this:

L32E (CPU)
PA4 (P/S)
IA8I/A
IA8I/A
IA8I/A
IA8I/A
OW8I/B
OW8I/B
IF4I/A
IF4I/A
OF4CI/A

Yesterday I had a strange failure in the first IF4I where it presented numbers in the first 2 channels raw input that were not possible. These even presented when the input connection was removed. So I replaced the the module --- and the fun began!

When powering back up the P/S would blink briefly and go out, I assumed this was an overload indication so I started looking into it further. While the total Current draw for the rack (875mA @ 5V and 490mA @ 24V) is well with tolerance for a single side of the P4A P/S - I see that I have 9 modules on a single side of the P/S. Documentation for the P/S and the modules all state that a maximum of 8 modules on either side of the P/S is all that is allowed.

I have 10 racks just like this, all built by a professional company 4 years ago. Had no problems until yesterday. Now I can't get this rack to come up with all 9 modules installed. While I started suspecting the last module of being defective, I can't know for sure with this 8 module distance specification looming over my head. I will reconfigure the racks and the program code to place the P/S in the middle if need be, but why is this a problem only now? How strict do I need to follow this 8 module distance?

Just my experience, I also have a Compactlogix installation that violates the 8 module distance, and it works successfully without any hiccups. In retrospect I would not design a system like this. I would follow the "rules".

When you installed the replacement card, did you make sure to specify the correct firmware?

I didnt get the chance to check firmware- It wont run long enough with the 9th module in place. Just found that in this document that there is also distance of 4 limit between the CPU and the P/S. It also says that you may get away with it for a long time but you risk module connection errors.

Found that if I slide the last module into connection with the thing running - that works.

Looks like I have just been running on borrowed time.

I haven't done the math on your specified system but the power supply should be placed for nearly equal, as possible, current consumption to the left and to the right of it. Could that be a factor?

As much as I would prefer that, or even to place Analog on one side and digital on the other, you cant do either. One of the specs is that the CPU can only have 3 modules between it and the P/S. That limits the possibilities.

We usually use IAB, removes the guess work (no math to figure). Just FYI for next time.

http://www.rockwellautomation.com/rockwellautomation/e-tools/overview.page#/tab2

James

I get it, and i have IAB but I am not looking forward to taking thsoe 10 cabinets apart and reconfiguring them after they have worked all these years.

TheWaterboy said:

I get it, and i have IAB but I am not looking forward to taking thsoe 10 cabinets apart and reconfiguring them after they have worked all these years.

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Understood, that's why I mentioned for use next time. It saves a lot of headache.

James

But.. for the record... I didnt build this one. Just gotta put that out there.

I'm sure like with most things the "8 cards per rack" rule is a recommendation for maximum life of the system. Probably when the PSU was brand new it could handle 9 cards. But the 8 module limit might have something to do with the fact that A-B setups have a design lifetime of 20 years, and they put the number at 8 because that is the number of modules they know to a reasonable degree of certainty the PSU can handle without failure for that amount of time. Either that, or the fact that every card draws a different amount of current makes it difficult to pin down an exact number so they draw the line at a fairly average number.

Moving the Power supply shouldn't affect the code at all. It's "invisible" in the rack setup so all of your slot numbers should stay intact.

FactoryTalktotheHand said:

Moving the Power supply shouldn't affect the code at all. It's "invisible" in the rack setup so all of your slot numbers should stay intact.

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I was not aware of that... Thats excellent news. Its enivitable this needs to be done so I went into the cabinets to see how bad it was going to be. Inside the nice tidy panduit channels are all the wires . . . nicely tie wrapped every 6 inches with no spare length. Sigh.