1771 ASB Failures

Hello All,

We have been struggling for the last couple weeks with a recurring Remote I/O communication problem. The problem is described below.

This is the network environment:
• PLC 5/40 channel 2A as the RIO scanner.
• Four 1771-ASB modules for 1771 racks.
• About fifteen 1794-ASB modules for Flex I/O racks (three recently converted from Block I/O).
• One 1336-GM1 module for a 1336 drive.
• Two Block I/O modules.

Here is an overview of events (some details are approximate):
• June 3 (in the middle of an extended shutdown): Two Block I/O modules are replaced by two Flex I/O racks. No apparent problems.
• June 4-5: Two additional Block I/O modules are replaced with a single Flex I/O rack. We had difficulty configuring the new rack in a way that would make it work (did not succeed yet).
• June 6: Communication is lost to all of channel 2A (red flashing light on the scanner). No adapter lights are on. Someone had previously noticed one of the 1771-ASB adapters had been faulted, but had been corrected by cycling power. Replaced that module, and the network returned to normal.
• June 7: Successfully configured the new Flex I/O rack. No apparent problems.
• June 19: Start-up from the extended shutdown. Communication is lost with channel 2A. Replaced the PLC5 processor, but problem was not resolved. Swapped the processor back out. Start pulling off RIO cables from adapters one at a time. When the cable is pulled from one of the 1771-ASB adapters, communication with other adapters is restored, and the channel light starts flashing green. Plugged the cable back in, and the fault immediately appeared. Replaced the adapter (a different one than before).
• June 20: Same channel 2A problem. Started pulling RIO cables. The fault disappears when we remove the cable from another of the 1771-ASB adapters, and returns when we plug it in. Replaced the adapter.
• June 23, ~8:00 PM: Same channel 2A problem. Cycled power to the processor and all remote racks, and problem appeared resolved. Replaced one of the Block I/O modules on the basis of a conjecture that it might be a problem.
• June 24, ~1:30 AM: Lost control of the 1336 drive (on channel 2A). The 1336 drive HIM was showing “Connecting …”, with fault lights on. Eventually the fault cleared and could be reset with the reset button for no obvious reason.
• June 24, ~4:30 AM: Same channel 2A problem. Started pulling RIO cables. The fault disappears when we remove the cable from another of the 1771-ASB adapters (the last remaining original one), and returns when we plug it in. Having run out of spare adapters, cycled power to the remote 1771 rack.

• June 28: One of the earlier replaced 1771 ASB adapter was replaced again after the whole channel went down again.


We are unsure why these modules are going bad or what is going bad in them as we don't have a way to test them. The only eye catching thing I could see is that some of the adapters are having some high numbers of reties on channel. Not sure if this is something that I should be concerned about.

Please advice.

I would suspect that the modules are not failing, but there might be a problem with the media (blue hose) and/or some changes that have made it more susceptible to interference. Are the cable lengths within specs? Are the terminations in good condition? Are the end nodes terminated with the proper resistors? Do any of the cable segments traverse noise prone (like VFD motor cables) areas?

Yes, this system has been running for over 15 years and we never had an issue. The only change made over the shutdown was replacing some of those Block I/O with Flex I/O. We also have the resistors in place on ends and terminators are good as well.

The other thing I noticed is the high number of retries for some of the modules in RsLogix. So I am not sure if that can help us track down something?

here are my thoughts.


cables do age and get brittle
connections get dirty and corrode sometimes they need to be cleaned and reseated.
the connections within your new equipment may not be seated properly, come loose, have trash in them from the manufacturing process, (I had that happed one time).
switch positions not seated properly / in their correct position.

just food for thought.
james

Thanks for your thoughts James. I do agree with that. Its just that all 4 of those 1771 faulted out after startup one after the other. So I was trying to see maybe it is something else.

We checked all the connections and terminals multiple time. It could be wiring but its very hard to point to where to start from.

• June 4-5: Two additional Block I/O modules are replaced with a single Flex I/O rack. We had difficulty configuring the new rack in a way that would make it work (did not succeed yet).

I would revisit the above again ( you probably have) this sounds like the catalyst.

1771-ASB's are very reliable in my experience, our plant has many and i have only seen 1 fail in 30 years, are you sure they have ALL failed? i agree with OkiePC, i suspect its another issue, if a process window exists try installing one of the suspected failed units to see if it works again after power down.

i would also revisit (and document) ALL your remote I/O configuration of the ASB's dip switch settings on all adapters (1771 & Flex) and I/O configuration in software. distance of Blue hose can be an issue and must be adhered to for reliability, i forget the limits but end resistors must be used and it needs to be a daisy chain config not mesh, incorrect layout will cause reflections. i suspect the addition of Flex I/O is the Issue somehow.

This is unrelated but may be of interest since you Have a flex I/O Adapter.

recently i have experienced some strange occurrences in my plant on a PLC 5/40E with remote 1771 I/O ( nearly 30 years since installed) and Remote Flex I/O ( 18 months Installed).

System was rock solid until the Flex I/O was installed, since then if a power sag/dip or outage occurs the PLC loses it's I/O config for the Flex I/O and we need to do an Auto Config to scan the Remote I/O adapters and then it recovers. this is very intermittent as sometimes its not affected. Battery in Cpu was changed recently, no other program issues.
we are going to install a small UPS on the power supply for the Local CPU Chassis to engineer this out.

Yesterday (in a 1771 remote rack) after a similar outage we had a 1771-IFE analog input module "Lock Up" it was doing weird stuff like an offset on 3 channels, could not change the config of the module etc even though the BTW/BTR instructions were not in error and seemed to be working. a simple power cycle of that chassis corrected this. this equipment on the Remote I/O is 20+ years installed. probably not related to the adapter link.

Never had a problem on any of this equipment in my 30 years at this plant until the Flex I/O adapter was installed.

Thanks for the ideas guys!