AB Flex I/O Problem


Thread Starter

R A Peterson

I received the following email from a colleague. Has anyone else heard this?

In talking with a customer this morning, they related the following problem:

On their machine, they were having random machine crashes. After eliminating all of the usual causes, an AB tech let slip that there is a known problem with the IV (sinking DC) input module.

Apparently, if this is the first module after the ASB, then the input bits can flicker on at random times. The product is being shipped without any
warnings of the problem.

The only fix is to put an output card as the first module, then everything will work out well.
Curses, our foul secret revealed ! The man responsible will be neutralized at once.

I think your A-B tech might have been confused as to the identity and details of the problem he mentioned.

There _was_ a problem with the 1794-IA16 (120 vAC input) module which was identified around October 1999 and resulted in a mandatory Type II (Retrofit) service advisory, numbered SP-3266. It affected rather a lot of shipped modules
and all sales offices and distributors worldwide were notified to participate in the replacement program.

The gist of that service advisory was that high noise conditions on the backplane of the FLEX I/O system would result in incorrect data from that
module, rather than faulting the module (which high noise _should_ do). The reason I think your tech was confusing the 1794-IA16 service advisory is that the most common occurrence of the problem was when the 1794-IA16 was in Slot 0,
meaning it is using the data bus pins closest to the power bus pins. If I was a field repairman without quick access to factory replacements, I'd recommend swapping the input and output modules, too.

When the problem was identified, shipments were stopped and all of the stock in our distribution center was reworked. Distributors were also instructed to return the modules for rework. A-B Tech Support can walk you through the
recognition of a good or suspect module if you mention the SP-3266 retrofit notice to them.

In DC systems, the most common cause of the "faulty input" behaviour that I see involves overloading that little 1794-PS1 power supply. It's only rated to 1 ampere and is really meant to just power the adapter and a few inputs. If
you're running some inductive output devices with the same power supply, I can imagine sinking inputs not being able to sink enough current occasionally and getting intermittent data.

It's possible I'm not in on any problems with the IV16, but I'm pretty sure your guy was confusing his statements with the retrofit advisory to the 1794-IA16 because of the similar description of Slot 0 susceptibility.


Ken Roach
A-B Seattle Sales Office
[email protected]