mysterious rack fault

PLC-5 remote I/O rack, intermittantly faults on the remote I/O adapter. Green lite blinks, adapter fault lite is red, I/O rack fault is red. We've changed the power supply, installed a sola power conditioner, changed the remote I/O adapter twice and finally changed the backplane. The frequency of occurrance has dropped in half, but the fault still happens. I want to test the blue hose (its daisy chaned and one of the runs is long) for possible damage after some recent contract work on the conduit, but I have no idea what a competent test would involve. Is there a tester that will check the impedance, do the transmission line length thing and test teh capacitance(mechanical engineer here- PLC newbie)? Any help is GREATLY appreciated!

I'm not sure about a tester you can use, but I can think of a few things to check:
-What is the baud rate set at & is it within the length specifications of your current set up?
-Are your termination resistors installed and are they the correct size for the baud rate?
-Make sure you don't have a rack address conflict.
-Verify the ASB and remote rack backplane dipswitch settings.

How long has this been going on? Do you go through spurts where it happens for awhile, then its ok for awhile, etc...or is it more continuious?

Thanks Dr, thats all good stuff I have to verify, I'll reply after I have checked off this items you mentioned. The fault is very intermittant, has been ongoing for a month. Prior to replacement of the remote I/O adapter for the second time, it was occuring as often as once per 14-24 hrs. Now occurance is reduced to about once every 3-4 days. I believe the baud rate is 9600, but I'll verify.

I would also try a time domain reflectometer (TDR) to check for cable problems.

Tech7

A good TDR will find bruises in the cable that no other method will detect. I've used one on an old coax based I/O system and it we were able to find the location of the "bruises" within a couple of feet!

I think AB has a special kit with a TDR designed to work specifically with blue hose.

Yeah, here it is ... looks pretty generic, like it will do it all, but may have specific instructions that apply to AB systems.

Brochure
http://literature.rockwellautomation.com/idc/groups/literature/documents/pp/1788-pp002_-en-p.pdf

Manual
http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1788-um002_-en-p.pdf

I've had a couple of similar problems pop up around here before, and one of them wasn't mentioned by Dr. Watson, so here goes....


One time, someone had cut the insulation back too far on one of the blue hose cables, and it was intermittently shorting out sometimes. Obvious looking back on it, but it was a little hard to find at the time....

The other time, someone had replaced one of the modules, and had the jumper settings set for a different baud rate.... Why it was working at all I have no idea.

Thanks a million guys- you've given me some ideas and I will pursue them- I'll post the results! Thanks again!

I once had an intermittent rack fault due to a loose screw at the 3 prong connector on the blue hose. I replaced the ASB module and never even looked at the connector screws. Just unplugged the connector from one unit & plugged it right into the new one. The wire was slipped up into the slot but the screw was about a half turn out. I try to look at the simple stuff first, but sometimes I miss one.
BD

we also have a fault that like this, on a lengthy RIO but it isn't random. it happens every Monday morning. AB manuals suggest
making sure cabling is correct and terminated properly, grounding
the chassis to specs, changing out RIO adapter, backplane, and/or
I/O cards one at a time, as any of these could be can contribute
to an I/O rack fault. I believe our problem is related to the fact
that the facility lighting and power equipment circuits are turned
on early on Mondays, and some type of power surge/ fluctuation is
occuring. You may want to check this.

May not be related to the original poster's problem, but katratzi, you might want to try these:

http://www.levitonhelpdesk.com/catalog/

I think the picture is of the DH-485 device, but the P/N & specs are for DH+/RIO device.