Odd behavior from analog output on PLC5 system

wildswing

Member
Join Date
May 2005
Location
Sault Ste Marie, Ontario
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281
Hey folks, this is my first posted topic and it's a spooky one. Got lotsa people scratching their heads over this one. Here's my "in a nutshell" version...

I'm using a PLC5 with an analog output card in a RIO rack (due to location). Occassionaly the card does not respond as expected. The 4-20 goes to a drive for speed reference. Sometimes the drive does not slow down as instructed by the logic. Histograms have confirmed that the integer in the BTW is changing correctly. At the same time we have a chart recorder watching the 4-20 at the drive. Once in a while the 4-20 does not change. We see no BTW error, BTR errors, rack faults, drive faults, RIO Que Full bits, RIO Channel Full bits. Tried moving the integer to another channel of the same card. We see the same behaviour there. The changes in analog value last at least 1 second in the logic. Value is programmed to go from from 85% for 5 sec, then 60% and 25% for 1 second each then zero. Most of the time we see the corresponding change in analog output and all works fine. Once in a while, there's a delayed change where it's 85% for 7 seconds or more then zero. Sometime we see a rapid change i.e 85% for 6.5 seconds, 60% and %25 each for only 1/4 sec. Scan time of the plc is 29 mSec. The output card has been set up with the standard BTR/BTW pair logic set for non continuous, either get's enabled when itself or the other isn't.

Ideas? Bad output? But we replicated the behaviour on another channel. Problem with something in the card that's common to all channels?

Any ideas would be appreciated. Sorry if I don't reply for a while. Vacation next week. Internet access is neither available nor wanted on the golf course.

My thanks for your replies, in advance,

Mark
 
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Be aware that on RIO, even for discrete I/O, throughput can be delayed considerably and it's asychronous to the program scan. e.g. your 5 second delay could be anywhere from 5 to 5.5 seconds plus the PLC scan time plus the block transfer delay depending on your RIO network configuration, processor type, and other factors. From 1771-ASB manual "Block transfer throughput is determined by a number of things, including how many adapters are on the network, how many block transfer modules are in each chassis, communication rate and number of block transfer words being transferred. Refer to your processor user manual to determine the worst case throughput and recommendations on how to optimize throughput." You'll have to dig deep in a/b literature to find this information. I have measured up to 0.125sec delay on a PLC5/40 with two remote 16 slot racks with only discrete i/o!

This is probably not your problem, but, I had a 1771-ASB series A in a series A chassis that suddenly decided to quit pasing analog values altogether after a shutdown. It still handled the discrete I/O just fine, but the programmed block transfers just stopped working and showed no errors anywhere. We went to a series B ASB and remote chassis and all was well. (Ours was not for analog, but for a 2760-RB module that was never intended for use in a remote series A chassis.)

Also, look at the channel diagnostic retries counter for that rack. The RIO link may be failing, but never faulting, adding additional delays to your system. You must have a diagnostic file associated with the channel to get this information. Here's a snapshot of one of mine with a noisy RIO link...
sample10.jpg


EDIT: Open up the .pdf for your processor and search for "throughput". There is a method to calculate worst case situations. That said, you still probably have RIO noise or comms. problems with block transfer timing/traffic to cause a two second delay you described.
 
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My best guess is that it is related to RIO network traffic; every once in a while you just happen to hit it at the wrong time and the module is missing the intermediate steps between 85% and 0%.
 
Thanks for the info guys. The drive was wired for discrete speeds today during some maintenance downtime. We'll see how that works over the next while.

OkiePC, how long has it been since you reset those counters. Our's have been running for 9+ years and this is probably the first time anyone's looked at them. Some were in the tens of thousands including the suspect rack. Keep in mind our blue hose has been down for changes/additions/deletions many times over the years. I've reset them. Let's see what they say when I get back from vacation.
Thanks a ton for the leads 🍻

:site:
 
Wildswing: I have never reset them. The machine has been running for about ten years since the PLC upgrade. I don't think the counters will reset unless someone intentionally does it, and I'm quite sure no one else would mess with them. On your machine, I would reset them, and then watch. A single retry on occasion should not cause a huge problem, but multiple retries in a short period certainly could cause your symptoms. One more thing I've seen cause RIO comms problems is using a blue hose shorter than ten feet. This happened on a different machine. We lengthened the cable to fix it.

EDIT: If your remote rack has been powered down, and the scanner wasn't, that could explain the huge counter values, otherwise, you probably need to look for causes of noise, bruised cable, etc.
 
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Thanks OkiePC. Yes, we've done a lot of work on that network since the original installation. Racks have been powered down and blue hose disconnected/rerouted all without disturbing the processor. I'm hoping that's where the large numbers have come from. I'll report back with my retry counts week after next.
 
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