Certain Tags Getting Bad Quality at Random Intervals

Hello all,

First off, I am running Ignition 7.9.6, and connecting to a SLC 5/05 PLC.

For some reason, certain tags seem to get a bad quality at random intervals, and then recover on the next scan cycle. It’s always the same tags, and all the tags from this PLC utilize the same scan class. I tried to bump the scan class up to see if it would help, but it didn’t seem to work (from 2,500 ms to 4,000 ms). The other tags we use from this PLC are working fine, and don’t have quality issues.

Has anyone ever run into this before? Any ideas how I can try to fix this?

Have you looked at the driver diagnostics page for this device? You might find your answer there. (Look extra close at the average response times and the number of reqests per time period.)

Here is the screen shot from the driver page:

Is there a different page with more info that I should be looking for? The tags i’m having trouble with are all in the 4,000 ms scan class, which seems to be doing ok, according to this data.

Your device is overloaded. You have 26 + 4(11) = 70 requests to handle every four seconds. You are actually getting only 10/second. So some of your 4-second tags are actually getting values every 7 seconds, and sometimes more as requests are used asynchronously for writes. That puts those over your stale limit occasionally.

That’s really crappy performance for a 5/05, btw. Do you have the message processing throttles turned on? Those would be S:2/15 and S:33/7. Make sure they are zero.


To solve this issue, I moved more tags out of the 1 second scan class and moved them to the 4 second scan class. Both of those tags were zero, BTW.

I am trying to grab data from a SLC 5/05 which we do not own the program to. OPC tag names are meaningless but we do have the electrical diagrams. Initially I added every tag that was available in the OPC Browser (3000+ tags). I then deleted all tags that were not physical IO which reduced my tag count to 93 although if you looked at the status for this device in the gateway, it said the tag count was ~250 and I had to use a 12 second scanclass to bring the load factor below 100%… I deleted the tags and the device then added only the 93 that I needed and all of a sudden I can use our default 1 second scanclass which gives me a load factor of 44%. Bit strange, just thought I would share.

I hope you now realize you should never do this. Also, did you check the message throttles noted above?

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