Tag quality alternating from good to bad - SLC-5/05

Hello. I’ve imported tags directly from a SLC-5/05 PLC using the ‘Browse OPC Servers’ button within the designer, but the tags are alternating between good and bad quality every 10 or so seconds. I understand that I can disable the overlay and continue working on the HMI, but I’m worried that bad quality tags will be an issue once the HMI is deployed.

Are there steps that I can take to correct this? I’m located roughly 20 miles from the PLC and an HMI running on RSView32 seems to be connected without any issues. I’ve attached a diagnostics screenshot. Thank you.

You’re subscribed to the tags from that PLC at a drastically faster rate than they’re actually able to be polled at. The tags you’re asking for get grouped into 375 requests and you’re asking for them @ 1000ms, which would mean you need to achieve a throughput of 375 req/s. The diagnostics indicate you’re getting ~8.6 req/s.

There’s no really great solutions for this right now, but here are your options:

  1. Use a significantly slower scan class for the tags coming from this PLC.

  2. Increase the ‘stale multiplier’ setting (under Ignition OPC-UA Server > Settings) to something so highly it effectively disables the stale detection the server does. A change in this setting requires the UA module to be restarted to take effect.

The way the stale detection in the server works is that it takes the rate a tag is subscribed at, applies the multiplier, then periodically checks to make sure the tag is being updated no slower than that. So in this case (rate=1000ms) your tags would have to be updated at least every 5 seconds (at the default multiplier setting) to avoid getting marked stale. It looks like in reality your tags are probably only updated every ~45 seconds.

So… either move them a 60,000ms scan class or increase the stale multiplier.

  1. Put an OPC-UA server at the remote site and then make a new connection from your gateway to the remote UA server to get your tag values. Odds are the slow polling is at least partially due to the latency of whatever connection you’re using to get to this remote PLC.

Thank you very much Kevin. I’ve implemented your scan class suggestion while I work on the design and all is good. I figured latency may play a role, as you have suggested, and I will set up a server at the remote site as soon as possible. Thank you for your help.