Write Only Tags

I have a fear that the answer is going to be one of those ones where I sorta shrink down in my chair and mutter…“oh right”.

One aspect of the overall design of a particular project is I am using Ignition as a “settings” interface to a plc. One where a new/replacement PLC can be installed in the machine, the ladder logic loaded, then the PLC brought online with Ignition at which point the tags that contained specific machine level settings would NOT read, but rather only write themselves to the PLC, thus making the new PLC the same as the PLC that was in the machine before (sorta like recipe’s I guess?).

The settings would be things like “Pump 2 in Rotation”, “Pressure Setpoint”, “min/max RPM allowed”.

I guess after asking, my question really does sound like recipe management. I was hoping to see a “write only” option in the tag definition where it doesn’t really care what the PLC addresses value is, it just writes a replacement value if they don’t match.



If you’re doing this within a transaction group, you can set the items to DB-to-OPC. This is what I currently do for recipe-type applications.

Whoops! I shoulda read a little deeper… :blush: Don’t think you can do it an an OPC tag itself, but you do up an expression tag with the value you want, and feed it back to an OPC Tag (There’s an option toward the bottom, underneath the spot where your tag would go…)

see here where I’ve asked for this same thing before.

There is such a thing as a write-only OPC tag. Kepware OPC servers support this by adding a ‘W’ to the item name. Why would you want a write only tag? If you have 50 or 100 sites multi-dropped devices on a radio, telco circuit, or RS485 you want may want to write to a specific discrete without including it in the round-robin read cycle - push-to-start or alarm acknowledge, for example. The PLC will always clear the bit, no need to read it back.

In your example I think you want to create memory tags that hold the setpoints and flags in addition to the same OPC tags. Mem tags are permanent in Ignition. You can place a button on one of your screens to copy the mem tags to the OPC tags if you need to reinitialize these PLC values. Or you could program the PLC to let your Ignition app know that it’s doing first scan and automate the transfer.