Depending on what the tags are doing, and how much latency the application can handle, a few approaches may be better suited.
1. Reference Tags - If the tags are truly 1 to 1 (as it seems to me from your OP), then Reference tags are probably the best approach. They will have the most clear setup and probably the least amount of latency available from just a standard Ignition install.
- Gateway Tag Change Event - If the tags are not 1 to 1, and/or the application will not be sensitive to a longer latency (100s of ms + scan class time ). This is the best script based approach in a standard Ignition install.
3. Expression Tags - If the tags are not 1 to 1 but can easily be combined into a single boolean value, then this is probably the best approach. Again this will have a clear setup and won't be accompanied by a heavier than scan class latency.
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Tag Value Change Scripts - Most likely the worst solution, though technically possible. There are caveats to using this approach that would rule it out for me personally, particularly if there are many of these tags, and the application is mid to large scale.
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Another Option - If you are able and/or the client is willing to, then I believe that you could use Phil's Automation Professionals' EtherNet/IP Communication Suite V2 Module. It has some optimizations that may allow this to be done as nearly to a PLC as possible.
Of course as stated the best option for this is to do some type of peer-to-peer messaging.
If the application is sensitive to more than low latency or has anything to do with safety, then I would be refusing to do it any way but through the PLC's.
EDIT: Turns out I should read the manual a bit closer. Neither Reference Tags, nor Expression tags, actually write values back to a different device.