Ignition Edge 8: Configuring Controller Tag Writes

Use an Ignition Edge 8 tag Value Changed event script to copy a source value from one controller to a writable tag in another controller. In the documented example, Circulation_VFD is the source and WriteValTo is the destination.

Supported Mapping Mechanism

The mapping runs inside Ignition rather than either controller program. A Value Changed script attached to the source tag calls system.tag.writeAsync(tagPath, value). The function takes the destination tag path and the value to write.

The valueChanged event executes when the source tag's data or quality changes. This means a quality transition can invoke the script even when the process value has not changed.

Confirm the Architecture and License

Confirm that both controller data sources are accessible to the Ignition Edge gateway and that the destination tag accepts writes. The evidence describes this as an Ignition solution requiring Edge Compute, but also contains a later statement that Edge Compute became included with the Ignition Edge 8 license. Because those statements may reflect different licensing dates or packages, verify the installed license before commissioning.

Ignition Edge 8.1 is described as allowing two device connections by default. The evidence also distinguishes device connections from OPC UA client connections, but it does not establish that every controller exposes an OPC UA server. Count the actual connection type used by each controller and verify licensing when the architecture exceeds the available connections.

Configure the Value Changed Script

1. Open the source tag, Circulation_VFD, and navigate to Scripting > Tag Event Scripts.
2. Select the edit control for Tag Event Scripts, then choose Value Events > Value Changed Script.
3. Insert system.tag.writeAsync(tagPath, value) in the scripting area.
4. Use the tag browser to select the writable destination tag, WriteValTo. Let the browser insert its complete tag path instead of manually guessing the provider or path syntax.
5. Use the event's currentValue parameter as the value passed to the write function.
6. Accept the script changes, save the tag properties, and confirm that the updated configuration is active.

The resulting call has this evidence-supported form; replace the symbolic destination with the path inserted by the tag browser:

system.tag.writeAsync(destinationTagPath, currentValue)

Set the Update Rate

Adjust the source scan rate to match the application's response requirement. The evidence states that there is little benefit in scanning faster than 250 ms. Treat that value as the documented design recommendation, then validate it against the process response and communication load.

Do not interpret the asynchronous write as a reason to shorten the scan rate. The event frequency follows source data or quality changes, so unstable quality or rapidly changing data can generate additional write requests.

Verify the Mapping

1. Observe Circulation_VFD and record its current value and quality.
2. Change the source value through the normal process or controller test method.
3. Confirm that WriteValTo receives the new value after the source event executes.
4. Check behavior during a source quality change because quality changes also run valueChanged.
5. If no value arrives, verify that the script and tag properties were saved, the browser-selected destination path is correct, the destination permits writes, both controllers are connected, and the installed license supports the configuration.

FAQ