Ignition valueChanged: Writing Custom Property to Memory Tag

Problem Overview

In Ignition Designer, a valueChanged event script on a component's custom property fails to write the updated value to a Memory tag. The two most common root causes are (1) using a string literal instead of currentValue.value to reference the incoming value, and (2) a Python indentation or line-continuation error that silently prevents the script from executing. A third cause — the tag path being marked read-only at the Tag Provider level — produces no script error but leaves the tag unchanged.

Root Causes

Corrected Script

The canonical valueChanged handler for pushing a custom property value to a Memory tag:

def valueChanged(self, previousValue, currentValue, origin, missedEvents):
    tagPath = "[default]Path/To/Your/MemoryTag"
    value = currentValue.value          # unwrap QualifiedValue → raw Python value
    system.tag.writeAsync([tagPath], [value])

Key corrections versus the broken version:

1. Remove quotes from the value assignment. value = "self.custom.Value" is a string literal. value = currentValue.value unpacks the QualifiedValue object that Ignition passes into every valueChanged handler.
2. Each statement on its own line. Python is whitespace-sensitive. If tagPath = "..." and value = ... appear on the same physical line without a semicolon separator, the interpreter raises a SyntaxError or silently misparses the block. Use four separate lines as shown above.
3. Consistent 4-space indentation. Mixing tabs and spaces inside the function body causes an IndentationError at runtime in Jython (Ignition's scripting engine).

Diagnosing Whether the Script Fires at All

Before fixing the write logic, confirm the event handler is being invoked:

1. Add a system.perspective.print() (Perspective) or print (Vision) as the first line of the handler.
2. Open Tools → Script Console or check the Designer console output tab. If nothing prints when the custom property changes, the binding that drives the property is not firing — debug the binding first.
3. Wrap the write in a try/except to surface silent failures:

   def valueChanged(self, previousValue, currentValue, origin, missedEvents):
       tagPath = "[default]Path/To/Your/MemoryTag"
       value = currentValue.value
       try:
           system.tag.writeAsync([tagPath], [value])
       except Exception as e:
           system.perspective.print("Tag write failed: " + str(e))
   

4. In the Gateway, navigate to Status → Diagnostics → Logs and filter on Tags to catch write-rejected messages.

Fixing Read-Only Tag Permissions

If the script fires and no exception is thrown but the tag value never changes, check the tag's security settings:

1. In Designer's Tag Browser, right-click the target Memory tag → Edit Tag.
2. On the Security tab, verify Read Only is not checked and that the active security zone grants Write access to the gateway user running scripts.
3. If the tag lives inside a folder, right-click the folder → Edit Folder — folder-level read-only overrides individual tag settings and is easy to miss.
4. After clearing the restriction, test with system.tag.write() in the Script Console first to confirm write access before re-testing the event handler:

   system.tag.write("[default]Path/To/Your/MemoryTag", True)
   

Parameter Reference: system.tag.writeAsync

For boolean Memory tags, pass Python True/False directly. Ignition's type coercion maps them to the tag's Boolean data type without explicit casting.