Instrument Interface, onBeforeParse not firing

We have dropped a serial controller on a window and added code similar to below to watch it go through the lifecycle. We can see it call the onReceive, and the onAfterParse callbacks, but it never calls onBeforeParse. Is there some setting that must be set so that onBeforeParse is fired?

Here’s the code:
value = u’I received data’
system.tag.writeToTag(‘MemTag2’, value)

The tag is bound to a field’s text.