There's nothing today, and it isn't easy if it isn't sent through something
like *VMEVENT or STARMSG.One idea we kicked around would be to have an option to the PIPE command to arm/disarm a generic event handler for external interrupts and then a stage that was called when the event handler fired that would produce a string like EXTnnn on its output, which could be handled as needed. As you say, not straightforward, but it might be an idea worth looking at. Something to put on the wish list.
