I have a light tied to the ALARM signal, but nowhere in the docs can I find what conditions set that signal.

So - what sounds the ALARM?

Using mc.mcCntlMacroAlarm activates the Alarm output. I believe writing to #3000 should also produce an alarm but I can't get it to work

So nothing native to Mach sets that signal? Limit switches? Loss of position info? EStop?

Shouldn't that signal be set by anything that takes Mach out of the ENABLED state?

Not that I am aware of. If you want it to function like that just set the alarm signal in the signal library when the output for machine enable signal turns off. That’s the great thing about Mach, you can pretty much make it do what you want

i add register for such
and some event that raise up the register
then in any M function before any action( special force output)i Check this registry value
and if...
then return