What includes MachPro and m4 reverse spindle?  I am unable to use m4 to reverse spindle.  mc command will work. RCATC works in reverse but m4 does not?

What do you have set to Spindle Forward and Spindle Reverse?
https://prnt.sc/vZgG7ANe2w00

Are you using any custom logic that overwrites the original M3 or M4?

No I am not doing anything specific for M3/M4. Today, it's working again. installed 19.2, and tried a M4S700 and spindle started. Yesterday I rewrote some code that autoload's and unloads a probe which is has a dock and pogo pins for electrical.

Sometimes, after an error with the MDI, you need to restart machpro. It will do nothing and you can't seem to clear the error until a restart.

In short, unmodified - the M4 command runs mc.MC_SPINDLE_REV in the core which fires the OSIGs Spindle On and Spindle Reverse.

"Sometimes, after an error with the MDI, you need to restart machpro. It will do nothing and you can't seem to clear the error until a restart."
Could you provide the Error code you get and what the MDI you're trying to run?