I built a manual control panel and hooked it up like this:

https://www.machsupport.com/forum/index.php?topic=44875.new#new

Now, any attempt to run the simplest of MDI programs (G0 X0.01) results in an error message "Move off limit switches before executing MDI"

Oddly, my limit switches are not connected (I'm debugging) so no switches are tripped.

What's going on here?

 

check on yours plug in if its use limit

So it turned out that I had done the smart thing and set up limit switches NC, so when I had the switches unplugged they registered as tripped.

PEBCAK.

Resolved.