Retrofitting a 1986 Maho MH400E

How can I drive an individual physical output pin from hal to check the voltage?

Easiest way is open a terminal and use halcmd:

halcmd setp big.long.pin_name true
halcmd setp big.long.pin_name false

you can get a list of the pin names with:

halcmd show pin *7i84*

Do I measure it with ground on TB1 (like I would power on TB1)? If so, an enabled empty output pin reads 0V.

I was planning to separate the vfield (200 pin with 24 volt) and vlogic (204 pin with 24 volt) power on 7i77, so the 7i77 cannot in any way output 24 volt in any output until get out of estop and the machine is on.

But this is not good thing for the health of 7i77, as PCW said.
John

You can do this but it would need a separate 24V supply (and switch its primary power)

Glemigobles wrote: Do I measure it with ground on TB1 (like I would power on TB1)? If so, an enabled empty output pin reads 0V.

Yes measure from TB1.8 ( note you should remove the I/O connector to measure 7I84
outputs as the screw does not reliably contact the output pin unless tightened )