I now realize that I should have been more specific in describing my setup and what has been tried.
I have two Emco machines, one VMC-100 mill with TM02 controller (this is the one I will try to retrofit linuxcnc on first) and one Emco Compact 6p lathe (very similar to 120p) with a TNM01 controller. So none of these controllers will work with the external keyboard.
What has been tried:
I power the keyboard with a 12DC power supply and the light comes on when the switch is thrown to the on position.
I made up a cable (as described in the pictures) to go between a prolific USB to serial adapter and the keyboard and set it up for 9600 7 databits, even parity and one stop bit.
I used Teraterm (same program I use to transfer files to and from the mill and lathe)
I pressed the key combination described in the manual to activate RS-232.
I couldn't see any output from the keyboard, no matter what knobs or keys I pressed.
I tried checking the signal levels on the keyboard port.
I get -8 V on pin 3
I get +8 V on pin 7
So I believe the keyboard is healthy.
What I have not tried yet:
I have not tried RS-485 because I have no experience or any idea of what the protocol might look like. I do have a RS-232 to RS-485 adapter that can be configured for two-wire protocol, but I could use some hints of what to do next if I made a cable up.
From a Swedish forum I could see that 4 years ago a guy had similar issues but he could connect the keyboard to his lathe and collect some data which lead him to believe that there is a initialization sequence after the control is powered up to activate the keyboard.
He did not investigate further and do not have any data left since the project. So this is where I hope someone could collect this communication and maybe I can decipher it.