Hi!
I have had this old LINK TECHOLOGIES MC420 A/S dumb terminal for many years… but without a keyboard.
Recently, I acquired a keyboard for it. 
Looks like it is a WYSE OEM, since all internal parts are branded as “WYSE”.
Its previous owner was using it through a TEENSY board with Soarer’s WYSE converter firmware.
I had to redo the RJ10 connector at the end of the cable, as he had been replaced with a DIN connector by the preview owner.
Found on a website this terminal pinout, and checked it through this schematics.
https://archive.org/details/bitsavers_wyseWY60Ma_4722450/page/n29/mode/2up
Connected the RJ10 side to the terminal (keyboard-side connector disconnected) and found (on the keyboard-side connector) those DC levels: 5V on pin2, ~2,2V on pin3 (clock), ~0,5V on pin1 (data). Looks like the “expected” values.
I then plugged in the keyboard and turned on the terminal with it connected.
Terminal booted and displayed MC420 OK message.
It waits for a “key” (any key)
Pressed a key and main screen appeared.
But now, nothing more happens.
Tried SETUP, control-SETUP, shift-SETUP, shift-control-SETUP, same combinations with MENU key, but “almost” no answers from terminal.
The “K” key lighs the CAPS message on screen. “D” key turns “COMP” message on. “ ‘ “ key turns HOLD message on. But nothing else works. Tried all keys.
What am I doing wrong?