![Image](https://i.ibb.co/PYS2sDP/59910750-925553564450805-4941789319237992448-n.jpg)
There is very little information available on the TRS-80 4P keyboard. Fortunately, the technical reference manual is available at http://www.classiccmp.org/dunfield/trs80/m4tech.pdf. It specifies an 8x8 key matrix and describes how the original controller functioned. The keyboard itself has a 16-pin connector and no ICs, so I believe the keyboard cable carries a parallel key matrix signal and the controller resides on the motherboard.
![Image](https://i.ibb.co/rZzy5ZZ/60468061-299196994344333-2078102664696037376-n.jpg)
Additionally, the case has analog brightness and contrast controls, so whatever I use as a keyboard controller has to be able to handle analog inputs.
If I'm right, I should be able to just connect the pins on the keyboard cable/case controls to some kind of microcontroller what can send the proper USB commands to the system. Suppose I'll have to trial and error the pinout, though.