This keyboard came in as part of a Digital DEV VT402 terminal. I have some information on that terminal here . After some time, I found that the keyboard of that terminal is also an interesting kind of thing.
DEC LK401 Hardware
DEC Keyboard has a serial interface. The connector to plug into a DEC VT420 terminal looks like an old telephone connector, with 4 pins, called RJ10.
An Intel 80C51 microcontroller handles most of the keyboard functions We can see also the Beeper and a 7.3728 MHz Quartz
Beeper, a 7475 and two LEDs. Datecode of 7475 is 29th week 1992
A SC23181, looks like a driver chip
Internals of the keyboard
DEC LK401 Interface
The keyboard uses +12V power and communicates via RS-423 serial communication at 4800 baud, 8N1. This is compatible with standard RS-232 serial ports in terms of both protocol and voltage level. All that needs to be done is to hook it up to a serial port and provide +12V power.
No, it’s not a “LK410” as I wrote in my drawing, it is a LK401…
Signal looks like the image below, which shows a single character, so we have values of about -6 volts for a ‘1’ and +6 volts for a ‘0’.
I tried to convert this keyboard into a USB keyboard, see here.