Reactivating Old Z80 Computer

I have found an old Z80 computer which was used in production in the 80’s. Specs seems to be a video card (or let’s say, a text card with video output), a Z80 MCU, 5,25″ floppy disk drive and a keyboard interface.

Front of the Z80 microcomputer with floppy disk, keyboard connector (lower right), reset button (right of keyboard connector) and the power button.
Front of the Z80 microcomputer with floppy disk, keyboard connector (lower left), reset button (right of keyboard connector) and the power button.

Connected to power, keyboard and TV the machine shows the usual monitor program. I used a good old PC AT keyboard, which seems to get power from the PC as I can see the power on self test of the keyboard (blinking keyboard LEDs). However, keyboard does not seem to work, no keystroke is displayed.

Inside of the Z80 computer. On the right the PSU and video card. On the left the 5,25" floppy and in the front the Z80 board.
Inside of the Z80 computer. On the right the PSU and video card. On the left the 5,25″ floppy and in the front the Z80 board.
Mainboard with Z80 MCU, floppy disk controller 765, DMA controller, RAM, ROM, logic chips and cables for keyboard (grey).
Mainboard with Z80 MCU, floppy disk controller 765, DMA controller, RAM, ROM, logic chips and ribbon cables for keyboard and floppy.

As you can see in the picture, the mainboard contains the Z80 MCU, the floppy disk controller chip D765AC, Mostek 8410 DMA controller, ROM (red label) and RAM chips. Keyboard is connected to the front panel using the small grey ribbon cable. The wide ribbon cable on the top is connected to the floppy disk.

Back of the Z80 mainboard.
Back of the Z80 mainboard.

Now the big question, why does the keyboard not work? Since it gets power from the connector I assume that it is the right keyboard type (PC AT). The keyboard works on other computers, so maybe the Z80 board is broken? Or are there older protocol variants?

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