Little Casino - Keyboard Interface

[MikeDB]

I've resurrected a Little Casino countertop video game which cleaned up nicely and replaced front panel incandescent lamps with red LED substitutions. (One of the lamp receptacles required a bit of rewiring for proper polarity with the LED substitute.)

I would like to do some experimenting with a keyboard connected to the back panel female DE9 connector.

I would be thankful if someone here can provide technical information on that interface.

By the way, for anyone interested I can share binary images of the 7 EPROMS from the game:
UA18.1, UB18.1, UC18.1, UD18.1, UE18.1, and two others with labels missing or partially obscured.

 

[MikeDB]

Back to the Keybd question I've done some digging and come up with the following for the DE9 (db9) connector.
There are only 4 leads wired to the DE9:
A. Pin 6 = Ground (supplied by Little Casino)
B. Pin 8 = +5V (supplied by Little Casino)
C. Pin 3 = Either Kybd Clk or KyBd Data (supplied by keybd)
D. Pin 5 - This is the one that has me baffled. I expected it to be the complement to Pin 3 (Clk or Data) but its lead is grounded within Little Casino via a ring lug which is secured to the chassis near the logic PCB mounting connector.

With that arrangement, the typical PC keyboard could never operate.

Interestingly, if I connect the KyBd Clock to Pin 3 and KyBd Data to Pin 5 (which is grounded), Little Casino boots up with a screen instructing the use of the keybd and awaiting user entry. But since the keybd data lead is now grounded the keybd can't send data to Little Casino. If I reverse the two connections Little Casino doesn't boot up to any screen.

Another mystery is that the first three leads listed above (Pins 6/8/3) are wired to a 6-pin molex type connector that connects to an interconnect bd to the logic bd (the other 3 leads of the molex are used for the volume control). So why was the lead for Pin 5 routed separately and grounded to a stud. Why not just common it with the lead from Pin 6?

Still digging, but so far 2 possibilities come to mind:
1. some other form of keybd is required; or
2. some mod was performed that caused the keybaord to be inhibited.