It is almost certain that the communication between the keyboard and the main unit of the SORD M68MX is serial communication via RS232C, as expected.
The pinout of the CHARACTER CRT port is as follows.
CHARACTER CRT port DB25pin
1 --- GND
2 --- Red
3 --- Green
4 --- Blue
5 --- BW
6 --- V-sync(negative logic)
7 --- H-sync(negative logic)
8 --- Unknown
9 --- SN75188N:3pin --- HD6850:TxD(TxD_CLK=76.9kHz)
10 --- SN75189AN:1pin --- HD6850:RxD(RxD_CLK=76.9kHz)
11 --- SN75188N:Vcc+ (+9V～+12V(Voltage unstable))
12 --- -12V～-9V(Voltage Unstable)
13 --- Unknown
14 --- GND
15 --- GND
16 --- GND
17 --- GND
18 --- GND
19 --- GND
20 --- GND
21 --- Unknown
22 --- SN75188N:6pin --- HD6850:RTS(active low)
23 --- SN75189AN:4pin --- HD6850:CTS(active low)
24 --- GND
25 --- Unknown
The communication speed may be 9600 bps or 4800 bps, guessing from the clock frequency. Stop bits, etc. cannot be determined by wiring information alone, so I plan to investigate by brute force.
