I checked around the HD6303RP and found that it was also using the SCI of the 6303. The following is the wiring diagram that I found out, although CTS and RTS are unknown, it is probably as shown in the diagram. Correction: Mr. Asakawa, a former SORD employee, gave me valuable information that it is controlled by DSR and DTR. The diagram has been corrected to match the information.
We also got a rough idea of the communication speed.
With 2.4576MHz input to the HD6303RP, we have four choices for the SCI speed.
1. 38400 Baud
2. 4800 Baud
3. 600 Baud
4. 150 Baud
Considering the human->keyboard->mainframe path, I'd say around 600 Baud.
I've been told by another former SORD employee in the comments section of the Japanese version that it's 38400 Baud. It's faster than I thought.
