In the end, I decided to remove the ROM inside the keyboard and read out the binary file.
The ROM used in this keyboard, TMM2464AP, is compatible with 27C64, so I was able to read it out with an inexpensive ROM writer TL866CS.
I was able to get the assembler code from the ROM binary file using the disassembler DASMx.
The first thing to note is what is set in the RMCR (the register that determines the communication speed) of the SCI of the HD6303. DASMx is very useful for attaching symbol names to addresses when disassembling with Sym files.
Thanks to this feature, I immediately found the address where the RMCR was set, and the value set was 0x05.
According to the handbook of the HD6303, 0x05 indicates that the communication speed is 4800 Baud when the clock is 2.4576 MHz. Also, by carefully reading the pages around this, I found out that the SCI of the HD6303 is stop-bit 1, no-parity.
However, it is hard to read the assembly code with almost no symbols, but I will read it little by little.
