STEbus 68B09E (GMT 1021)

2024-05-18

Had a first attempt at disassembling the ROM. 

I assumed the 6522 was being used in a clever way to provide a serial interface. Which would probably be a headache to reverse engineer.

The ROM is marked "Autobaud" so there should be some serial I/O going on. But looking at the board, there are no RS232 buffers. 

Looking at the code, some of it tests bits 1 and 0, which are status bits in the 6850, so maybe it is using 6850 chips on an separate board? 

That's all the work I fancy doing today. I think buzzing-out the signals going to the PAL and the PROM chips may give further clues to the memory mapping.

Read the installed ROM today, had a 24 pin NMC27C16Q-45 CMOS PROM installed, need to have a look at the binary to see the address range and if there are any clues on the installed application. Chip label reads S1000 Autobaud checksum F935