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STEbus 68B09E (GMT 1021)

STEbus 68B09E by GMT Electronic Systems

keithKeith
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Richard Marsh Bagged this for just £10.
No manual.

The board looked a little sparse. Then I noticed it doesn't have the usual 6850 UART or the 6840 counter timer. I guess those functions are done by the 6522 chip, though perhaps not as well.

The glue logic is one DMPAL16R4 chip and three N82S153 PROM chips. The latter should be trivial to reverse engineer. Apart from the fact they are soldered to the board

I look forward to disassembling the ROM to see which addresses it is accessing, and the way it is using the 6522 as a UART.

GMT_6809_CSF935-src.txt

2024-05-18 First inspection. Found a hex word/byte/nibble output routine but have not figured out where the 6522 VIA is in memory.

plain - 54.29 kB - 05/18/2024 at 15:36

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GMT_6809_CSF935.txt

Source file of .DW and ASCII strings

text/plain - 10.41 kB - 05/17/2024 at 01:23

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F935.BIN

Binary file from the EPROM Checksum F935

octet-stream - 2.00 kB - 05/16/2024 at 19:38

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  • Disassembly

    Keith • 05/18/2024 at 17:13 • 0 comments

    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.

  • ROM Read

    Richard Marsh • 05/14/2024 at 18:57 • 0 comments

    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

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