6541 Mystery board. By Lee Davison.
Not such a mystery

The only identification on this board is the board logo and the labels on the EPROM and the PAL.

I still don't know exactly what this is but I can now make a few guesses. I believe the board was made by Sigma Designs who have made a few disk controllers with the SDC prefix. I can identify all the chips except the missing 40 pin DIL and that I suspect is a Western Digital WDxxxx floppy disk controller.

The 34 way header on the right hand edge of the board would seem to be the port for the disk drives though it's not the standard PC type drive as there is a power connection to some of the pins that would usually be grounded. Going by the code two, double sided, drives are supported.

Thanks to Roberto for reverse engineering the equations to the PAL, this has let me work out how the devices are addressed. The EPROM goes from $F800 to $FFFF even though it's a 4k device, the RAM appears from $F200 to $F7FF which is only three quaters of it's 2k size. Some of the unused area, $F080 to $F0FF is taken by the WDxxxx chip.

The smaller 26 way header seen on the bottom right of the board is the host interface, this is a byte wide port and looks like a 65xx peripheral to the host device.

Larger pictures of the board are here..

  • A 640 x 480 jpeg image.
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  • A 1024 x 768 jpeg image.
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  • A 1200 x 900 jpeg image.

    • Firmware

    I've done a dump of the 2732A 4k EPROM and it appears to contain 6501 code. Though it's a 4k device it's only addressed from $F800 to $FFFF and it seems to have two identical 2k copies of the code. You can download it as either a binary dump or Motorola S record.

  • Download the EPROM as a Motorola S record file.
  • Download the EPROM as a binary dump.

    • With the guess at a WDxxxx disk controller and Roberto's PAL equations it only took a few hours to disassemble and comment the code in the EPROM. As I don't have an R6541AQ assembler to hand I can't check that this is bit correct, but it does make some sort of sense. It can be downloaded here .

    I've also read the AMPAL16L8APC that does the address decoding. It's here as a jdec file.

  • AMPAL16L8APC as a jdec file.

    • If you recognise this board or what it is from please e-mail me . Thank you to everyone who has sent suggestions and data so far.

    Last page update: 21st February, 2005. e-mail me