Hi Everyone,
I have recently uncovered a minor issue with the MSDOS board V2 where the PC speaker function doesn’t work.
After a little digging around I discovered that the PC speaker control port should be I/O port 61H (bits 0 & 1) whereas on both V1 & V2 of the MSDOS board
It is incorrectly mapped to ports 44H-47H. (See link below)
There are two other small issues, Pin 4 of the speaker connector P51 should be attached to the +5v rail as the pc speaker connects to pins 1 and 4,
the speaker driver IC U47 will only sink current and not source it so the +5v on pin 4 is necessary to get sound.
The other issue is that I might have expected gate U60D to be a NOR gate so that its output goes positive when all I/O conditions are met by active lows (the 74LS74 is positive edge triggered), but if memory serves me correctly S100 bus data should also be valid on the end of the I/O write cycle so therefore it’s not a real issue.
I have made up a little work around which kind of works for me (Mario sound working through PC speaker) using a GAL 16V8 in place of U33.
The documentation and relevant PDS & JED file is all zipped up in the archive linked below.
If anyone implements this patch I would be interested to know how you get on,
Regards
David Fry