Reply 3020 of 3122, by kdehl
I haven't followed this thread for a while; I'm glad to see there has been some progress by others on this stuff!
I have just recently finished reverse-engineering the somewhat simplified GuSar board, without the DRAM chips and I have some questions.
Are there any other differences from the original ArGUS board but the lack of DRAM chips?
I did notice, as someone pointed out here earlier, that some of the address pins of the SIMM were reversed and I fixed that.
I understand that the EPROM is optional, so I might just skip it for the first revision. But for future reference, if I decide to just use a regular 29F800 instead of that now-unobtainable IW78C21M1, there must be some modification of the board if I understand it correctly. RA20 and RA21 are weakly pulled low, which makes no sense for the ROM chip. According to the datasheet for the AM78 they are used as inputs during reset and I assume this is why they are pulled low by default.
They are connected to ~RESET and ~WE on the ROM, which don't make muse sense either.
I have a vague feeling they are not used at all, but put there for future revisions of the AM78... that could use larger ROMs than the 1 MBytes that are addressable by a 20-bit address bus. But that is just my guess.
So my question is: if I just pull ~RESET and ~WE high at all times, a regular 29F800 ROM should work, shouldn't it?
Thanks for all the help!
Once I figure this out, I will order a test batch of five boards and keep maybe one or two for myself. Then I will sell the rest, as finished boards. If there is still demand for them, I plan on making more of these and sell to anyone who wants one.
As a side note, I should point out that I have made other sound cards before; I made my own Sound Blaster clones, heavily inspired by Snark Barker. I have a few ISA boards for sale on eBay and all the MCA boards have sold out before I manage to put them on eBay. 😀 Nowadays the CPLD I use for those (XC9572XL; same as Snark Barker) are nowhere to be found, so I can't really make any more for the next year or so.