is my 486's DMA controller shot?

[Mike Chambers]

i've been having this problem with my 486 DX2/66 while quite some time now. i can't seem to communicate with any expansion cards that require shared memory to operate. for example, i can only use my 3C503 in it if i turn off mapped memory and just it via PIO.

same situation with my SB16 in there. it can do stuff like reset and it will play FM synthesized stuff just fine, but it won't do any DMA PCM playback or recording. i've tried 4 different sound cards that all work in other machines, they all do this. i've used several network cards as well, same deal.

it's getting pretty annoying. games are no fun without PCM sound, and PIO networking is like brutal-slow.

 

[Chuck(G)]

Its possible that there's DRQ/DACK problem. I'd think that if the DMA controller were toast, memory would go bye-bye pretty fast (Ch0 of the DMAC is used to refresh DRAM).

A bit of probing with a 'scope or logic analyzer might show something funny. Are you overclocking the bus?

 

[Mike Chambers]

nope, no overclocking on the bus. something i just found out actually, i was just able to get PCM sound playback in Mortal Kombat 3, but i had to tell i have an SB Pro - not an SB16. nothing can find a high DMA channel, only low 8-bit. i had to set the options manually too. everything i try to use 16-bit sound on errors. (including windows) it can't seem to get detected. and then there's still the NIC issue. this is really strange. it's been going on for the last 4 or 5 months.

 

[Chuck(G)]

Can you at least find the 16-bit (high) DMA registers and read and write them? If not, there's a problem in either the selection/address decode circuitry or the DMAC itself.