DMA port "update" ideas..

[exxos]

I started a seperate thread for this...

For those not aware... https://www.exxosforum.co.uk/forum/viewt ... 500#p11552

We finally solved the "idea" behind the STE DMA buffers. Basically the 1772 does not "let go" of the bus until , up to 150ns later...

1772.jpg

The Odd thing is the STFM really should have never worked with a hard drive attached. It is possible as the 1772 delay is from 20-150ns, that maybe some batches of 1772 worked, when others did not due to a higher delay...

If I remember rightly, Atari hard drives did not suffer from issues, but 3rd party drives sometimes did.. What we do know is the Atari hard drives had buffers on the inputs... and a fair amount of logic on the buffer enable line... of course on the STFM, if the buffers did not isolate the DMA, the bus would conflict.. So this could explain those 3rd party drives not working if they didn't add the "delay buffers".. All food for thought I guess....


Looking at the STE circuit...

ste.jpg

The DMA port gets "delayed" by one clock cycle after FDCS goes high. This makes sense as the 1772 can hold the bus for that time. So if the DMA port was active, it could actually short out the 1772 bus to the DMA bus (same bus different DMA devices).

What I don't like is the "delay" on the DMA port which could attribute to DMA issues on the STE..

So my thought here is a patch board on the 1772 to add in a 74LS245 bus transceiver to isolate the 1772 once the DMA de-selects the 1772 via FDCS. This way, there is no bus conflict and no delay on the DMA port. It would also not matter the "delay" (20-150ns) of the 1772 as it would be isolated automatically.

As a side thought, it may still be a idea to add buffers on the DMA port itself (like the STE does but without delay).. Though I am still in 2 minds if its really needed or not yet... Really they would be there to protect the DMA chip more than anything.. cheaper to replace LS chips than the DMA... So I am leaning towards adding the buffers in later revisions...

[Icky]

As a side thought, it may still be a idea to add buffers on the DMA port itself (like the STE does but without delay).. Though I am still in 2 minds if its really needed or not yet..

So the reason I like the idea of having buffers on the DMA port too is based on the discussion you mentioned that some devices had them built in and others didn’t.

Yeah I saw Atari drives have the buffers, again odd that ste have same buffers again... I guess it could explain a little why Atari drives seemed to work better than 3rd party ones back in the day. I guess most issues were with pre-ste machines as I assume 3rd party drives may not have the buffers and could fail to function correctly.

Makes sense now that some drives don’t work well.

[exxos]

So the reason I like the idea of having buffers on the DMA port too is based on the discussion you mentioned that some devices had them built in and others didn’t.

Yeah, I guess in the spirit of retro and fixing things, buffers should really be added..

[exxos]

Looks right on paper :)

1772.png

transceiver-octal-bidireccional-tri-estado-74ls245.jpg

[rpineau]

looks right to me too :)

[exxos]

Here is the PCB file..

f3119fb02d8314a8bf43854323a01737.png

1772_BUFFER.zip

OSH is $17.25 for 3... Little expensive I think particularly people only want one PCB... I could get a small batch of them done for my store, I could list them without signed for postage keep costs down.. This will of course be little cheaper for people I think.. But of course people overseas this could take 2-3 months to get the stuff..

What do people want to do ?

[PhilC]

Happy to order one from you with the other bits I need.

[Icky]

Happy to order one from you with the other bits I need.

+1

At this rate the STF Remake will be littered with piggy back boards. At least this is going to give you a good solid foundation for the BETA board.

[exxos]

Happy to order one from you with the other bits I need.

+1

At this rate the STF Remake will be littered with piggy back boards. At least this is going to give you a good solid foundation for the BETA board.

OK, will order 10 of the things....

Yeah Going to have stuff bodged all over the place on the thing ultimately... Just wait until the FPGA stuff kicks into gear...

Of course this 1772 board was one of those unforeseen mods.. didn't think we would be having to debug the original circuit design as well..

[troed]

I thought Icky got floppy and DMA working without a bodge?