exxos's Atari, Amiga & retro forum

@rubber_jonnie If all else fails, I know someone who has been advertising IMP DMA chips for a reasonable price.
The NOS ones might have sold out now but there may be second hand ones still.

Hope it's not as bad to fix as that Mega of yours!

JezC wrote: 18 Sep 2021 20:57 @rubber_jonnie If all else fails, I know someone who has been advertising IMP DMA chips for a reasonable price.
The NOS ones might have sold out now but there may be second hand ones still.

Hope it's not as bad to fix as that Mega of yours!

Cool, thanks. I'll have to go through my stock, see what I have.

If I come up dry I'll get back to you 👍

Thought this machine would be a good candidate for testing my full test harness:


This allows a full test of the hardware using the diagnostic cartridge.

As expected, DMA is a fail.....

Interesting, looking at the underside of the mainboard, the DMA socket appears to be factory fit, certainly the only time I've ever seen one like that in any of my machines.

Also interesting is that the FDD cable does have signs of rework, so I need to buzz that out and see what's what.

Having tried two alternative DMA chips now, it seems that there may be more than meets the eye here, as it is still not working and reporting a DMA issue.

Time for schematics and tracing! Maybe even a WD1772 swapout. Let the games commence!!

Great, I seem to have 3 dead DMAs to my name ATM :(

In a couple of my stock STF/M models, I've also got the DMA in a factory fitted socket, so quite rare but not as rare as hens teeth...

Can't remember if you already checked the ACSI port functionality on the new addition...might help limit any troubleshooting to the Floppy if the hard drive works?

Other potential vulnerable part might be the sound chip maybe?

Edit: oh dear, just seen the latest update...bad news... hope that is the extent of the problem.

JezC wrote: 20 Sep 2021 13:36 In a couple of my stock STF/M models, I've also got the DMA in a factory fitted socket, so quite rare but not as rare as hens teeth...

Can't remember if you already checked the ACSI port functionality on the new addition...might help limit any troubleshooting to the Floppy if the hard drive works?

Other potential vulnerable part might be the sound chip maybe?

Edit: oh dear, just seen the latest update...bad news... hope that is the extent of the problem.

I just did a bit more testing, and actually all of the DMAs work just fine, but the results are weird. On my H4 and my new machine, when I run the DMA port test, it fails. I have another machine on which the DMA port test works just fine, so tested all the 'faulty' DMAs there and they all pass, which suggests maybe a problem with the connectors, so I will have a closer look at that.

Also, all my machines fail with my Ultrasatan connected, in that they boot and then fail with 2 bombs before getting to the desktop. I think I need to whip out the bus resistors and replace, see what that changes. H4 shouldn't have that problem though, so that's a bit weird. Might just try booting from FDD with a blank SD card, see if that changes anything as the US works fine on my MSTE.

@JezC Quick update, using a blank SD card and booting from floppy seems to be OK. Bus resistors still need doing though, but will partition the SD card and see what's what.

On the STFM I put the TF536 into changing the main bus resistors actually made things worse rather than better. I think it depends upon the chip. I know that the original chip got worse and worse with the US connected until it wouldn't read anything with the US fitted (but worked fine without it). I can't remember if I tried with a real ICD host adapter or not.

stephen_usher wrote: 20 Sep 2021 14:28 On the STFM I put the TF536 into changing the main bus resistors actually made things worse rather than better. I think it depends upon the chip. I know that the original chip got worse and worse with the US connected until it wouldn't read anything with the US fitted (but worked fine without it). I can't remember if I tried with a real ICD host adapter or not.

As it goes, the blank SD card works fine, but when you try and boot using HDDRIVER is when the double bomb appears, so I tried with PPeras driver and can now boot from the US.