exxos's Atari, Amiga & retro forum

So pulled the mega out to test a few things with the TF536 only to find that the floppy had died.. great :roll:

So ive tried quite a few things and its very nearly working now.

Firstly something killed the DMA chip, so it now has a nice new IMP DMA fitted instead.
Next up fitted the pullups on the WD1772 and found the 74LS06 also faulty, so that was changed and pullups fitted to that (using good non atari drive)

Removed and fitted floppy header to rule out bad floppy cable.

BTW, the Mega has had the Mandatory fixes done.

This is what happens:

Boot with Tos1.04 and try and format a floppy. It gets all the way to track 80, goes back to 0, then 80, 0 80 and then comes up with an error disk write protect etc. Does this every time.

Boot up with Diag Cart and go to floppy test. Using disk exerciser it formats fine BUT if I use the quick test which formats track 0 and 79 it comes up with a read error and error code F6 IIRC.

Things I've checked:

Metered all tracks from WD1772 to floppy, ls06 and DMA, fixed one but rest are fine.
Scoped out the WD1772 and it behaves as expected as far as I understand at least.
Checked floppy and ls06 waveforms and they are all in sync for WD and RD.

The bit that I don't know is what I should expect the WD1772 , DMA etc to need during the finishing of formatting. I suspect its a problem reading or writing data but I can't work out where at the mo.

Any tips guys?

Doesn't the IMP DMA need a diode in the 5V rail ?

exxos wrote: 04 Sep 2020 22:52 Doesn't the IMP DMA need a diode in the 5V rail ?

Don't know but will check. It seems to be working otherwise?

PhilC wrote: 04 Sep 2020 22:57 Don't know but will check. It seems to be working otherwise?

Well you said the floppy wasn't working and that goes via the DMA.. :lol:

@exxos I thought there was something I needed to do but couldn't remember what

Does it need to be a specific type of diode?

A 1n4001 will probably do.

I've fitted the suggested diode to pin 40 of the IMP DMA but still have the same problem.

So I can't find a lot of info about the DMA, but I'm presuming that the following happens, WD1772 converts serial data into parallel data on the DAL lines to the DMA and then this in turn is sent out to the CPU using the Data lines. So I'll give those a check next as the disk check on Diag Cart still shows F6 and I still get Drive A is not responding in TOS.

So it seems you don't need D5 connected between the DMA and the CPU for the Mega to boot then :lol:

Annoying bit is that's still not the issue.

Do you have a stock atari drive to test on it

Tom

Tomswork wrote: 05 Sep 2020 20:58 Do you have a stock atari drive to test on it

Tom

Not to hand, ill have to Rob one out of another ST