exxos's Atari, Amiga & retro forum

Steve wrote: 16 Apr 2022 20:03 There is also one more, the 1172 pullups: viewtopic.php?f=17&t=1395
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Well, it helped Exxos with a 'known working' drive which was having write protect issues. Sometimes these things can be related.

Where Exxos said that this specific fix (and not the pullups at the DMA bus) helped in such situation? But anyway, a drive having write protect issues it not really a good working drive.

@ijor

This issue can cause the drive not to be found, intermittent read/write issues etc This issue is likely present on any machine with a 1772/Ajax floppy controller using a LS06 type buffer.

I'd say corrupted text would fall into the category of a read issue.
Hey I am just trying to help the guy, can you leave me alone now?

Steve wrote: 17 Apr 2022 09:55 Hey I am just trying to help the guy, can you leave me alone now?

Sorry Steve. I apologize if I sounded too harsh. I realize you are just trying to help. I didn't mean to be offensive in any way.

I'm also trying to help. And for this purpose I am trying to guide the OP into possible solutions. IMHO, adding pullups on the floppy signals is not very helpful in cases like this where the problem seems to be the computer, and not the drive. I am sorry again that I probably didn't express myself friendly enough.

@ijor no worries :)

Okay, this morning's investigation:

Continuity along paths to the floppy look good. I've tested paths from all pins on the 1772, be it the 1772 to a wire stuck in the other end of the ribbon cable, the 1772 to the 7406, the 7406 to the other end of a wire stuck into the other end of the ribbon cable, the 1772 to the DMA, or 1772 to 5v/ground/reset. All pins in and out of the 1772 look like they have continuity to where they should. All lines on the ribbon cable that should be connected to something appear to be connected to that thing. (I haven't exhaustively checked lines to/from the DMA yet. The main data lines appear good.)

Also I opted to try the DMA pull-up fix mentioned up-thread. On the bottom of the board, I've thrown in a 1k resistor network. I've done the same as far as I can tell from what I see in other photos.


(It's not clear in the photo but the top pin broken out is indeed the one marked as pin 1, and the resistances look good so I know I've got contact on the 8 other pins.)

And the result is: no change!

I've never had an "untested" purchase so stubborn as this one. :D

So far I've neglected the reset fix, and at some point I'll have to try it. (So far I've declared it unrelated, but, fewer other options remain.) I haven't tried the 1772 pull-up fix yet either.

Editing to add: this purchase was deliberately bought untested, so if it never works, it's not a loss. I bought it figuring I'd use it for parts. Floppy aside, it was clean enough and booted so easily that I've been compelled to see if I can make it fly again!

Steve wrote: 16 Apr 2022 15:58There is also one more, the 1172 pullups: https://www.exxosforum.co.uk/forum/viewt ... =17&t=1395

Going back to this fix, quoting from the post itself:

The short version of this fix , is that the original Atari floppy drives had internal pull ups on some of the signals

I'm only trying to use original ST drives here, so I guess this is not necessary?

@stween
Yes but further on in his comment Exxos says:

It is possible original drives could suffer as well, but this is a very new fix and the "bug" has only just been found.

Hey I'm the type of guy that tries everything until there is nothing left to try. Don't tell ijor ;D

stween wrote: 17 Apr 2022 16:56 ...Also I opted to try the DMA pull-up fix mentioned up-thread. On the bottom of the board, I've thrown in a 1k resistor network. I've done the same as far as I can tell from what I see in other photos...

Is this a typo? 1K is way to strong. Should be 10K.

Just to recap, have you tried another DMA yet, i've lost track a bit!! If not, then you should at least try it. I spent an age on one of mine and ultimately a DMA swap would have fixed it with the exception of the bodge wires I had to add where the WD1772 had been badly reworked..

Also, have you got an oscilloscope, and if so have you scoped the signals at the WD1772 during diag cart tests and then at the FDD to see if they match?

IIRC there are some logic gates along the way, possibly inverters (You'd have to check the schematics to be 100% sure) so be aware signals may flip/change along the way to the FDD.

If you can scope the data lines at the DMA and floppy, you should be able to see if they look right. On the one I mentioned that failed fast and had a bad DMA, the signal was quite weak at the DMA data lines and looked more like they were floating than pulled up.

If I understand things correctly, with the "1772 pull-up fix" and using a drive with internal pull-ups as well, the overall pull-up will be stronger -because one would ends up with 2 pull-ups in parallel for each line-.

@DoG , very likely a typo indeed, looking at the pictures I think the resistors on the pictures of the fix, I believe it is [grey/black/black/brown] i.e. 8K (because decoding in reverse direction would give [red/brown/black/black] i.e. 210.