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 Post subject: Weird Seagate MFM drive
PostPosted: July 24th, 2022, 13:54 
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I'm trying to make a Seagate ST1100 83 MB MFM drive working.
It behaves in quite a strange way however; when I format with only head 0 it formats flawlessly without any bad sectors, but when I use more than that one head, it generates a lot of errors - SpeedStor even gives up because the error table fills up.
What puzzles me though, is the fact it throws such errors even when LLF'ing with head 0, which otherwise works flawlessly on my Myarc HFDC (TI/99 controller).
Also, when using a Seagate S11M controller on a PC, I'm not able to enter the correct parameters, so SpeedStor is only able to access the system defined 615 cyl/4 heads etc, not the 1072 (or 1024 for that controller) and 9 heads.
It seems to me it's a logic error, not a physical one, since it makes no sense it should have problems with every single head/platter except for the first one...and also the fact SpeedStor throws these errors at head 0 too, where the HDFC formats it flawlessly...?! (which, by the way, also works fine when writing files/dirs, etc.
Could it be the PCB perhaps?


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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 25th, 2022, 16:37 
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When the drive and controller are idle, what are the voltages on the head select bits at the MFM interface? (HS0 - HS3).

What are the same HS voltages on the drive's MFM edge connector when the MFM control cable is left disconnected?

Is the terminator resistor pack installed?

When you say that "the HDFC formats it flawlessly", do you mean that this controller formats head #0 flawlessly but still has problems with the other heads?

Is it possible that some controllers treat HS3 as a "Reduced Write Current" signal, thereby causing the wrong head to be selected? What happens if you place insulation tape over this pin (pin #2)?

https://stason.org/TULARC/pc/hard-drives-hdd/seagate/ST1100-84MB-3-5-HH-MFM-ST506.html (the pinout diagram has an error at pins 2 and 4)
https://old.pinouts.ru/HD/co_ST506_pinout.shtml (this diagram is correct)

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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 10:08 
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Location: Denmark
Thanks for your reply!

I haven't got the voltage readings, but I'll see if I can get these measured. The controller should be sound anyway, since it works with other drives.

I've tried different combinations with and without the terminator; it does make a difference but when it seems to format properly on all heads, it ends with a track 0 error... that is, if both drives are terminated which they shouldn't be but usually works with the HFDC. If I terminate them properly however, i.e. the furthest drive only terminated, it formats with head 0 and the rest goes too fast as if it can''t format properly.

Yesterday I came across the issue with pin 2 however; I wasn't aware of that and that MAY make a difference. It shouldn't try to access heads 9+ if I tell it to format with 8 heads only of course, but if it sends a RWC signal on that pin it obviously can make some havoc.

The biggest challenge these days with MFM drives I think, is the lack of information available. Luckily there are many guys around still possessing quite some knowledge about them. Even though I've used them since the 80's myself, I realize there's still much to learn. :-D

Yesterday I swapped the PCB with another one I had at hand, and it behaves better now. I'll try disabling pin 2 first and foremost and see what the result will be!


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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 19:23 
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It's getting weirder. Now heads 0 and 1 won't format properly, but apparently (judging from the speed at which it happens) it's now able to format the next six surfaces (and probably the seventh, although the controller doesn't support 9 heads).

Unfortunately that also spells doom for it as it is, since track 0 is obviously unavailable.

It seems to me there's something going on with the PCB; perhaps a slight difference with the voltage as you mentioned, a short circuit, unwanted resistance or the termination.

If I could somehow swap heads on the ribbon cable; I've seen that done on another drive but I won't risk ruining anything on it since I don't know the exact pinouts on that one (and there are more than just for the 9 heads and ground, 32 in total IIRC...

Perhaps I should just ditch the disk, but I have a strong feeling it IS possible to resolve that issue somehow. Seems to me that the PCB swap made it able to format completely different surfaces, and for that reason I've removed and reseated the ribbon cable, cleaned it with IPA, etc... but to no avail.
Covering pin 2 also seem to make no difference whatsoever.

It reminds me of a 251 I bought many many years ago. That too wasn't able to format, so I messed around with it having nothing to lose. It turned out that if I shorted two legs at the termination, it suddenly formatted properly - but I had to wait until the very last moment before I executed the format command, or the program wouldn't recognize the drive. And afterwards the short should be removed right away. But it got it working, and I've used that drive for many years now, formatting it if needed with the above method, haha :-D


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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 19:45 
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I don't know which 32-pin cable you are referring to, but if it's the 34-pin MFM control cable, then there are 4 binary coded head select bits. These bits select head 0 - 15. It is not a case of 1 pin per head.

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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 19:57 
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No, what I mean is the ribbon cable on the drive itself, which connects to the head stack.

Anyway, since I was able to format properly with head 0 initially and now 2-6 but not 0 and 1, I'm certain the drive internals are sound and the culprit is something PCB related. I'll see if I can get a voltage readout soon. Thanks so far! c",)


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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 20:11 
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Perhaps I could test it by modifying a control cable so that head 8 and 9 are selected instead of 0 and 1. That way.I should be able to format it with 7 heads, at least.


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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 21:21 
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From what I can see, it's a termination issue... just as with the ST-251.

Here's the entire story: I have two Myarc Geneve's with each a HFDC controller, and with two different format programs - the original MDM5 from Myarc and a program called CFORM.

CFORM formats with heads 2-7 but not 0 and 1, if I (erroneously, I know) leave the resistor pack on in the middle of the chain. It won't if I remove it, however (!)

MDM5 has the opposite problem - it won't format at all and exits right away if I leave the resistor pack on, but formats head 0 - and head 0 only - if I remove it as I should. As soon as I raise the head count with just one, it exits.

So something is going on with that termination. Any suggestions? :shock:


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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 21:26 
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I think it's a case of 3 wires per head, plus grounds for shielding.

If you need some insight into the electronics ...

https://mirrorservice.org/sites/www.bitsavers.org/pdf/cmi/
https://mirrorservice.org/sites/www.bitsavers.org/pdf/seagate/mfm/
https://mirrorservice.org/sites/www.bitsavers.org/pdf/
http://www.hddoracle.com/download/file.php?id=304 (IBM AT 20MB HDD circuit diagrams)
http://users.on.net/~fzabkar/PC-AT/FDC_HDC/ (IBM AT FDD/HDD controller)

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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 21:32 
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I would measure the voltages at the resistor pack.

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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 21:52 
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Thanks for your suggestions and links! I'll take a look at that.

What's the voltages at the resistor pack supposed to be, do you know these values?


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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 22:04 
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Page 20 of the following document shows a resistive divider with 220 ohm on the high side and 330 ohm on the low side. If the cable is disconnected, then I would expect the inputs to sit at 3.0V.

https://mirrorservice.org/sites/www.bitsavers.org/pdf/seagate/mfm/ST-506/ST506_Preliminary_OEM_Manual_Apr81.pdf

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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 22:11 
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Okay, thanks!

Eh, perhaps I've misunderstood something. I thought the odd pins were facing away from the head stack etc, but after reading I get the impression that pin 2 is facing outwards...?


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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 22:22 
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All odd pins on the 34-pin cable are grounded.

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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 22:24 
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Yep, but are they the ones facing the head stack or on the front of the PCB?


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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 22:25 
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I don't have access to a photo of the PCB, and my memory doesn't go back that far.

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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 22:29 
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Okay, I'll see what I can find out. Thanks for your help so far; even though I've used this interface for more than 3 decades, I've never been that far into the subject.


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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 27th, 2022, 22:39 
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If this is your PCB ...

https://www.computerbase.de/forum/attachments/img_20201018_150803_5-jpg.1008787/

... then the odd pins (grounds) must be on the other side of the PCB.

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 Post subject: Re: Weird Seagate MFM drive
PostPosted: July 28th, 2022, 12:17 
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Yes, that's the one. Now I've covered that pin, but it doesn't make any difference. Also, I guess it would only affect it if the program actually sends a RWC signal; otherwise the last head select should be low of course.

I'll try different combinations as well as measure the termination voltages and see what the outcome will be.

Thanks a lot for your suggestions etc! :-D


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 Post subject: Re: Weird Seagate MFM drive
PostPosted: August 18th, 2022, 4:15 
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fzabkar wrote:
If this is your PCB ...

https://www.computerbase.de/forum/attachments/img_20201018_150803_5-jpg.1008787/

... then the odd pins (grounds) must be on the other side of the PCB.


I got a bit further now. I what I've learned is correct, then the heads are inactive when the pins are high, and when they go to low, they become active.

My measurements reveals a slight difference between three of them; 2.99V on two and 3.20V on the 3rd. H3, however, is sitting at 0.16V which would indicate it tries to activate head 8 along with the others; if so, it's no wonder it won't format properly. c".)

That's the one I modified the output signal for... so either that's done incorrectly (which shouldn't be the case), or the other drive, which has the termination, sets it to low via the resistor pack.

Thus, I'll try again with that drive only, so it relies on it's own resistor pack.
Perhaps you have some suggestions?


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