From: B Degnan (billdeg_at_degnanco.com)
Date: 2007-03-22 22:34:38
>
> The RWC and WP refer to cylinder numbers where these parameters kick
> in. In the case of the ST-251, there is no Write Precomp (820 ==
> 820), and neither is there with the TM602S and TM603S (153 == 153).
> The problem is that starting at cylinder 128, the ST506 expects the
> controller to do write precompensation. I'm a big foggy on the
> specific mechanism, but it has to do with timing and bit density on
> crowded or looser tracks. In any case, you won't have reliable
> operation if the drive expects it and the controller doesn't provide
> it.
>
> In a desktop PC of the era, you told the controller what to do (within
> limits) and it did it. In the case of the D9060/D9090 drives, there's
> no mechanism to turn that on or off at a certain point. There _is_ a
> head-count jumper (4 vs 6), but it only changes the geometry table. I
> do not know how one tells a SASI<->MFM controller how to engage or
> disengage WP, but since the 6502 ROM code presently does not
> manipulate that parameter, if it _is_ possible to twiddle it from the
> 6502, the twiddling certainly won't fit as a patch amidst the present
> firmware. If one had compilable source, it might be possible to cut a
> new ROM rev, but I haven't seen a treatment of the D90x0 firmware at
> that level of detail.
>
> In practice, the thing to do is to find any drive that does not
> require write precomp below cyl. 153. Newer drives than the ST506 and
> the TM602S don't get hung up on reduced write current, which is why an
> ST-225 or ST-251 still work. They are internally smarter than an
> ST506. Different drives than that might or might not do well.
>
> Sorry there isn't an easy answer for this, but the ST506 is sort of in
> a class by itself.
>
> -ethan
>
> Message was sent through the cbm-hackers mailing list
>
Very interesting.
At the bottom of this page...
http://nemesis.lonestar.org/computers/tandy/hardware/storage/mfm.html
...there is text about putting a piece of tape on pin two of the control
cable. Maybe I will try that.
Bill D
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