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WD1770 vs WD1772 (XF-551 drive)

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Hi All,

I was experimenting with a WD1770 instead of the WD1772 in my XF-551 5.25 inches drive. I thought the conclusion about the compatibility of the WD1770 was that it should work as long as you are not doing 720K in 3.5" floppies (no matter if using HyperX or Woodley firmware).

However, I got reading and booting problems with some MyDOS 360K 5.25" floppies when using WD1770... with some, others were read correctly.

I was reading about the differences b/w WD1770 and WD1772 and it looks like the steps rates available are the only difference. Not really fully understanding the step rate concept, but having an idea of what it is, could a weird step rate selection during formatting in some 360K 5.25" floppies be the cause of the inability to read those diskettes?

Why we need to select a step rate during formatting in some DOS in the first place? how that affect the reading of that floppy in other disk drives / other firmwares? 

and finally... what is the last word regarding this WD1770 compatibility with XF551?

Thanks, Mauricio

Edited by manterola

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I don't have a personal experience with the WD1770. From the datasheet the only differences are the step rate *AND* the settle delay.


Step rate is the delay between step pulses sent by the FDC to the drive mechanism. The stepper motor takes some time to move the head. If two consecutive pulses are sent too soon one after the other, the disk might ignore it or might step incorrectly. Most 3.5 drives can step faster than 5.25 ones, typical step rates are 3ms and 6m respectively. I'm not sure how this would produce a fatal incompatibility, although it is conceivable ...


The WD1772 was designed with the newer 3.5 drives and has faster stepping rates. When using 720K disks the firmware will likely use the faster, 3ms stepping rate. If the FDC is a 1770 though, what would happen in the worst case is that the drive would step slower, even slower than with a 5.25 disk. This should not be fatal unless this makes the firmware to timeout. Similarly, the 1772 has a smaller settle delay. If the firmware does use the settle delay because it might be enabled or disabled (I'm not sure, but seems the stock XF551 firmware doesn't use it), this again could conceivable produce a timeout if the firmware doesn't expect it would take that much time.


Note that the step rate issue should be relevant for 3.5 disks only, but the different settle delay might affect 5.25 disks as well.


I don't see how formatting with a different step rate could create any kind of incompatibility unless again, a timeout was provoked and the firmware didn't complete the formatting correctly. In other drives, like the 1050, a different step rate would provoke a different track skew. But the XF551 uses the actual index pulse, so the tracks are always formatted aligned to index no matter what.


The different timing could, of course, affect copy protected disks. But most protected disks that need strict timing don't work on the XF551 either way ...


Conceivable there is another difference between the WD1770 and the WD1772 that is not documented ..


Edited by ijor

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In theory a format without settling delay might create a track that starts out misaligned - though doesn't the XF wait for the index pulse before commencing the track?  That would more than compensate for a delay in most cases.

I played about on the ST with different step rates - with some of them you can get harmonics that introduce vibration.

Can't remember though if I did it to do write or formats, probably only reads.

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