WD1770 vs WD1772 (XF-551 drive)

[manterola]

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 March 4, 2020 by manterola

[ijor]

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 March 4, 2020 by ijor

[Rybags]

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.

[macsonny]

Was it ever validated that a WD1770 could be used in place of a WD1772 in an XF551?

[Dropcheck]

I know the WD1770 is much easier to acquire and cheaper.  From all I've heard and read it might work in the XF551 with a 5.25 drive, but your mileage might vary depending on a number of issues, some of which you can't easily control.  For that reason I don't recommend anything other than a WD1772PH chip.  I would rather have a disk drive that worked reliably every time, otherwise it's not much use to me. 

[TGB1718]

There is another difference, looking at the datasheets, 

 

 

[Rybags]

Those are two different things.

 

[Peri Noid]

14 hours ago, macsonny said:

Was it ever validated that a WD1770 could be used in place of a WD1772 in an XF551?

I don't know if it was validated but @zaxon puts WD1770 in his rexreations of XF551 drives, both 5.25" and 3.5" versions. And they work fine unless you want to run Hyper-XF firmware - then you need WD1772, WD1770 is not fully compatible with it.

[+kheller2]

The only thing I can find about the WD1772-02 vs -00 is “enhanced digital data separation”

The XF firmware uses the 6ms step flags so exactly why a 1770-00 doesn’t work at times in a mystery. I wonder if a 1772-00 has the same flakiness. 

 

However, the preliminary data sheets show different step tables AND a different rate, 2,3,5,6 vs 6,12,2,3... probably just another typo.... but maybe -00 is different from non -00 and -02.

 

The -02 data sheet also talks automatic pre compensation "The outgoing Write Data stream is delayed or advanced from nominal by 187 nsec [vs the -00 at 125] [and is controlled by some table in the datasheet]." 

 

Preliminary: 

 

-00 final:

 

[_The Doctor__]

Use the second table not the first

step rate is one of the important differences

write precomp is only provided for certain disk drive mechanisms that need it, and I believe not only for the XF's mech. Some mechs do this for themselves the XF mech does not in this case.

 

[+kheller2]

On the 1050, precomp is used after track 20, I believe. It’s been a while since we did the source code discussion so I could be misremembering.  As for the different FDCs in the XF, I don’t think anyone has posted specifically what rev was used when trying the 1770. 

[w1k]

somewhere on net i found WD1773.. its different, or better?

[+kheller2]

4 hours ago, w1k said:

somewhere on net i found WD1773.. its different, or better?

Different, and not a drop in.  

[ijor]

All MFM FDCs devices support write precompensation. The 1050 already enables write precompensation for the inner tracks on MFM (enhanced or double density).

 

[_The Doctor__]

On the XF, Is write precomp handled by the mech's PCB or by the chip that is on the XF's PCB?

Edited February 25 by _The Doctor__

[ijor]

30 minutes ago, _The Doctor__ said:

On the XF, Is write precomp handled by the mech's PCB or by the chip that is on the XF's PCB?

It's handled by the WD1772 FDC, which is in the XF's PCB.