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wd1772 vs. wd1770 vs. wd1773

Well, they're very similar chips. The 1986 WD databook on bitsavers goes into detail, but in essence the 1770 and 1772 are fully pin-compatible, with the difference being the 1772 has faster step rates. The 1773 has the same step rates as the 1770 (and the 1793) but has a ready/enable precomp input versus the motor on output of the 1770 and 1772. The 1770 should be a drop-in replacement, but with slower step rates.
 
Note that there are varying speeds of the WD1772 available. Some of the later ones can be clocked for high density. Other than for the step rates, the 1770 and 1772 should be identical, at least for the -00 revisions.

The 1772-02 is a bit different Datasheet
 
Western Digital often used the numbers after the main part number to differentiate between very different parts. The various WD1100-XY parts used on the 8X300-based WD1000 controllers are good examples of this, where the XY was really the main part number. So it wouldn't surprise me that WD1772-00 and WD1772-02 might have differences.

Does anyone have a pointer to where the 1770 was tried in an ST? I'd love to read the write-up.
 
Well, there is a FAQ page at http://www.atarimania.com/faq-atari-400-800-xl-xe-what-is-the-atari-xf551-disk-drive_29.html that states some XF551 drives in late production used WD1770 instead of WD1772. As the ST isn't my area of expertise, and since I don't have one myself, can't be for sure. But if Atari actually did use the 1770 themselves as a substitute in another WD1772-based product it could mean that it might work in this case.

Then again, maybe not.... But see https://atariage.com/forums/topic/291342-xf551-35-clone/

I know a bit about the controllers, but not a great deal about the ST itself, so keep that in mind if you try the 1770.
 
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It's possible that 1770 works nice in ATARI XL/XE floppy drive, but it's tested that it won't work in ST, steprate is too slow for TOS.

As the 1773 is not pin compatible you can't use it in ST.

The best 1772 is the 1772-02-02 because it can be clocked temporary to 16 Mhz what makes HD floppy in ST possoble. If you clock it permanently to 16 MHz it will die because of heat. But if you make logics that it only clocks to 16 MHz if the floppy drive reports a HD diskette (rewired pin 2 of the shugart bus) and drive-select/motor-on is active, then it works mostly. If you want to make it reliable, then use ATARI AJAX controller, this one is 1772 compatible and can support 16 and 34 MHz (Extra Density ED drives) permanently. AJAX is hard to get. AJAX is the standard floppy controller in TT, Mega STE and Falcon.
 
1ST1, do you know if anyone has patched TOS for the slower step rates of the 1770?

And, as you say, the 1773, used in the Gate-array versions of the TRS-80 Model 4 and 4P, is just simply not pin-compatible.
 
Why doesn't a slower step rate work? The drive should be a bit noisier in seeking, but otherwise should function. I threw an ISA card together with a 1770 on it and it worked fine.
 
TOS expects a fast stepping. There are tools to slow down steps in TOS, for example for 40 track 5,25 inch drives required, but they first have to be started from floppy. You hava a chicken egg problem. You would have to modify TOS ROM. And several copy tools also expect fast steps, that you can't patch.
 
You don't have to care about compatibility on a closed platform. Atari ST owners weren't swapping in random floppy controller cards. The machines were all exactly the same, so programmers wrote for that and tried to get the best performance possible.
 
I'm not following; you issue a seek and an interrupt is issued when the head reaches the specified position. You have a deadman timer in case the drive goes not-ready or whatever, but that can be seconds, not milliseconds. (I know how the 1770 works--I use one in one of my cards).

Please explain why software has to be impatient for a drive to step to a cylinder. Or is this one of Atari's notable Tramiel "shortcuts" where interrupts aren't used?
 
The WD1772 is controlled by a DMA-Controller (which also controls the DMA/ACSI port for harddisks). If you look at the circuit diagram of the ST you can see that. The interrupt is connected to the 68901 interrupt controller, but the DMA controller does not see the signal. So if you implement fast floppy routines through DMA... It's also possible to set the DMA controller to a pass through direct FDC register mode and using the Interrupt, but by default the fast DMA floppy routines aren't doing that.
 
Looking at the 520ST's schematic (since we've not said WHICH ST we're talking about, I just picked the first on on Archive.org that came up), it's a bog-standard WD17xx-style DMA connection (TRS-80 Model II/12/16/16B/6000 use the same basic setup, using a Z80 DMA) where the DRQ of the FDC is connected to one of the two DMA request inputs of the Atari-custom DMA controller. PDF page 14 of http://www.atarimania.com/documents/atari_520st_service_manual.pdf at the second paragraph goes over the basic process, which is completely driven by the WD1772's DRQ byte timing. Now, for the 1772, by the time the DRQ is involved you've already completed the seek to a track and are reading data; the DRQ timing has nothing to do with the step rate at that point in the typical service routine. In this case, the IRQ from the WD1772 interrupts the CPU at the end of the transfer, with the byte transfers happening by DMA. This is virtually identical to the TRS-80 Model II setup, just with a different DMA chip and an older WD controller.

So the TOS driver software apparently just isn't set up to wait long enough for the seek to complete, with the 1770's step rates just not supported by the built-in delay in the OS. We can complain all we want about the OS not being more intelligent in its step rate support, and we might even have a case due to some of Atari's own products using the WD1770 in place of the WD1772, but it's a simple fact that that horse has already left the barn, and the OS isn't set up for any but the WD1772's step rates.
 
I really appreciate the thread! A couple of years ago I was given a naked Zaxxon xf551 clone PCB and populated it myself. I did some looking into the differences between the specced 1772 and the 1770, finding here and elsewhere that even if the drive did not work perfectly, it would show signs of life using the 1770, which costs nearly nothing. The 1772 is like 40 bucks! I'm please to report that the XF551, at least this clone, works perfectly as a 40 track, ds/dd, 360k controller using either a 3.5" or 5.25" mechanism. I used the HyperFX 5.25" rom version, as several attempts with several 3.5" rom versions all failed with strange results. So, you can use the 1770 controller in your drive so long as you do not intend to actually try and format or read an 80-track disk. That will require installing the 1772.
 
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