Floppy controller questions

Tony Duell ard at p850ug1.demon.co.uk
Tue Aug 23 18:39:41 CDT 2005

> In message <m1E7h9n-000IyXC at p850ug1>
>           ard at p850ug1.demon.co.uk (Tony Duell) wrote:
> > Also be aware that many HD-capcable 5.25" drives turn at 360 rpm all th=
> e=20
> > time. Which means the data rate if you put a DD (or for that matter an=20
> > SD) disk in them is 6/5 times what you'd get with that disk in a DD dri=
> ve=20
> > (which turns at 300 rpm). You may well need to provide a suitable clock=
> =20
> > for 300 kbps, for example.
> Ick. Time to find a 24MHz crystal oscillator :-/

Indeed. As you've probably spotted by now, some of the all-in-one-chip PC 
disk controllers need a 24MHz clock for just that reason.

> Divide by 3 gives 8MHz, div24 for 1MHz, div48 for 500kHz, div96 for 250kH=
> z.

There's a well-known divide-by-3 circuit using a couple of JKs...

> And none of those are powers of two. Yay. I think I'll just stick to 8MHz=
>  and
> 1M/500k write clocks.
> > I always thought they were invented to make my job more painful than it=
> =20
> > should be ;-)
> Only if you're trying to reverse engineer something :)

Or repair it. I have a lot more luck finding an obscure TTL part than 
finding an ASIC, or the right CPLD + the data to program into it + 
somebody who can actually program the thing

> I hate ASICs too, unless they're fully documented down to gate level in s=
> ome
> way. The CBM 1541 is a great example... Version 1 was all TTL, v2 had an =
> to do the GCR coding, v3 had an ASIC that did the GCR *and* analogue
> interfacing IIRC. Ick.

I found the Sams repair manual for the 1541 many years ago in a 
second-hand bookshop in Bristol. Of course I bought it. It covers 3 
versions of the drive (which they call the 1540, 1541, 1542, although 
AFAIK those names are not all official). There's some data on the ASIC 
(which replaces a load of TTL + a ROM in the original version. I don't 
recall there being any analagoue ASIC stuff in those drives, but IIRC the 
1570 and 1571 do have a hybrid circuit that's part of the analogue side, 
and is custom/unrepairable.

> > data separators. And I've seen a board of logic (the original IBM=20
> > controller, for example). But I've never seen a real 765 linked to an=20
> I really should try and get a copy of the IBM PC Technical Reference manu=
> als
> at some point.

Be warned that later versions of the _PC_ technical manual don't contain 
data on the I/O cards. IBM moved that into the Options and Adapters 
Technical Reference.....

> > Unless you are using the original Sony 3.5" drives, which turn at 600=20
> > rpm, and thus use twice the data rate you might expect, 5.25" drives an=
> d=20
> > 3.5=A3 drives use the same data rates for the same modes.
> All new-ish PC-type floppy drives here. I don't think I've got any of the
> original Sony drives - most of mine are MFD920s, with one or two MFD520s =
> (I

I would be suprised if you had any. The units I am thinking of are twice 
the height of a PC drive and have a 26 pin interface connector. There was 
a later one, could be half-helght (although I've only ever seen it with a 
full-height paneL), and a 34 pin connector, which could be combined power 
and data (solder-link selectable).

They were mainly used in HP machines and drive units. although Apricot 
used them too at one point. 

If you support the HD data rates, then you can use these drives, at least 
with DD disks.

> > HD disks (1.2Mbyte, 1.44Mbyte respectively) in their approriate drives=20
> > use the same data rates as 8". SD and DD disks are half of those rates=20
> > _apart from the 360rpm issue I mentioned above).
> Easy way to solve the 360rpm issue: Find the SPEED_SEL pin on the motor
> controller, lift the pin then wire it to the edge connector.

There may even be a jumper on the drive logic board for this. Problem is 
finding it ;-)

> I
> > FWIW, 3" are the same as those too (there never was a HD 3" disk AFAIK).
> Heh. Amstrad 3" "indestructable" floppies. Those bring back some memories=
> ...
> Mostly dealing with disintegrating drive belts.

Actually, I think the 3" disk is mechancially better than the 3.5" one. 
I've never had an Amstrad drive, I used the Hitachi ones (I even have the 
offiical service manual for the double-head version), which had a 
direct-drive spindle motor.


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