If you love your old drives, you'll operate the stepper at it's optimum
rate, which should be essentially noise-free. If you can hear your drives,
you need to tweak the step rate, usually upward. One alternative is to ad a
mux or whatever to change the speed to the FDC when the step pulses are
being transmitted, as nothing else is happening then. Many drives need
rates somewhat faster than what the standard controller issues, hence, it's
a good idea to consider a fix, involving clock selection via the to match
the drive select.
In the case of the 1771 and 179x series it's possible to build a really neat
circuit I've seen but never tried to match, which uses the /TEST pin on the
FDC to cause the device to put out its pulses much faster, allowing them to
be accumulated externally in a counter, which, drives a DAC which drives a
VCO, which drives the counter as it downcounts the number of steps, thereby
slewing the head assembly. This could lead to an interesting but lengthy
discussion.
The point is that if you want your drives to suffer the least possible wear
from off-rate stepping, you'll do SOMETHING to reduce vibration due to an
incorrect step rate. The vibration is on the same axis as the eventual
misalignment that will result.
None of this applies to the 1770/1772, which have reasonbly fast but not
fast enough step rates.
Dick
-----Original Message-----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: classiccmp_at_classiccmp.org <classiccmp_at_classiccmp.org>
Date: Friday, February 04, 2000 1:41 PM
Subject: WD1770 help needed
>> Hello all
>>
>> Sadly, the WD1770 fcd IC in my home-brew fd i/o board finally gave up the
>> ghost. Fourteen years of service is admirable -- when it finally
croaked, I
>> felt like weeping.
>>
>> Additionally, in violation of my policy of *always* keeping spares of
>> discontinued components which I use -- I have no spare.
>>
>> Does anyone know of a source for these? Or the 1772 (I believe Tony
Duell
>> once mentioned that the 1772 could replace the 1770 in most cases -- but
I
>> could be wrong).
>
>The 1772 appears to be identical to the 1770 apart from the step rates it
>uses. If your drives can take said higher rates, then the 1772 will work
>in your controller.
>
>The 1772 (although not the 1770) was, I believe, second-sourced by VLSI
>(the company that made most of the ARM chips), as it was used in Acorn
>Archimedes machines. The 1770 was used in later version of the BBC micro.
>
>The 1772 (I think) was used in Atari STs.
>
>I am not suggesting you raid a working example of any of these machines
>for the chip, but maybe somebody has a dead ST with a working 1772 or
>something.
>
>>
>> Also, I understand that the C64 floppy drives (1581?) used the 1770 but
I'm
>> not a commie and can't immediately verify this. Anyone out there with
junk
>> C64 drives who wants to liberate this IC -- to a good home??
>>
>
>The 1570 and 1571 (5.25" CBM drives that could read GDR and MFM disks)
>have a 1770 in them. Older CBM drives (1541, PET 5.25" drives) do not. I
>don't know about the 1581.
>
>Again, I'd not want to raid a working unit for the chip. But if you have
>a broken drive, it's a possible source.
>
>-tony
>
Received on Fri Feb 04 2000 - 14:58:51 GMT