Need Shugart 851 manual

From: Richard Erlacher <>
Date: Wed Mar 6 01:43:25 2002

see below, plz.


----- Original Message -----
From: "Tony Duell" <>
To: <>
Sent: Tuesday, March 05, 2002 4:57 PM
Subject: Re: Need Shugart 851 manual

> >
> > There are several versions of the '80x manual, but, unless I completely
> > my guess, at least one of mine has pin 2 defined for RWC. Now, several of
> The service manual I have shows pin 2 as not used. There are schematics
> for 4 versions of the logic board, and _none_ of them seem to have a RWC
> circuit. The earlier versions use discrete components for the write
> driver and as far as I can see there's no circuit to reduce the write
> current at all. Later versions use an ASIC which might incorporate this
> function (the pin '+I2 Sel' (pin 8) looks possible, but it's just tied to
> ground on the schematics I have )
I haven't used an 80x drive, myself, since about 1978, but since I designed
numerous FDC's back in the '70's and early '80's, I had to support that
function on pin 2 of the cable. Several drive makers kept track of the head
position with a couple of up/down counters back before ASICs and
microprocessors were common. I'm not sure which versions of the 80x schematic
I've looked at, but it's pretty certain I didn't do it after 1980. By that
time the negative supply wasn't used any longer and the SHugart folks may have
decided they didn't need to reduce their write current. I can't imagine those
guys passing up any possible avenue for increasing their drives' effectiveness
or reliability, though, and that would certainly have been the effect.
> > It would be really interesting to know which version of the drive the
> > folks intended to be used here. For most purposes, the RWC signal
> Indeed...
> > internally by the drive itself is perfectly adequate for detecting track
> I've never seen an SA800 (or SA850 for that matter) that does that. Some
> later drives have a second slotted optoswitch (in front of the track 0
> sensor) to detect when the head is past cylinder 43, even later ones
> which use a microcontroller for the seek control simply used that to keep
> a cylinder count and set a port pin appropriately.
several drives generate that signal on the drive by counting the steps using
the step and direction signals. No physical sensor is needed.
> > thereby freeing up the signal on pin 2, but only the later versions of the
> > Shugart drives used that. The connection to pin 2 was removed from the
> > at about the time that they stopped using a negative supply, IIRC.
> At least some versions in my manual need a -ve supply, but none of them
> seem to have the RWC input. Maybe I'm missing something obvious, but I
> can't spot it...
I haven't concerned myself with a Shugart 80x manual since '70-something. As
a designer of FDC''s, and not of drives, I was concerned with supporting all
the functions that were straightforward to provide. Whether a given drive
required that function or not was of concern, since it could be ignored if it
wasn't used, and it could be bypassed if some other function was needed in its
place. It's very possible I could be mistaken about the RWC being required by
the 80x series, but I'm quite sure it was used on the earlier drives, e.g.
90x, and on later drives from other vendors. The provision of a TG43 signal
on the MFM-capable disk controllers of the late '70's suggests that it might
have been used by the drives, though it certainly was needed to enable write
precompensation. Reduced write current was normally used over the same range
of tracks that required write precompensation.
> > Needless to say, it would be interesting to know whether this controller
> > even uses the head select signal that requisite for two-sided operation.
> I am sure it does NOT.
The reason for my interest is that it's a terrible waste of a two-sided drive
putting it in an application incapable of benefitting from its capabilities.
I've struggled to dispose of all the single-sided drives I've had over the
past decade. At this point, I'm not sure that I have any functional
single-sided drives left, but I do still take some time and trouble with the
two-sided ones.

It seems as though what Joe really needs is to know what was originally
intended for a hookup between the controller and the drives, and, as I've
mentioned, it appears to me that something's missing. It's not terribly
important whether the drive uses RWC if one doesn't know what the relationship
between the apparently 100-conductor cable Joe described and the "normal"
50-conductor drive cable was intended to be. As I've said before, it looks to
me as though there's intended to be an intermediate circuit of some sort.
Received on Wed Mar 06 2002 - 01:43:25 GMT

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