I don't feel it necessary to defend what was an obvious memory failure.
Eric Smith (much younger than I, and an Apple-owner, unlike me at the time,
hence more likely to have paid close attention to the discussion because it
affected him and his interests) has already pointed out that I must have
been suffering from "old-timer's disease" when coughing up the account of
those discussions, though I've corroborated the vague recollection with
another old-timer who was there at the time as well, but who also was not an
Apple-owner at the time.
Since the time of these discussions, I've had occasion to purchase, for a
pre-defined purpose, several Apple-II's, some of the wreckage of which and
one (maybe more) functional unit of which is serving as a doorstop or spacer
between shelves, or some such function. I've nonetheless NEVER looked
inside an Apple disk drive, nor have I pondered the schematics beyond
noticing that there were such things among the paperwork we got with the
half-dozen or so of these units. I never even got particularly familiar
with them. I'm certainly not an expert on Apple's hardware, software, or
anything else about them. I merely was attempting to recount what I seemed
to recall about a specific discussion I witnessed. Clearly the passage of
some 20+ years has muddled my recollection.
As for the tristate multiplexers, that was an error probably influenced by
the fact I'd just plugged a half dozen of them in to three S-100 memory
boards I'm giving to some guy in Minnesota who offered me something for
them, so I populated them with the requisite IC's, including the 'S257's. I
just looked back at the emails I've received in the past month, and couldn't
find the specific reference to the part in question, but I must have slipped
a couple of numbers in the course of replying to the email in which this
subject came up. If they are, indeed, '259's, then they are, as you
suggest, addressable latches.
Now, I'll hapily accept responsibility for having introduced some error into
the discussion of this topic. I guess I'm just going soft in the head. . .
That's clearly evidenced in the amount of time I've spent on the discussion
of Apple products.
Dick
-----Original Message-----
From: Tony Duell <ard_at_p850ug1.demon.co.uk>
To: Discussion re-collecting of classic computers
<classiccmp_at_u.washington.edu>
Date: Thursday, April 08, 1999 8:48 PM
Subject: Re: Heatkit 5 1/4 floppies
>>
>> Let me qualify this, first of all, with two bits of fact . . .
>
>I'll also post some facts. Actually, I'm suprised that nobody else has
>done what I've just done, and looked at (a) the Disk II circuit diagrams
>and (b) an actual Disk II. Neither are particularly rare.
>
>> (2) - I remember lengthy discussions among those members of the Denver
Area
>> 6502-Users' Group who were presumably qualified to discuss the
intricacies
>> of the internals of APPLE's disk I/O routines at a level I neither knew
nor
>> cared about, beyond the superficial details I gleaned from the several
and
>> varied sessions discussing that set of details. Now, I attended these
>> weekely and typically 4-hour long meetings for several years, and KNOW
the
>> guys who were hashing out the details of the hardware and software in
>> question knew what they were talking about, so I accept that as fact.
If
>> it wasn't true, no harm done, but I doubt that was the case. These
fellows
>
>If they claimed that the positioner was a normal DC motor and not a
>stepper, then I'm afraid they didn't know what they were talking about.
>
>Every Disk II that I have ever seen (and I've been working on them for
>some 20 years or so) has a stepper as the head positioner. The circuits
>show this, with a '259 addressable latch on the controller card to
>provide the drive signals and a ULN2003 on the 'analog board' to drive
>the windings.
>
>I'll believe that a drive with a DC motor positioner exists when somebody
>shows me one in operation, or provides reasonable evidence (schematics,
>software to drive it) that it exists.
>
>> The scheme with the tristate multiplexers came later, I believe, than the
>
>What tri-state multiplexers? I can't find a tri-state multiplexer in any
>part of the Disk II.
>
>> was among them. Having said that, I would point out that, given a
software
>> scheme sequenced the stepper, it is just as possible that one could have
>> read the diskettes written a half track off by fiddling with the stepping
>
>Indeed, and that was done for some copy-protection schemes.
>Quarter-tracks might be possible as well...
>
>> sequence. I doubt, however, that Wayne Wall would have allowed the waste
of
>> several sessions of the meetings he so firmly controlled back in those
days,
>> if the assumptions presented as fact in those discussions had not been
>> verified.
>>
>> The helical cam I remember didn't have a groove, but rather, a ridge or
>> "fence" in the shape of a helix, which was tracked by a small,
>> spring-loaded, roller bearing. This worked quite well, but, because of
>
>The one I have here has a groove. There's a ball bearing in that groove,
>with a spring leaf on top of it fixed to the head assembly. As the disk
>rotates, the ball bearing tracks along the groove and moves the head.
>
>-tony
>
Received on Fri Apr 09 1999 - 00:21:15 BST