1771 floppy controller questions

From: Greg Ewing <greg_at_cosc.canterbury.ac.nz>
Date: Tue Nov 20 18:47:34 2001

Richard Erlacher <edick_at_idcomm.com> wrote (concerning
plugging a 1771 in backwards):

> I've done that on one of those "late nights" on more than one
> occasion.

I discovered a more creative way to destroy a 1771 once. It was on a
highly personalised system based on a Dick Smith Super 80 (not System
80!) for which I had built my own disk controller. A friend of mine
was attempting to build a colour graphics display, and I had the
prototype connected between my system and a TV (via RF).

There was a lot of herringbone stuff on the screen, and I wondered
whether hash from the computer was getting into the RF modulator
through the power supply, so I decided to try filtering it with an
inductor, to wit, one winding of an old audio output transformer that
I had lying about.

It didn't help, so I disconnected it. With the power on. The disk
drive head went clunk, and the motor turned on. And stayed
on. Permanently. At that point I realised that I had neglected to
anticipate the effects of inductive kickback.

I couldn't get a 1771 to replace it with at the time, so I got a 1791,
which looked like a better part anyway (capable of double density [1]) so
I wasn't unhappy about having to make a few changes to the circuitry,
such as devising an external data separator.

More of a challenge was when I discovered that, of the 4 kinds of data
mark supported by the 1771, the 1791 could only read two of them. And,
of course, the one I had chosen to use (in my unique home-brewed DOS)
was one of the ones no longer supported, so I couldn't read any of my
disks! Recovering from that situation was an interesting exercise...

Footnotes:

[1] I never made use of it, though, since my 2MHz Z80 couldn't
keep up with feeding the data buffer fast enough. :-(

Greg Ewing, Computer Science Dept, +--------------------------------------+
University of Canterbury, | A citizen of NewZealandCorp, a |
Christchurch, New Zealand | wholly-owned subsidiary of USA Inc. |
greg_at_cosc.canterbury.ac.nz +--------------------------------------+
Received on Tue Nov 20 2001 - 18:47:34 GMT