Date   : Tue, 07 Aug 2001 01:24:20 +0100 (BST)
From   : Brian Widdas <brian@...>
Subject: Re: Slight [technical] Disc Drive Question

On Sun, 5 Aug 2001, Thomas Harte wrote:

> Baring in mind that the WD1770 is designed to be connected to only one
> drive, and that two drives are possible because of an extra Acorn
> implemented drive control register which sets which drive is currently
> connected to the WD1770, I was wondering what state the various lines assume
> on the drive not currently selected. Is it likely they assume a default
> state (i.e. write gate disabled, motor disabled, no step pulse) or that they
> maintain their state unchanged? Perhaps some lines are shared, maybe not
> directly (since having both drives share the same write gate for example
> would be disasterous) but with some strange logic? Does anybody know?

This didn't sound quite right to me, so I've just had a look at the
circuit diagram.

The 1770 is designed to control two drives, which it does through a couple
of drive select lines. The chip itself may not be designed to drive two
drives (I don't know the specifics of it), and there is some buffer
circuitry on the lines, but the actual drive select is performed by the
1770.

The question about the state the non-selected drive's lines takes is not
applicable to the situation, as both drives share the same set of lines.
That is to say, both drives receive the same motor on/off, step pulse,
head select, etc, but only one of them actually does anything about it -
the other drive, not being selected, ignores them.

If you look on a disk drive, you should find a drive select jumper
somewhere. This will let you switch the drive between being drive 0 and
drive 1. You can also keep all the drives the same and change the cable
over, which (although I haven't checked), is what I believe the twist in a
modern PC's floppy cable is for.

Brian
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