Date : Tue, 07 Aug 2001 06:42:41 -0700 (PDT)
From : Thomas Harte <t.harte@...>
Subject: Re: Slight [technical] Disc Drive Question
> 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.
I don't mean to be argumentative, but the datasheet which has been knocking
around on Commodore FTP sites for years and has recently appeared on the BBC
Documentation Project (http://members.magnet.at/marku/bbc/) also claims that
the 177x family has 28 pins which consist of chip select, (cpu interface)
read/write, two address lines, 8 data lines, reset, ground, power, step
direction, (drive interface) read, ready, motor, write gate, write data,
track 0, index pulse, write protect, double density select, data request and
interrupt request. Certainly no drive select.
Either way, that doesn't alter the physical connections expected by floppy
discs, so just ignoring for a second where the drive select lines come from
(they certainly aren't selectable with any WD commands, but
http://www.cloud9.co.uk/james/BBCMicro/Documentation/wd1770.html has a list
at the bottom of where the select registers can be found on various BBC
models), I have one further question :
> 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.
Does it ignore their changes in state, or ignore their value? E.g. if I
switched drive such that a drive became deselected with its motor on, I'd be
right in thinking its motor would stay on, but that even if its write gate
was enabled it wouldn't actually pay attention to the write line and would
just corrupt whichever track the head was currently over?
-Thomas
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