Date   : Fri, 21 Oct 2005 14:32:45 +0100 (BST)
From   : Pete Turnbull <pete@...>
Subject: Re: Dual Floppies On A Master

On Oct 21 2005, 12:41, Richard Gellman wrote:
> Pete Turnbull wrote:
>
> >With a simple set of pullups, it'll float at about
> >5V, and with a termination network, typically at about 3V (DEC
> >equipment and some others use 3.5V or thereabouts on various busses)
> >
> I knew it was somewhere in the middle, hence I said "around" 2V ;) I
> always thought they were biased towards the lower voltage, so that in
> the event of a "mis reading" by the equpiment, it appeared nearer to
> logic 0 than logic 1? I admit to taking guesses here...

Most of these busses are open-collector and use active-low signals, so
it makes more sense to keep them at a high voltage since that's the
inactive, or logic-zero, level.

> >>Something to keep in mind, the floppy connector, unlike say, an IDE
> >or
> >>SCSI connector, is not a bus - i.e. it doesn't have "data" lines,
> >>there's no strobe line, REQ/ACK, etc.
> >
> >I disagree, it's a perfectly ordinary bus, in the sense used by
> >electronics engineers.  It's a set of signal/control lines to which
you
> >can attach several devices in parallel, along its length.  It
happens
> >to be an asynchronous bus, and there is no clock on it.
> >
> I meant its not a bus in the conventional digital sense, i.e. no
A0-A15,
> D0-D7, etc (example being more of a processor bus than a hard drive
> bus).

But that's not especially "conventional"; there are plenty of busses
not organised like that.  Moreover, of those that are, very few indeed
have strobes or REQ/ACK signals.

> Hard drives for example are higher level and "addressable", such
> that the drive is given a command, does it, then allows the result to
be
> read in - exactly like it were a chip on the system board. Where as
the
> floppy drive interface puts the procedure in the hands of the
computer -
> the procedure being "go back until you hit track 0, then go forward 4
> times, then enable the bitstream....". In IDE/SCSI devices this is
done
> by the drive itself. This is the difference I was illustrating :)

OK, but that's nothing to do with busses.  Your PC memory bus doesn't
work like that, nor does your PCI or ISA bus (though DMA devices on it
*might*).

> >>(Oh, and the lil' red light on the front? Tied to the motor enable
> >circuit).
> >
> >That depends on the drive.  Normally it's tied to the select line,
> >actually, but that may in turn be linked in some way to the "motor
on"
> >line.
> >
> >
> I'll conceed the possibility, and admit to not having seen every
drive
> in existence, but the ones I have seen the LED comes on and off
> *precisely* with the motor spin. In my mind, this makes more sense,
as
> the red LED would then indicate more of a "Drive busy" rather than
> "accessing disk" light. This could of course, all just be
coincidence,
> and I could be talking from betwixt mine buttocks...

No, most activate the LED when the drive is active, as determined by
the drive select.  You can set up many standard-interface drives so
that MOTOR ON turns on the motor regardless of drive select, but you
won't get any data out of it like that, and it would be foolish to have
that state turn on the LED because then all the drives in the set would
have the LED on at the same time.  So the norm is actuially the way I
described, with the LED controlled by drive select, and usually the
motor comes on with drive select, or things are arranged so that the
head only loads with drive select active.  Drive selects are the only
signals that are unique to the individual drives on a bus; the MOTOR ON
and other signals are all in parallel.  Perhaps you've spent too much
time looking at PC drives, where the signal often called "Motor On" on
the interface is actually one of the drive selects.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York