Date   : Fri, 21 Oct 2005 14:41:06 +0100
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: Dual Floppies On A Master

Richard Gellman wrote:
> Termination of floppies is a valid point though. I remember many 5.25" 
> drives simply couldn't be "doubled" because the resister pack was either 
> absent, or more frequently hard wired (doubling involving cutting 
> resistors on one drive from the board).

I expect it's particularly important for such as the BBC where the cable 
length tends to be quite long (because several inches is "wasted" 
beneath the case).

Maybe that's the only reason double drives in a PC tend to be less picky 
- typically they have pretty short cable runs.

> 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. Its a *very* low level interface, 

as you qualified in another post, the data's essentially a serial bus, 
with various other control lines paralleled. It's just a simple bus 
arrangement with very little in the way of protocol (in a similar way, 
ST506/412 interfaced hard disks on the earlier PCs and other systems 
were much the same - it's only much later that hard disks became 
intelligent as chip costs came down)

> (Oh, and the lil' red light on the front? Tied to the motor enable 
> circuit).

Usually! Some drives do very strange things...

> In fact it all make me wonder why the circuit board in floppy drives is 
> so complex. I'm sure most of it is handled by the computer - surely just 
> some current stepping would do the job?

Hmm, I guess there's head stepper and spindle motor control circuitry, 
head amplification, processing of signals from the write protect switch, 
and density select switch, head load circuitry (where applicable), 
control of signals based on write gate line, track zero detection (to 
clean up signal from sensor), index hole detection (to clean up signal 
from sensor), reset circuitry (so the drive doesn't wipe data on power 
on/off; often a problem on very old machines!) etc.

Not complex overall, but I suppose a handful of components for each job 
adds up! I suppose there's enough logic (15 - 20 gates) that they could 
justify a custom chip on later drives to do all that work.

cheers

Jules