Date   : Fri, 21 Oct 2005 12:41:09 +0100
From   : Richard Gellman <splodge@...>
Subject: Re: Dual Floppies On A Master

Pete Turnbull wrote:

>>resistances. This causes the bus lines to "float" at roughly 2V.
>>    
>>
>
>It may be the lazy way but the pullup resistors used are the standard
>method for 5.25" and 8" floppies.  The common 220/330 ohm and 180/220
>ohm termination networks used elsewhere (eg SCSI) actually set the
>lines at around 3V or 3.5V, not 2V -- that would be far too low. 
>
>If
>  
>
>>its terminated, it'll float at around 2v.
>>    
>>
>
>No, much higher.  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...

>  
>
>>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
>
>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). 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 :)

>  
>
>>(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...

-- Richard