Date   : Fri, 21 Oct 2005 09:57:32 +0100
From   : Richard Gellman <splodge@...>
Subject: Re: Dual Floppies On A Master

Tim Fardell wrote:

>On Thu, 20 Oct 2005, Jules Richardson wrote:
>
>  
>
>>Hmm, interesting. I suppose a lot of modern SCSI cards and external 
>>boxes manage (often badly) to auto-terminate, so maybe it's a similar 
>>thing for the floppy drive world - there must be a way of electrically 
>>detecting that you're the last device on the bus. No idea how though!
>>    
>>
>
>Isn't the termination simply a bank of resistors between all the data 
>lines and ground? If so, wouldn't it be fairly easy to detect the presence 
>or absence of such termination? 
>  
>
Thats the cheaper (read: lazier) form of termination, and it works 
*most* of the time. Proper termination actually involves two resistor 
banks, one tieing the lines to ground, and one to +5v, usually at offset 
resistances. This causes the bus lines to "float" at roughly 2V.

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'm not sure how modern 3.5" 
floppies terminate, but I'm guessing they don't bother too much about 
auto-detecting on the really recent drives (who wants two 3.5" floppy 
drives on a PC these days?).

If you have the equipment to hand, you can test for termination by 
plugging in power and checking the voltage on one of the input lines. If 
its terminated, it'll float at around 2v.

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, 
that basically consists of motor enable (switch on), head step, head 
direction (when the step occurs), track 0 detect, sector 0 detect, read 
bit stream, write bit stream, write gate (to synchronise writes to the 
correct points on the disk), side select (:0 and :2 to our DFS 
afficianados), and an access clock to keep the drive timing in line with 
the computer.

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

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?

-- Richard

On another note, you've all inspired me. I'll shall experiment with my 
Master 128 and its Dual Drive stand (the bridge over the system type) 
and see if I can replace drive :1 (a naff cumana drive) with a 3.5" variety.