Date   : Fri, 04 Apr 2008 00:29:51 +0100
From   : pete@... (Pete Turnbull)
Subject: Wires cut in floppy ribbon cable?

On 03/04/2008 22:19, Kris Adcock wrote:
> Hi Jules - thanks for your reply!
> Jules Richardson wrote:

>> I can't remember off the top of my head how the BBC's expected cylinders, 
>> spindle speeds, head type and interface data rate compare to 5.25" drives in 
>> the PC world; I'm not sure that the two are particularly compatible (at least 
>> without changing drive jumpers) - plugging in something that's been pulled 
>> from a PC might not work. I *think* BBC drives are always 360rpm aren't
they - 
>> whereas PC drives are 300RPM for DD and 360RPM for HD...

No, all BBC 5.25" drives, and virtually all 3.5" drives (even on PCs) 
are always 300 rpm.  Most PC 5.25" HD-capable drives are capable of both 
speeds, but some are either configured or even manufactured to only run 
at 360 rpm (which is the same as old 8" drives).  PCs are generally 
capable of handling double-density at 300 kbps instead of, or as well 
as, 250 kbps.  That exactly compensates for the difference between 300 
rpm (needing 250kbps) and 360 rpm (needing 300kbps).  However, PCs 
always run 5.25" drives at 360 rpm for high-density (HD) at 500kbps data 
rate, and that's why you get fewer sectors and less capacity on a 5.25" 
HD disk compared to a 3.5" HD disk, which uses the same 500kbps data 
rate but only runs at 300rpm.

One problem with plugging a PC drive in to a Beeb is that the drive is 
probably jumpered to be drive 1.  Using the twist in the (PC) cable 
doesn't help to make it drive 0, it just makes it drive 2.

Another potential problem is the Ready signal, and some PC drives don't 
support that.  Luckily, the common Beeb interfaces don't use the Ready 
signal from the drive but instead derive one from the index signal (the 
general method used on drives, is that a drive is considered ready if 
the motor is running, the drive is selected, and the drive sees two 
consecutive index pulses within a time window that suggests the drive is 
up to speed.  The Beeb's 8271 circuit does a very similar thing).

Other problems sometimes comes from the speed/density settings.  Most 
80-track double-sided drives you see nowadays are HD drives sold for the 
PC market, and are sometimes set to always run at 360 rpm.  Much less 
commonly they may be jumpered for the appropriate write current for high 
density (only), but often they're jumpered to use HD settings when 
there's no signal on pin 2 -- and the Beeb doesn't supply an appropriate 
signal on pin 2.

> I suspect there will be some rolled-eyes and tired sighs from some 
> people on this list with more comprehensive knowledge, when I say that 
> for me, interfacing drives with Beebs (especially when using new drives, 
> or mixing 5.25 with 3.5, or picking the right DFS) seems a bit of a 
> black art.

Sadly, it *is* a bit of a black art, partly because some drives are 
manufactured specifically for the PC market, and may not have all the 
signals and jumpers that perhaps we think they should.  Things are not 
helped by the amount of erroneous or incomplete information on the net 
as a result of misunderstandings of that.

I didn't roll my eyes at *you*  ;-)   IBM, maybe :-)

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York