3.5" Altair CP/M

[Chuck Guzis]

On 3 Jun 2007 at 2:38, Grant Stockly wrote:

> BYTES REMAINING ON B: 69K

Doesn't sound too far off from right.

Consider that the Tarbell's probably using 35 cylinders on one side 
at the 250KHz clock single density.  Let's run some numbers for 128 
byte sectors, of which we can safely put 16 of them to the track:

35 * 128 * 16 = 71680 bytes.  Now, if we take a 1K = 1024 bytes, 
that's 70K.  Reserve a track or two for a boot track...

You'd be better off calling your 3.5" drive an 8" unit to get 250K 
(SS) on a disk.  Okay, you're going to say, but what about the deal 
with the 1.44MB drive being 300 RPM and not 360?  Won't that cause a 
problem.

Yes, but--you can fix the problem--several ways. 

About a year or two ago, I wrote to Herb Johnson on his 
retrocomputing site about how to modify some models of Teac FD-235HF 
drives to run at 360 RPM.  You see, for many years, a lot of Japanese 
equipment requires a 360 RPM drive and many manufacturers have a way 
to accomplish this--even if they don't document it particularly well. 
 I believe that FD-235HG's bring the speed select out to the edge 
connector, but they're harder to find on this side of the pond.  
Almost all NEC 3.5" HD drives can be modified for 360 RPM use.  

The other alternative is to swap the crystal on your Tarbell 
controller for something that's a bit slower to make the drive look 
as if it's running at 360 RPM.  Say, for sake of argument that the 
crystal is 8 MHz.  By substituting a crystal for (8*(300/360)) = 
6.6666...7MHz, things will work as expected, albeit somewhat slower, 
but still faster than the 5.25" mode.

You could also modify the formatting software to lay out a longer 
track (i.e. increase the number of end-of-track "run out" bytes) or 
even format the diskettes using something like 22Disk, which doesn't 
care if the physical track is too long.

You could also modify the formatting software and CBIOS to handle 80 
track 16-sector disks.  Not as good as simulating 8" drives, but 
better than 60K.  You could also modify it to use larger sector 
sizes.  I don't know if the Tarbell controller handles double-sided 
drives, but that's another possibility.

Early 5.25" drives were really pretty wimpy things compared to the 8" 
heavy iron. :)

Cheers,
Chuck