3.5" Altair CP/M

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