On 10/29/24 12:09, Tony Duell via cctalk wrote: > I wonder if the same heads (with different diameter pulleys on the > stepper motor) were used both 96tpi and 100tpi drives in some cases. > You'd get away with it if the head was made for 100tpi I think.
Somewhere in my pile, I have a TM-100-4M with a paper label under the door latch that says "96 tpi" (it's not). Micropolis, of course, used leadscrew positioning, rather than taut-band, (something that they were very proud of) so the difference there was substantial. I seem to recall that the Micropolis (the "M" in TM-100-4M) employed a slightly different pinout for DS3 and DS4 selects. MPI also made 100 tpi drive variants; I don't recall if Teac ever did. The location of track 0 is radically different in the 96 tpi and 100 tpi conventions--there's about a 6 track offset. 100 tpi drives were also spec-ed as being 77 track (like their 8" relatives). As regards reliability between the two conventions, that's pretty hard to nail down. Micropolis was an early adapter of GCR encoding in their own controllers, while the rest of the world, by and large, used FM or MFM encoding. So, for example, one could fit 12 sectors of 512 bytes on a track using the Micropolis technique, while the limit for MFM was 10. My feeling is that using the same encoding, the difference between the two, reliability-wise, was negligible. > I've had floppy disks that will reliably format and work in 40 > cylinder drives (data readable at least a year later) but which threw > up errors if you tried to format them in (standard data rate) 80 > cylinder drives. I never got to the bottom of that. There, the degausser (bulk eraser) is your friend. >>> 3.5" drives tend to be 80 cylinder, 135 tpi. A few Japanese-origin systems retained the 77 cylinder color of 8" drives. In addition, the rotation rate was 360 RPM, not 300 as most of the West uses. I suspect that it's the reason that the so-called "1.2M) 5.25" drives also spin (default) at 360 RPM. Indeed, there were Japanese-origin 5.25" drives that were not capable of supporting the lower data rates of "360K" drives. In retrospect, this was an eminently common-sense approach: no difference in disk format regardless of physical embodiment. For whatever it's worth, Chuck
