29 Oct
2024
29 Oct
'24
7:55 p.m.
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