Re: Generic 1.44Mb on PS/2 systems (was "Auction: Nice Model 77 on the bay")

Hi Peter,
...The stepping stone on 3.5" is the reversed function of the "Media
size" function on PS/2 - brought in contact with Pin #2 called "-
High Density Select". On PS/2 the FDD controller does a "blind
read" of the first sector and gets the media byte and sets the
controller speed and number of sectors accordingly. On
genericas the switch on the FDD front detects a hole (not
pressed = 1.44MB) or not (pressed = 720K) and signals the
condition *to* the controller. If you leave out the -High Density
Select pin it will however work in most cases. Many drives have
the trace to pin 2 broken anyway (like the Sony MPF920 I have)...
An ALPS and NEC 1.44Mb drive I have here seem to be out, but the
Sony, Teac, Panasonic, Epson, & Mitsumi work great! All of those passed
reading both 720Kb & 1.44Mb diskettes. More on other tests below.

...Another so far ignored topic is the use and handling of the Pin
#34 "Diskette Changed" on the interface. As far as I recall it has
no less than 3 different purposes - either one, but not all 3 at
one time, but the PS/2 use the lesser used...
Changeline support works on all functional drives tested here
(Model 50Z planar; I'll test the Model 30 8086 later)...

My experiments with the Sony failed in most points. The drive
is detected, identified as 1.44MB / 3.5" and if addressed it
starts spinning the media ... but fails to read from it.
I have a NEC drive FD1231T which is "made to fit" already,
but I haven't got the chance to test it so far.
Write-protect is sensed on all functional drives too. I'm out for
a few days, but once back I can show the schematic for possible
reduction (7407s not needed to buffer internal drives maybe). The
circuit should work great, but is a little beyond trivial.
David
David@xxxxxxxxxxxxx

.

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