HD drives and the PCjr

From: Fred Cisin <cisin_at_xenosoft.com>
Date: Wed Apr 10 12:07:03 2002

On Tue, 9 Apr 2002, Cameron Kaiser wrote:
> What is the highest DOS I can run on a PCjr?
ANY! DOS!, NOT WINDOZE 3.10 or above; NO WINDOWS95.
But each successive version of DOS was bigger. If you don't further
expand the memory, then later ones (5, 6.2x, 7.10, etc.) will be too
cramped to have enough space to run most programs.
I recommend NOT using anything before 2.00.
And, if you use the Qumetrak 142 drive that IBM used, those drives are too
slow for 2.00, which is why 2.10 was created.
Thus, PC-DOS 2.10 is the ideal version of DOS to use unless/until you
expand the memory.
If/when you expand the memory to 640K, then I'd recommend running
6.2x. <RANT> 6.2x is the only product in Microsoft's history where the
primary design goal was to improve reliability rather than adding bells
and whistles. </RANT>

> And how can I tell the
> difference between a 720K drive and a 1.2MB 5.25" drive? (See, I really *am*
> ignorant about old PC systems. ;-)

Model number is the easiest way (although Tony might prefer tracing the
signal from the write-current level pin)
If you find a drive with an asterisk on the front, it is a very late model
360K. If there is no asterisk, then it is a 360K, 720K, or 1.2M (there
also exist a few "weird" ones)

> Is a 3.5" disk really practical in the jr?
720K is.
1.4M would require a different disk controller. Although a
controller to do 1.2M (5.25") and 1.4M (3.5") could be relatively easily
designed and manufactured, I don't recall ever seeing one for the Jr.

The 720K is fully hardware compatible with the Jr. You would need to use
either special patches to the OS, or use PC-DOS 3.20 or newer (using
When used with the old style mounting brackets, it can bolt in in place of
a 5.25.
A "dual" or "combo" drive could be used to give you both a 3.5 and a 5.25
in the stock Jr case. The 3.5 would operate as a 720K, and the 5.25 could
be used as either 360K or 720K, although I expect that you'd have to write
a device driver to get the 360K mode (double stepping) to work, and you'd
have to jumper the drive for 300RPM spindle motor speed, since the Jr
doesn't provide a 300K data transfer rate to be able to use the
360RPM as the AT did.

Grumpy Ol' Fred        cisin_at_xenosoft.com
Received on Wed Apr 10 2002 - 12:07:03 BST

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