PC6300 cmos clock strangeness

[Bill Mayhew]

The PC6300 clock is odd.  The design predates the IBM AT computer,
and thus is not wired in a fashion that is compatible with the AT.
In order for the clock to work correctly, it is advisable to run the
machine with AT&T-branded msdos release.

There are some bugs in the AT&T dos.  Early releases of 3.2 reset
the RTC seconds register to 00 every time the machine was rebooted.
I believe 3.2 relase 1.02 or higher did not have the flaw.  A patch
was circulated on the net to fix early releases of 3.2.

DOS 3.3 does not have the RTC set bug.  3.3 also knows about the
newer AT&T machines that do have AT-compatible clocks.

IBM DOS on the 6300 goofs up the time of day while DOS is running.
The 8253 timer is programmed by the BIOS to tick at a different
rate on the 6300, which makes the clock run about 10% fast when the
machine is booted from IBM-branded DOS.  It is possible to reset
the 8253 timer on the fly with a couple of outputs to the control
port.  I recall a long time ago, I used DEBUG < TFIX in my
AUTOEXEC.BAT file to affect the fix when I ran IBM DOS on my 6300.
I don't recall the numbers any more.  TFIX just contains the
commands as you would have typed them manually into DEBUG.

Another thing about the 6300.  The RTC clock battery is a
nickel-cadmium battery.  The machine needs to be run for a few
hours every couple of days or so to prevent the battery from going
dead.

==Bill==
-- 

Bill Mayhew  Northeastern Ohio Universities College of Medicine
Rootstown, OH  44272-9995  USA    216-325-2511
wtm at uhura.neoucom.edu   ....!uunet!aablue!neoucom!wtm