ingoldsby@calgary.UUCP (04/12/87)
Hear ye, hear ye all owners of old colour computer disk controllers that
have had troubles making them work at 2Mhz (with OS9 level 2).
I have been complaining that I couldn't use double sided drives with
my Coco 3 and old disk controller under OS9 Level 2. I presumed it was a software
problem since the side select is determined by enabling `drive 4' and I thought
that the logic driving drives 1-4 was all the same. It isn't. I began doing
some hardware debugging and noticed that the drive 4 enable comes from a
different chip than drives 1-3. It turns out that all the drive selects pass
through CMOS D latches. Drives 1-3 through one chip and drive 4 through
another. Upon checking with a logic probe, I discovered that the side select
(drive 4) line was occasionally sticking ON, even though a signal was being
sent to reset it. As most of you know, CMOS is very slow, especially at 5 volts.
Since OS9 Level 2 doubles the clock speed of the original Coco, it appears that
the CMOS sometimes can't keepup (everything else should be OK). To compensate
I replaced U5 (74LS04), U4 (74LS00) and U9 (74LS02) with the 74ALS series
equivalents. This helps in two ways. ALS series logic is faster than LS and
this meant that ALS series would get the control signals to the CMOS latches faster.
Also, ALS logic has slightly better drive capacity than LS so it performs
superior in driving the highly capacitive loads presented by the CMOS. Since
only 1 of the two CMOS chips in my computer was too slow (they both use the
exact same control signals) I reasoned that it might be performing near the
maximum end of specifications and replaced it in the hope that the new one would
be closer to nominal or typical specs. In any case, my controller works just
find now, even with 80 track double side drives.
If you are having problems with your controller, try the above solution. If
the problem is not solved it may be necessary to make wiring modifications to
replace the CMOS with TTL or Hi-speed CMOS D latches. Does anyone know why
they used CMOS anyway?
Good Luck.
Terry Ingoldsby
ihnp4!alberta!calgary!ingoldsbyknudsen@ihwpt.UUCP (04/15/87)
In article <873@vaxb.calgary.UUCP>, ingoldsby@calgary.UUCP (Terry Ingoldsby) writes: > Hear ye, hear ye all owners of old colour computer disk controllers that > have had troubles making them work at 2Mhz (with OS9 level 2). > > If you are having problems with your controller, try the above solution. If > the problem is not solved it may be necessary to make wiring modifications to > replace the CMOS with TTL or Hi-speed CMOS D latches. Does anyone know why > they used CMOS anyway? > Terry Ingoldsby Nice sleuthing there. Didn't know you could hack HW too! I wonder if you couldn't just replace the 74C chips with 74LS directly (no wiring mods). Why did they use CMOS? Good question. I wouldn't want it near an external bus -- easily damaged by static, and rather wimpy on the drive as you point out. Maybe Tandy wanted the rise/fall times on the bus to be sluggish, to cut down on RF emissions. Our FCC was very tight about that stuff on the early home computers. You probably know about the 3 capacitors and 1 resistor that should be removed from Coco 1s to help them run the Speed Poke. Sort of like bypassing/removing the smog control gadgets on car engines (does Canada have that stuff yet? Do you need it yet?) I have a friend who'll appreciate this info you posted. Thanks, mike k -- Mike J Knudsen ...ihnp4!ihwpt!knudsen Bell Labs(AT&T) Delphi: RAGTIMER CIS: <memory failure, too many digits> " ~E(x):[is_lunch(x) && cost(x)==0] "