Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!utgpu!water!watnot!watmath!clyde!rutgers!mit-eddie!uw-beaver!ubc-vision!alberta!calgary!ingoldsby
From: ingoldsby@calgary.UUCP
Newsgroups: comp.sys.m6809
Subject: Making old CoCo Disk Controllers work at 2 Mhz
Message-ID: <873@vaxb.calgary.UUCP>
Date: Sat, 11-Apr-87 20:00:39 EST
Article-I.D.: vaxb.873
Posted: Sat Apr 11 20:00:39 1987
Date-Received: Sun, 12-Apr-87 20:04:56 EST
Organization: U. of Calgary, Calgary, Ab.
Lines: 36
Keywords: CoCo 3, Disk Controllers, 2Mhz troubles with OS9 Level 2



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!ingoldsby