As time passes, more and more H89 computer owners are discovering
that it is feasible to upgrade the Heath H89 computer to run at a clock
speed of 4Mhz, instead of the standard 2Mhz. The potential throughput
improvements in processing capability and console speed are impressive.
And best of all, the necessary modifications can usually be performed
for as little as $25, depending on the speed of the RAM chips in the H89.
This article is intended to provide some general information about
the conversion process, without trying to include every conceivable patch
or trick, since that would be a Herculean undertaking!
The 4Mhz conversion includes the following basic steps:
1) Replace the Z-80 cpu chip which operates at a 2Mhz clock speed with
a Z-80A cpu chip which operates at 4Mhz. Z-80A chips are available
for as little as $7 to $8 in quantities of one (check BYTE magazine).
2) Replace the H-17 Disk Controller ROM (444-19), which assumes 2Mhz
operation, with a new ROM in which the 2Mhz dependencies have been
removed. The modified ROM can be used at EITHER 2Mhz OR 4Mhz.
The modified ROM can be obtained for $7.50 from Najay Systems.
(See the address at the end of this article.)
3) Replace none, some, or all of the 64K of RAM chips, as necessary.
This is the tricky part. Depending on the speed of the RAM chips
currently in your H89, you may have to replace all 64K, in the worst
case. This is usually the last thing to check; if you can run ok
at 4Mhz without touching your RAM, you're home free. You may be
able to determine the speed of some of the chips by contacting the
chip manufacturer and providing the part number. Otherwise, you
may wish to test your RAM in blocks of 16K and replace each set of
16K that is too slow. If money is no object, simply replace all 64K!
Some 4116 dynamic RAM chips are stamped with "dash numbers" such as
4116-2, or 4116-3. Unfortunately, these do not always have the same
speed definition from manufacturer to manufacturer, so beware!
The common 4116 RAM chip access speeds are summarized below:
(*) Advertised in a recent issue of Microcomputing magazine.
From the table above, it is apparent that 150 or 200 nanosecond
access time chips are preferred, 250 nsec chips are marginal, and
300 nsec or slower chips probably won't work. Some chips rated
at slower speeds MAY perform reliably at faster speeds, but why
take a chance?
In some cases H89 owners have also had to replace the 2 2114 RAM
chips on the cpu card with faster parts, but this is the exception
rather than the rule.
4) Modify the H89 cpu card so that the CPU, ROM, and RAM see a 4Mhz
clock signal, while the H88-1 disk controller card sees a 2Mhz
clock signal. This can be accomplished in several ways, some more
reliable than others. Najay Systems provides instructions on how
to do this for $7.50 (see below). This modification involves
cutting 3 traces and adding either 2 jumpers and 3 wires to a switch
or 3 jumpers without a switch. The switch allows you to easily
go back to 2Mhz operation, which is helpful when troubleshooting.
5) Make any necessary patches to the HDOS or CP/M software you use.
When I performed the 4Mhz mod, I was very surprised at how few
patches were required to run at 4Mhz. If you use the standard Heath
H17 SY DVD, the DG Electronics H17 SY DVD, or the Ultimeth V2.0 H17
SY DVD, NO patches are necessary for 4Mhz operation with either the
standard 100K hard sectored 5" drives or the 400K Tandon drives.
The HUG SY DVD (Ultimeth V1.0) will NOT work at 4Mhz. The Heath
CP/M BIOS V2.2.02 or V2.2.03 MUST be patched to work at 4Mhz. The
best collection of patches I have seen for CP/M is those compiled
by Bill Moss. These are available on the MicroNet HUG Bulletin
Board, and may be published in a future >CHUG newsletter, if there
is sufficient interest. The Livingston Logic Labs BIOS-80 for
Heath CP/M with the 400K Tandon drives has also been patched for
reliable 4Mhz operation, and recently shipped copies need no patches.
6) This step is optional, but is highly recommended if you wish to fully
realize the increased H89 console throughput capabilities available
at 4Mhz. If you have not already done so, you should now increase
the H89 terminal logic board clock rate to 3Mhz and the baud rate
from 9600 baud to 19200 baud. It is usually unnecessary to replace
the Z-80 chip on the terminal logic board to run at 3Mhz, but you
may wish to install a Z-80A to be on the safe side. Operation of the
terminal logic board at 4Mhz appears possible, but of dubious value
at the present time (a faster H-19 ROM will probably be required and
the 2114 display RAMs may have to be replaced with faster parts).
The switch settings for 19200 baud rate are as follows:
(*) The H19 rom doesn't seem to handle this combination reliably.
As you can see, the baud rate is the limiting factor on console
throughput at 9600 baud. Increasing the baud rate to 19200 with
a 2Mhz cpu clock and a 3Mhz terminal clock provides about a 20%
improvement, but the console is now cpu-bound. Only at a 4Mhz
cpu clock rate and a 3Mhz terminal clock rate is the full speed
of the 19200 baud rate realized.
One other note: I understand that the newer H-19A's and H-89A's
have significantly different internal printed circuit cards than
the older H-19's and H-89's. I have yet to hear of anyone who
has succesfully modified the newer models, but chances are that
the process will be very similar.
In conclusion, it is obvious that this article has omitted some of
the technical details of implementing the 4Mhz conversion. This
was intentionally done to keep it down to a reasonable size. If you
decide to go ahead with the conversion, I think you will be very
impressed with the results. Programs such as PIE, INVADERS, all
of the commonly used assemblers, compilers and basic interpreters
really come to life at 4Mhz and saving time is what computing is
all about! As mentioned above, the modified H17 ROM and detailed
instructions on modifying the cpu board can be ordered for $15 from:
George Najarian
Najay Systems
3136 Vermillion St.
W Covina, Ca. 91792
(714) 594-9564
P.S. Najay Systems is reportedly working on a newer, easier to
install 4Mhz modification kit which is a small PC board which
plugs into a socket on the H89 cpu card and may allow a software
selectable 2 or 4Mhz clock frequency with NO trace cuts or jumpers
required (of course it will cost more!). And for those with
even more money to spend, there's the new DG Electronics SUPER-89!
