. Modifying the H89 for 4Mhz Operation . . by John Stetson . . 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: . . Access time Clock Speed Approximate Cost(*) . ----------- ----------- ---------------- . 450 nsec 2Mhz ....... . 300 nsec 3Mhz $11/16K . 250 nsec 3-4Mhz $12/16K . 200 nsec 4-5Mhz $13/16K . 150 nsec 5-6Mhz $16/16K . . (*) 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: . . H89/H19 terminal logic card: . . Switch 401: 0 1 2 3 4 5 6 7 . 9600 baud: 0 0 1 1 . . . . . 19200 baud: 1 0 1 1 . . . . . . H89 cpu card with MTR-89: . . Switch 501: 0 1 2 3 4 5 6 7 . 9600 baud: . . . . . . 0 . . 19200 baud: . . . . . . 1 . . . The modifications to change the terminal logic board clock . rate are as follows: . . H89/H19 terminal clock speed selection: . . 2Mhz: jumper JP10 pin 2 to pin 5 . . 3Mhz: cut JP10 pin 2 to pin 5 . jumper JP10 pin 1 to pin 5 . . 4Mhz: cut JP10 pin 2 to pin 5 . jumper JP10 pin 5 to U413 pin 9 . . To see the reason for increasing the terminal logic board clock . rate, consider the following unscientific benchmark tests I ran: . . H89 4Mhz benchmark tests: . . List a 128 sector ASCII file on the console: . . 2Mhz H89 cpu, 3Mhz terminal, 9600 baud -> 36 seconds . 4Mhz H89 cpu, 2Mhz terminal, 9600 baud -> 36 seconds . 4Mhz H89 cpu, 3Mhz terminal, 9600 baud -> 36 seconds . . 2Mhz H89 cpu, 3Mhz terminal, 19200 baud -> 29 seconds . 4Mhz H89 cpu, 2Mhz terminal, 19200 baud -> 22 seconds (*) . 4Mhz H89 cpu, 3Mhz terminal, 19200 baud -> 18 seconds . . (*) 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! . . EOF