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NOCONA ELECTRONICS • Box593 • Nocona, TX 76255

Sprinter II

Triple your CPU speed with this board from Holmes Engineering

Models l/lll, PMC-80, LNW80 Jim Klaproth, Associate editor

It seems that nowadays the owners of TRS-80 microcomputers have an unlimited selection of addon hardware to enhance their tiny machines. Any day now, I expect to be able to upgrade my Model I to a Model III just by plugging in a board. My Model I is already so crammed full of hardware modifications that it is becoming difficult to even get all the guts back in the case. However, I had never taken the plunge and installed a speed-up modification. When I saw the ad for a new speed modification that simply plugged in, already had the faster Z-80B chip on it, and allowed operation at 5.3 Mhz, with an unconditional thirty-day money back guarantee, I simply had to try one out.

A slow clock One of the limiting factors of the Models I and III is the slow clock speed of the the CPU. The Z-80 microprocessor is rated for 4.0 megahertz (Mhz) operation, but Radio Shack chose to run it at 1.77 Mhz on the Model I and 2.03 Mhz on the Model III. By simply allowing the Z-80 to run at its full rated speed, we can see a two-fold improvement in actual computing time. Taking it one step further, if we triple the clock speed we should get a three-fold improvement. The only way to accomplish this is to replace the Z-80 chip with a Z-80B, which is rated for 6.0Mhz. Enter the Holmes Engineering Sprinter II. What this board does is to take the original clock signal, which is at 10.6445 Mhz (Model I) and send it through a programmable divide-by counter. This counter then divides the 10.6445 Mhz pulses by 2,3,4,5,6, 7,8 or 9 to arrive at 5.32, 3.55, 2.66, 2.13, 1.77, 1.52, 1.33 and 1.19 Mhz respectively. These clock pulses are then routed to the Z-80B mounted on the Sprinter II, which replaces the original Z-80 in the TRS-80.

A software gear shift

The actual clock frequency is determined by a simple software command that can be given in BASIC or machine language. The BASIC command is simply: OUT 254,(X), where X is an integer from 0 to 15. That means there are actually sixteen separate combinations available to the user. There are eight different clock speeds, each with the option of running with or without wait states. Running with wait states means that a small delay will be added to every memory access, slowing down the actual speed by about 10%. Holmes guarantees that any computer will run at 3.55 Mhz with wait states. Anything above that speed, it is hit or miss.

Hardware limitations

Every machine is different and if even one component is marginal, it may interfere with high-speed operation. The maximum speed that an unmodified TRS-80 will run is 5.32 with wait states. When I called Holmes to get the information on the device, they asked what my configuration was. I told them I had a suffix G keyboard with 3 chip ROM set, expansion interface with buffered cable and DIN plug, with a mix of Motorola and NEC RAM chips. The technician told me that my machine would probably have to be modified in order to run at 5.32 Mhz, but to try it out unmodified first. I did, and was pleasantly surprised to discover that it worked perfectly!


Installation was a snap. The Sprinter II simply plugs into the socket after removing the Z-80 from the computer. Then there are four wires to attach to the computer circuit board. This is accomplished with solderless clips that simply push on. If you are skillful with an iron, you can cut off the clips and solder if you wish. Several photos are supplied with the documentation to aid you in locating the proper points to fasten the wires. It took me about twenty minutes to do the job. There is one modification to the board that is recommended for all machines that involves simply cutting a trace on the circuit board. There are other modifications in the documentation, but none of them applied to my machine. If you have trouble getting it to work properly, Holmes will recommend solutions or they will modify your interface for only $25.

Testing the machine at various speeds involves simply issuing the command OUT 254,(X) and then running a program to see if it crashes. My machine ran at every speed except at 5.32 Mhz without wait states. I have tested out every one of my programs using 5.32 Mhz with wait states and none of them have crashed. I am also happy to report that Holmes has provided an option of hardware switch to disable or enable the slowdown of the clock during disk accesses. Since NEWDOS/8O will run successfully with no slowdown, I decided to order this option with the unit. By setting SYSTEM option BJ = 3 and disabling slowdown, I can now run at three times normal speed all the time. It seems to speed up disk I/O considerably and is required to run eight-inch disk drives in double density.

Holmes also has an option of an on-board parallel printer port. This is for those who do not have an expansion interface. It is the same as the Centronics port on the expansion interface; however, the edge connector is different. It uses the same connector as the Model II does, so a Model II cable will work. Holmes will supply a cable for only $19.50 versus $39.95 for the Model II cable. It uses the same commands, LIST and LLIST, as the standard printer port and no initialization commands are necessary as in the speed-up modification. Holmes also makes a 48K memory expansion board that plugs right into your keyboard.

After running all of my programs without a single glitch at triple speed, I can say without reservation that I highly recommend this product. The improvement in computing speed is fantastic! Graphics games in BASIC are greatly enhanced but sometimes delay loops will be needed to slow them down. All in all, I was very impressed with the quality of the hardware, the helpfulness of the staff, and (best of all) the performance of the product. Now, Holmes, how about that Model III converter board? ■

Ed. note — The Sprinter II is available from Holmes Engineering, 3555 South 3200 West, Salt Lake City, UT 84119. The price is $99.95. The optional Parallel Printer Port is $24.50.

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