My Motorola MEK6800D1 Computer System

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I bought my Motorola Evaluation Kit MEK6800D1 on July 2nd, 1976 for C$267 -- my first ever "computer"!!

This was shortly after I started my first job after graduating from the two-year diploma course "Computer Programming and Systems" at BCIT. While buying the MEK6800D1 at Bowtek Electric (a Motorola distributor at Oak St. and 8th Ave. in Vancouver BC), I also saw a notice posted on the wall asking if anyone would be interested in forming a computer club.

(Click on a picture for a larger version.)

icon to link to photo of top side of MED6800D1 board icon to link to photo of bottom side of MED6800D1 board
Pictures of a MEK6800D1 similar to mine (68-72KB .jpg).

I don't remember all the reasons for choosing the MEK6800D1. I was interested in the Motorola 6800 microprocessor, partly because it seemed to have a nice instruction set that was influenced by the DEC PDP-11 (which I was also interested in), that it used memory-mapped I/O, and that it was big-endian. (Was this preference due to having spent the previous 2 years programming an IBM System/370, or because a memory dump of 16-bit or 32-bit big-endian values was easier to read than a little-endian dump?) In hindsight I should have bought a SwTPc 6800 computer kit for my first computer, but didn't. Instead I chose the MEK6800D1 single-board evaluation kit, maybe because the upfront cost was lower?

The MEK6800D1 was released in 1975 as a standalone system for the evaluation of the MC6800 microprocessor and its related peripheral chips. Two MC6820 PIA devices are included: one is available for general use, while the other is used to implement a bit-banging serial port for the RS232C or TTY current loop terminal interface running at 110 or 300 bps. (These bit rates are set by adjusting two potentiometers in an R/C oscillator connected to an MC14536 programmable timer.) The board supports up to six MCM6810 SRAM chips, providing 128-640 bytes of SRAM for user programs and 128 bytes of SRAM for the MIKBUG firmware, which is in a mask programmed MCM6830L7 ROM (p.2-169). An MC6850 ACIA was also included in some versions of the kit to support an additional serial communication channel, but its 16x bit rate clock needed to be supplied from an off-board circuit via connector P2 -- an MC14411 bit rate generator chip with crystal controlled clock was suggested.

An 86-pin Motorola EXORciser bus (EXORbus) connector allows for further expansion. Refer to Table 2-6 of the M6800 EXORciser User's Guide for more details on the bus, and to the assembly instructions for the evaluation kit.

Of course the MEK6800D1 wasn't a complete computer, as it didn't include any power supply, keyboard, display, or cassette tape interface, unlike the 6502-based KIM-1, or the later and more expensive MEK6800D2. But over the next two years I built it into a useful, though limited, computer system.


Before assembling the MEK6800D1 kit I had hardly any electronics training or experience, and my electronics tools were mainly an inexpensive analog multi-meter and an ungrounded 27W soldering iron, both from Radio Shack, and a 100/140 watt soldering gun. I remember building a frequency counter on perf boards from an article (maybe this one - pp.48-49) in one of the electronics magazines, but this might have been after I assembled the MEK6800D1, not before. (The frequency counter didn't work very well, possibly due to bad solder joints, intermittent shorts, damage from static electricity, or defective surplus parts?)

After assembling the MEK6800D1 board, the next task was to design and build a custom linear power supply for it. Many of the components were surplus parts from a mail order supplier. It wasn't beautiful, but it worked, supplying:
 unregulated: +10V, +16V, -16V
  top heatsink: +12V from a 7812 regulator in a TO-220 package
  middle heatsink: +5V from an LM309AK regulator in a TO-3 package, rated up to 1 amp
  bottom heatsink: -12V from a (part number faded) regulator in a TO-3 package
  side heatsink: +5V from an LM323K regulator in a TO-3 package, rated up to 3 amps

I'm not sure why I used a more powerful regulator and better heatsink for -12V instead of for +12V. I probably added the second +5V regulator (the side unit) later when I installed the power hungry SRAM and graphics cards.

(Click on a picture for a larger version.)

icon to link to photo of front and side of power supply for MEK6800D1 icon to link to photo of inside of power supply for MEK6800D1
My power supply for the MEK6800D1 (161-177KB .jpg).

Next I needed a way to communicate with the MEK6800D1. Although I drooled over a Teletype ASR-33, as it would provide keyboard, display, printed hard copy, as well as non-volatile storage (via paper tape), all in one device, the price of even a used one made me hesitate, and it was slow, noisy, and heavy. So, a few months later I instead bought the SwTPc CT-1024 TV Typewriter II kit (with serial interface, computer controlled cursor option, power supply, and keyboard), then assembled it. It could display 16 lines of 32 uppercase characters. I used an external RF modulator to connect the CT-1024 to a 12" B&W AC/DC TV. (I had planned to modify the TV for composite video input, but never did.) The CT-1024 and RF modulator generated lots of RF interference. I added aluminum sheets to the outside of the wooden enclosure I had built, but at the time I didn't understand how to design proper shielding, so it wasn't very effective. But it was exciting to see that whatever I typed on the keyboard was displayed on the TV screen!

After connecting the power supply and CT-1024 to the MEK6800D1 board, the little 6800 microprocessor finally came to life, allowing me to play with its MIKBUG monitor, and enter and run simple programs that I hand-assembled. I had been waiting for this day for over 13 years, ever since my interest in computers was first kindled!!!


By this time the WCCS computer club had formed and was very active. One of our members designed an 8KB static RAM board for the EXORbus. I bought a bare board, located the components for it, and assembled it. But now I needed a way to connect the two boards together, so I built a wooden chassis with three 86-pin card-edge connectors wired together. Now sporting a total of 8.25KB SRAM, my computer was beginning to be capable of some serious processing!

A computer isn't very useful without some kind of non-volatile storage, since typing in the object code of a large program every time the power fails, or a software bug clobbers the program, isn't enjoyable. I designed and built an interface/controller to control two cassette tape recorders, similar to the SwTPc AC-30, based on a circuit design I found in a magazine article (maybe this one?). It worked, but wasn't always reliable. Any read error would require me to rewind the tape and start from the beginning, so it was very time-consuming to do software development with it.

I bought an Oliver Audio Engineering OP-80A manual paper tape reader. I can't remember exactly how I connected it (to the MEK68800D1's 6820/PIA?) or what I used it for, but it might have been to read a bootstrap loader (that controlled the dual-cassette tape interface/controller) into memory? I punched the paper tape on an ASR-33 at the local Byte Shop computer store (on Burrard St. near 6th Ave in Vancouver BC), where I had bought the tape reader.

The May 1977 issue of Interface Age magazine included a thin plastic record (a "floppy ROM") containing Robert Uiterwyk's 6800 4K BASIC in Kansas City standard audio format. I must have had problems getting the connection between my turntable and tape interface working correctly, because I remember that more than once I carefully and slowly typed in the complete object code (from the listing in the magazine) into RAM, then saved (or tried to save) it to cassette tape.

I created and used my own very crude text editor. But I don't remember having a 6800 assembler for this system, so I probably just did lots of hand assembly. I soon had memorized the machine code values for most of the 6800's instructions! However, it's possible that later on I typed in and used the object code for the Tiny Assembler for the 6800 by Jack Emmerichs, as published in the April and May 1977 issues of BYTE magazine.

Getting a printed copy of what I created with the text editor, or generated with the programs I wrote, would be nice. So in February 1978 I bought my first printer, a 40-column (uppercase only) SwTPc PR-40, for C$380.

Seeing some other computers, such as the new Apple II, generate graphics, made me want to do the same. I bought a bit-mapped monochrome graphics board that was originally designed to plug into the KIM-1, since the 6502 bus timings were similar to the 6800. This board was probably the 320x200 resolution K-1008 Visible Memory board from Micro Technology Unlimited (refer to MTU catalog). I built an adapter board to let it plug into the 3rd slot of my EXORbus chassis. When not displaying graphics it could also be used as an 8KB SRAM board. Software development on my system was awkward and slow with the unreliable tape cassette storage, so I never did much with this graphics board. I tried to run some of Bruce Artwick's 3D graphics code in BASIC (the forerunner to his Flight Simulator program) that was published in the October 1977 issue of Kilobaud Magazine.

I had problems getting the graphics board to run reliably, possibly due to the MEK6800D1 not using a crystal to generate the 6800's clock signals. Instead it used two one-shots with potentiometers to generate the non-overlapping phase 1 and 2 clocks, but I didn't have any oscilloscope to calibrate the frequency and phase widths. (Maybe that's when and why I built the frequency counter? But it wouldn't have told me the relative widths of the two phases.) In hindsight I should have built a crystal-controlled clock circuit, an example of which was shown on p.4-4 in the M6800 Microprocessor Applications Manual which came with my MEK6800D1. (The SwTPc MP-A2 used a circuit similar to this.) The MC6870A clock chip, which was a hybrid device including a crystal, crystal oscillator, and TTL and two phase MOS drivers, had become available by this time, but I don't know if I was aware of it, and it was expensive at around U$35 each.

Moving On

By now I was beginning to be dissatisfied with my MEK6800D1-based system. In order to be more productive in developing software, I needed to add a floppy disk controller and drives, more RAM, and possibly more I/O ports. EXORbus boards were either difficult to find, or very expensive, but I didn't want (and didn't yet have the skills) to design and build and test all these boards from scratch. In any case, my system only had two expansion slots, both of which were already occupied, and both of which were connected to the unbuffered bus from the 6800 microprocessor chip on the MEK6800D1 board. So, almost exactly two years after buying the MEK6800D1 board, I bought my SwTPc 6800 computer system, which I probably should have done in the first place.

Recently I discovered that a 6-slot EXORbus expansion motherboard [1] [2] became available in 1980 for the 6502-based AIM-65 single-board development computer. If something like this had been available in 1977, but for the MEK6800D1 to plug into, I might have been interested. But it was rather expensive.

The only part of my MEK6800D1-based computer system that I still have is the power supply. Unfortunately I can't find pictures of any other parts of the system. I vaguely remember selling the three boards and the chassis for a few dollars at a computer swap meet decades ago.

I also kept (and still have) the thick Motorola M6800 Microprocessor Applications Manual that came with the MEK6800D1 kit, which gives all the information necessary to design a complete point-of-sale terminal with keyboard, display, bar code scanner, floppy disk controller, modem interface, printer interface, etc. It includes schematics, source code (in 6800 assembler), and explanations of how it all works. It also shows how to interface a memory board to the EXORbus (page 4-20 and pages 4-65 thru 4-87). This book was a real treasure trove for its time!

Restoring the Power Supply

In May 2020 I started restoring the power supply. Since it hadn't been powered for many (maybe more than 25) years, reforming the ten Sprague 5000µF 30VDC electrolytic capacitors (date code 7608) seemed like a prudent course of action. The bus wires blocked access to many of the screws holding the solder lugs to 8 of the 10 capacitors, so I had to cut off the bus wires to remove the capacitors. Physical inspection didn't show any signs of bulging or leaking electrolyte. After washing off almost 44 years of dust they all looked brand new again.

icon to link to schematic of electrolytic capacitor reforming circuit icon to link to photo of electrolytic capacitor reforming circuit board I built a simple circuit for controlling and monitoring the reforming process for one capacitor at a time. It starts at +5V through a 94kΩ series resistor, increasing to +12V, then +24V, then switching to a 10kΩ resistor. When the leakage current settled at a low level, I considered the reforming done and discharged the capacitor through a 2kΩ resistor.

After 4 to 12 hours of reforming, all ten capacitors had a leakage current under 6µA when supplied with +24.3V thru a 10kΩ resistor, which is excellent. I also measured the charge and discharge time of 2 of the 10 capacitors to estimate their capacitance -- both were still better than nominal. But I don't have any equipment to measure their ESR, which can be degraded even when the capacitance and leakage are OK. I couldn't test any of the 8 other smaller capacitors (tantalum and other types) in the power supply because they are soldered directly to the voltage regulator leads.

I built a new set of bus wires to connect the 8 capacitors in the +10VDC (unregulated) circuit, since I had cut the original set to remove the capacitors. But I didn't finish the restoration at this time.

Over a year later, in August 2021, I discovered that one of the ten capacitors had leaked lots of electrolyte. Maybe the heat from soldering the bus wires to the solder lugs last year damaged it? The thick copper bus wires drew away so much heat that I had to use my 100/140 watt soldering gun instead of my 65W temperature-controlled soldering iron. (The solder lugs were screwed to the capacitors while soldering, to hold them in position, but I had used thin cardboard on both sides of the lugs to minimize heat transferred to the capacitors. But I hadn't used cardboard 45 years ago when I soldered the original set of bus wires.) I hoped the other capacitors were still OK, and decided to rebuild the +10VDC circuit with only 7 of its 8 capacitors.

icon to link to photo of top side of power supply test load icon to link to photo of adapter cable for power supply test load icon to link to photo of bottom side of power supply test load I built a new power supply test load board and cables to be able to measure the ripple in the input voltages to the regulators while under load. Jumpers allow me to select the +5V load from 333mA to 3.33A in 333mA increments, and the +12V or -12V load from 120mA to 1.44A in 120mA increments. The 5W resistors only dissipate approx. 1.5W to keep them from getting too hot.

A few weeks later, after cleaning the rest of the power supply and re-installing 9 of the 10 large capacitors, and a cursory inspection to see that everything else looked OK, I turned on the power -- and was immediately met with a very loud "snap" sound, and the release of some magic smoke! Uh, oh! Fortunately all three voltage regulators on the front are connected to a terminal block, making them easy to remove and re-install. Closer inspection found that a capacitor (probably a tantalum) on the input of the +12V regulator had exploded. I replaced it with two 0.1µF 100V ceramic capacitors in parallel.

Now turning on the power didn't result in any sound effects! All four regulated output voltages (no load connected) were within acceptable limits: +4.95V, +4.99V, +11.68V, -12.14V. But I was surprised when I used my DMM to measure the unregulated input voltages, as they were much higher than the labels on the front panel indicated: +14.22V vs. +10V, +26.4V vs. +16V and -26.4V vs. -16V. Were they always that high? The 26.4V at two of the large electrolytic capacitors is much too close to their 30VDC rating, especially considering how old they are. I'm not sure what the maximum allowed input voltages were for the original voltage regulators, but newer data sheets indicate I'm still within their absolute maximums, just that the increased power dissipation reduces the maximum output current they can supply without overheating. But the high voltage might explain why the old tantalum capacitor exploded?

On 2021-Sep-11, after having the power supply turned on for several hours, I used my oscilloscope and new test load board to measure the voltage, ripple, and noise on each of the unregulated and regulated supplies. The regulated voltages were measured at the test load board, with a total of 28" of wire between the power supply and test load, which caused the voltages to droop somewhat. I'm not sure how accurate my noise and ripple measurements were on the regulated voltages, so won't report all of them:
 +10V @ 1A load:   +13.05V to +13.21V, 160mVp-p ripple
 +5V (side) @ 1A load:  +4.87V (+4.97V close to regulator), 12mVp-p ripple, 2-5mVp-p noise?
 +16V @ 120mA load:  +25.85V to +26.01V, 160mVp-p ripple
 +12V @ 120mA load:  +11.64V
 -16V @ 120mA load:  -25.90V to -26.05V, 150mVp-p ripple
 -12V @ 120mA load:  -12.02V

I consider these results to be acceptable. I didn't replace the thermal paste between the regulators and heat sinks since I don't plan to use this old power supply much. This completes a successful restoration!

In the future I will try to remember to use my variable autotransformer with this power supply to reduce the 120VAC input voltage to 90VAC in order to reduce the voltage applied to the regulators. This will reduce stress on the capacitors, and reduce the heat dissipated by the regulators, hopefully making this power supply last longer.

Last revised 2024-Apr-04 12:23 PDT.
Copyright 2019- David C. Wiens.

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