My Motorola MEK6800D1 Computer System

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 the electronics distributor's store (name?? 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.

Pictures of a MEK6800D1 similar to mine (68-72KB .jpg).

I don't remember all the reasons for choosing the MEK6800D1 as my first computer. 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, 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. Two MC6820 PIA devices are included: one is used to implement a bit-banging serial port for the RS232C or TTY current loop terminal interface running at 110 or 300 bps (with the bit rate selected by adjusting two potentiometers), the other is available for general use. 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 to support an additional serial communication channel, but its 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 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, nor any keyboard or display, 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 had very limited electronics experience, and my electronics tools were mainly an inexpensive analog multi-meter and ungrounded soldering iron, both from Radio Shack. I remember building a frequency counter on perf boards from an article in one of the electronics magazines, but am not sure whether this was before or after I assembled the MEK6800D1. (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 project was to design and build a power supply for it. Many of the components, such as the large electrolytic capacitors, were surplus parts from a mail order supplier. I haven't powered it up for over 20 years, so don't know whether it still works.

(Click on a picture for a larger version.)
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 keyboard and serial interface, 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 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 a cassette tape interface/controller to control two tape recorders, similar to the SwTPc AC-30, based on a circuit design I found in a magazine article. 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 record player 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 them. 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.

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 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.

In 1980 a 6-slot EXORbus expansion motherboard [1] [2] became available for the 6502-based AIM-65 single-board development computer. If something like this had been available in 1976, but for the MEK6800D1 to plug into, I might have been interested.

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. Some of the components were sold for nominal sums decades ago, the rest were probably just put into the garbage before I realized they would be worth something years later.

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!

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Last revised 2020-Apr-17 14:20 PDT.
Copyright 2020 by David C. Wiens.

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