Motorola MEK6800D2 computer

This Web page last updated date Jan 10 2015; links modified and added Aug 26 2019.


[Motorola 6800 D2 manual]

I acquired this 6800 D2 Evaluation Kit many years ago. In March 2013 I was going to show it at the Trenton Computer Festival 2013, but it was rejected because it was not running, and I had no time to check it out. But I took time after the event to repair it, and I was surprised that it came up without a lot of fuss. The photo at left shows the User's manual and is also a photo of the keyboard & display of the D2 kit. I sold this kit in Aug 2013.

This is part of other vintage computer repair and restoration work I've done. I did some work on a related 6800 system from SWTPC, but it's not had power as of mid-2013. Also look at a Motorola EXORset system I acquired in 2018. I provide a 6800 cross assembler in C. - Herb Johnson

D2 board

[Motorola 6800 D2 CPU] [Motorola 6800 D2 kbd/disp]

To the left is the D2 CPU board. It has the usual sorts of things that an 8-bit CPU family had in the 1970's. The 6800 CPU chip, the 6810 128-byte RAM, the 6820 PIO parallel I/O chip. Also a Motorola 6830 mask-programmed JBUG ROM, and a Motorola brand crystal/clock chip in the metal can. There's a serial UART chip, called an 6850 ACIA,

To the right is the display and keyboard, connected to the CPU board by a flat cable. There's six 7-segment displays, and a hex keypad with function keys. The red flatcable on the left of the display board, connects to the white connector on the right of the CPU board.


[Motorola 6800 D2 operation]

When I got the board, a chip was missing, and it was an odd one - 74155, a kid of 3-to-8 decoder. I was fortunate to have a used "pull" from a collection I bought last summer. Some of the through-holes on the PC board seemed to need some solder, and a few components had cold-solder joints upon visual inspection. I made these repairs. After some fuss with wall-warts, I decided to use an old open-frame power supply for the 5 volts needed.

Much to my suprise, when I powered up the first time, there was a display, and some response! I checked for loose chips in their sockets, and when I tweaked a few of them I got some instability. A few rounds of such work and the board appeared stable. I punched some keys and got some results - I took a photo QUICK before it blew up!

Of course...I had not read the user's manual in about 30 years, so I didn't know how to run it. But like many "evaluation board" kit manuals, the keyboard functions were described and not hard to use. "M" is for memory, "E" is for escape (from the current operation), "R" shows the registers (as dumped into memory), "G" is for go or execute at. You can also set breakpoints and remove them, and step through execution. The JBUG monitor (varient of MIKBUG) sorts all this out, and the source is listed in the manual. The schematic for the CPU is here and the schematic for the keyboard/display is here.

The manual even has a short program to add some values, and with a bug. Of course, you load the program, walk through it, and debug it - all described and simple enough to follow. Like many vintage computer manuals, you can find copies at bitsaver's various mirror sites (but see below).

I sold this kit in Aug 2013.

2019: 6830 JBUG and SMITHbug

In April 2019 I was asked about an assembly source for JBUG and/or a source for the 6830 ROM which contained that monitor program. I had forgotten, that the Motorola 6830 was a 24-pin ROM which was "mask-programmed"; the bits were actually designed in when the chip was produced. And, the 6830 IC has a "pinout" of address and data lines, which are different from common UV-erasable (and user-programmable) EPROMs. It's hard to find a 6830, much less one with the JBUG monitor as Motorola produced other 6800 monitors for their 6830's. A Web search found an example of replacing the 6830 with a PC board including a UV-EPROM.

On JBUG sources. I looked at bitsavers and found the links had been moved since my last D2 search; I've corrected my links. Thanks to the Internet Wayback Machine for providing alternatives. Just in case,here's a hex dump of JBUG and here's a binary of JBUG. Compare them against the 6800 D2 manual's listings. In mid-Aug 2019, Greg Simmons kindly hand-entered the JBUG source into an ASM file. He made some modifications to accomodate some modern 6800 assemblers. I also modified his JBUG source to be suitable for my A68 6800 cross assembler. I've ZIPped up both versions and Greg's notes into this ZIP file.

SmithBUG: Software Works Company and Ed Smith produced SmithBUG for the 6800, sometime in the late 1970's. Here's more information about SmithBUG and how Mike Lee brought it to my attention. - Herb

- Herb Johnson

Herb Johnson
New Jersey, USA
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