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6800 CPU

I am no longer active in this hobby for the foreseeable future. 
I will no longer maintain or update the website, but I will leave it accessible to the web for as long as possible (years).

 

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6800 Processor Board

I built this board originally around January, 1977. Although I was already an experienced electronics hobbyist, my experience with computers was limited to keypunch and time share systems, and a little (whatever I could get) at the local university. I had no experience programming in machine or assembly languages. I had no trouble assembling the computer, and other than a small glitch due to a shorted line on the video card (direct from the factory--not a solder bridge!), the machine worked the first time it was switched on.

I promptly called DG to order up some software, and was told that the only thing they sold was for the 8080 and Z80. I felt robbed! I was told that there would be some titles forthcoming, but none available just yet. I was pacified. I waited, but DG never came through. Nothing was ever offered for the 6800 by DG. :(
 

2008-05-27a_116.jpg (1743140 bytes)
6800 Processor Board, Sort-of Restored

I chose the 6800 in the first place because I had read several articles on microprocessor architectures, and after careful consideration, concluded that the 6800 instruction set was more understandable. And for me, it was. I found it quite easy to write software for the 6800, and although I wished (in vain) for some DG software, I got along fine without it, though not without trying to exchange my 6800 at one point for a Z80! In a letter from Dick Bemis, my proposal was declined, though I was offered a Z80 for bargain price, my piggy bank was too small at the time, and I made due with the 6800. I'm glad I did. :)

The 6800 was a dynamic processor, that is to say, when paused, all address and data lines were invalid. It was capable of single-stepping, but required latches to hold over the values for examination. This made the 6800 a poor choice for any computer that required a front panel. Motorola intended this CPU to load startup data from a ROM--they offered a standard ROM operating system they called MIKBUG. The DG computer did not use this ROM, and as a result, was incompatible with most off-the-shelf software available, without modification. Life was tough for us poor DG 6800 owners!
 

2008-05-27a_117.jpg (1895378 bytes)
6800 Processor Board, Solder Side

Restoration

From the first time this card was powered up, until today, this board has always worked. However, over the years, I modified this board extensively. I added a piggy-back RAM card of my own design, I added a MIKBUG ROM for greater software compatibility, and I over-clocked it by adding a variable speed clock. (Mostly unsuccessful--the improvement in speed was insignificant, the DG 6800 ran pretty much as fast as the chip would go.) These are just a few of the mods I made--I did literally dozens.

Back in 1986, I backed out all of my modifications in favor of the original design. In the mid-nineties, I decided to do a cosmetic restoration, and cleaned up the board as seen here. Can you spot any of the remains? Probably not many, most of my mods were done by pulling chips and plugging in the mods in place of various existing chips.
 

6800-6500_layout.jpg (737053 bytes)
6800 Processor Board, Layout

6800 Card Information

6800_CPU_Construction.PDF - My original CPU documentation, the schematic is in poor condition I'm afraid, the result of unending use. I now have a better copy, but have yet to do the scanning. One day...

See my Document archive for more related files.
 

 


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Copyright 2008 Bryan's Old Computers
Last modified:
October 16, 2009