#6 TRS-80 Color Computer 3 Motherboard Review / Teardown

[@gregclare]

Nice video. Just a small correction (3:50): The 74LS245 does not buffer the Address Bus! It's an octal bi-directional tri-state buffer commonly used to buffer the Data Bus I/O, to allow driving the multiple devices normally connected to the CPU’s data bus pins (the 6809 could only directly drive 4x LS TTL inputs). The MC6809 was indeed the most powerful 8-bit processor. Internally it implemented 16-bit functionality with 16-bit arithmetic & 16-bit Accumulator (D), and even had a Multiply instruction (MUL) which multiplied the two 8 bit accumulators (A & B), with a 16 bit result (in just 11 clock cycles!). But to be fair, it was still fundamentally an 8-bit class processor. I remember saving-up to buy my first 6809 CPU chip to build my own wire-wrap computer, as a powerful upgrade to my prior MC6800 based designs. LOL. I even recall as a teenager in the 80’s writing 6809 assembly code, being amused by the 6809’s ‘SEX' (Sign EXtend) instruction nmemonic. :-)

[@boblabinne]

Very great. I like this stuff explained. This board is mostly unconventionnal compared to other regular 8 bit computer. Anyway, I had one in the 80's and I liked it. In french Canada, this COCO 3 was more popular than the Commodore 64. Mainly because we had a full french big Microsoft Basic manual to make our games or softwares. Now, with only some fundamental logic knowledges and electronic knowledges... We can finally understand what's in there.

[@RetroLogicLaboratory]

Thanks for the comment! Radio Shack had its issues, but they did a very good job supporting French Canada. The Coco and most of the other TRS-80 machines had an English and French version you could order. I remember also a few games available in French as well, "Mots Magiques" is one the comes to mind. I am not surprised they were popular in French Canada!

[@JamesJones-zt2yx]

The 808x and Z-80 may have run at a higher clock rate, but they took many more clock cycles to execute instructions. A 2 MHz 6809 could outrun the 4,77 MHz 8088 on the original IBM PC.

[@Starchface]

The 6809 was also a pleasure to program. The 8/16-bit accumulator was very handy and the instruction set made sense, without the idiosyncrasies of the 6502. The 6809 was easily the best of the 8-bit CPUs.

[@krisc1243]

thanks for uploading a great video. just curious, what would be the command (BASIC) to control I/O port? ie) &h out (numer) ...

[@davesakievich9657]

The 6821 IC that you were unfamiliar with was actually a 6822 PIA or the commercial version of the PIA and was used because it eliminated most of the 'noise' related keyboard errors due to the long lead length associated with the keyboard. The 6822 worked better in the 'noisy' factory or commercial setting. The CPU or the 6809, 68A09 and the 68B09 were, in many ways, far superior to the original 'PC' or as it was called back then, the IBM computer. The 6809, running at a measly .89 MegHertz, could run circles around the PC (with many graphics programs), then running at 4MHz, or nearly 5 times the COCO's system speed because of its built in Multiply op code. The A and B versions of the 6809 were CPU's that would run properly at 1.5 Mhz and 2 Mhz respectively. I also have circuit diagrams and information that will allow the 68B09 to run correctly at up to 4Mhxz.

[@davesakievich9657]

The second 'PIA' is the equivalent to a 6822 which is just a 6821 that was designed for use in an 'industrial' location because it could handle a more noisy environment. It was used because the ribbon cable for the keyboard would sometimes not function properly in some environments.

[@sangbeta2043]

You could play a cassette audio file from a phone, if it was in WAV format, and could play loud enough. MP3 compression would probably mangle the data.

[@WalterGreenIII]

There are no 12v signals used in the color computer 3. The joysticks are 6 bit, not 8. and the relay only clicks due to changes in the logic levels during power up and reset, it is actually only under software control when "CSAVE", "CSAVEM", "CLOAD", "CLOADM", "MOTOR ON", "MOTOR OFF" and the equivalent machine language sequences are in use. Other that that the relay is not used.

[@derek8564]

trip down memory lane...used to have one

[@pm71241]

Even though I had a 6502 back in the days, I've later worked with OS/9 ... and I would say that what matters is not whether the 6809 was 8 or 8/16 bit. What matters is that it could run an OS like OS/9

[@amnesie6615]

Wow this was interesting! Thank you!

[@RetroLogicLaboratory]

Hey thanks for the nice comment! Glad you enjoyed the video!

[@broneetube]

Is this a modified board? Why so many chip sockets? Is this an earlier version? I can't imagine that later versions would have sockets, since that would just increase the cost. My CoCo3 had virtually no socketed chips as I recall. I added a CPU socket myself when I upgraded to a 6309 processor.

[@RetroLogicLaboratory]

Good catch! You know what, I used a brute force repair technique on the board and replaced all the off-the-shelf logic chips hence the sockets. That said, the board in the video is an early one, GIME is dated 1986 and the RAM chips were all Made in Germany...

[@tenminutetokyo2643]

Replace the huge cap with a new Nichicon one - $4. The new ones are about 1/2 the size. You can also replace the 245s with modern 645s which are direct replacements.

[@landmind666]

My memory is fuzzy, but didn't the coco 3 have something in common with the nes, i remember playing some almost direct ports of some nes games like robocop?

[@tekdragon]

well the NES had a 6502 variant CPU and the coco3 had a 6809. they are similar chips in some ways though very different in other of ways. there WERE several NES game clones made for the coco3 like "Thelda" (Zelda) and "The Contras" (Contra) and a number of others. but these were clones that used entirely different code and hardware to play

[@alhartman66]

You DO NOT have to throw out the logic board if the GIME chip is bad. There are a few people working on a replacement for the GIME chip that also add additional video functionality to the system.

[@alhartman66]

The relay is not for resetting the computer, it controls the remote jack on a tape player for saving and loading tapes.

[@grandrapids57]

Good video. I know where that tie is from... I have the same one.

[@RetroLogicLaboratory]

Nice! Gotta keep the tie manufacturers in business during COVID. :)