Tandy CoCo 2

Discussion in 'Retro & Arcade Worklogs' started by aXLe, Apr 17, 2016.

  1. aXLe

    aXLe Member

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    A friend offered me a couple of Tandy CoCo 2's with accessories, and I picked them up today - thanks Greg :)

    CoCo is short for Color Computer, and the CoCo 2 was introduced in 1983 as an evolution of the TRS-80 Color Computer that preceeded it.

    Included were the 2 computers, a printer, a Multi-Pak interface, 2 joysticks and a game cartridge.

    These computers are both model 26-3136B Korean-made PAL units dating from around 1983 to 1986, and have a 6809 microprocessor and 16KB RAM which can be upgraded to 64KB by swapping out RAM chips.

    [​IMG]



    Computer #1 - This one is in very good cosmetic condition, with minor yellowing.

    [​IMG]


    Computer #2 - Not quite as nice as #1, but should clean up very well :

    [​IMG]


    Mulit-pak - allows 4 cartridges to be installed at once :

    [​IMG]


    2 Joysticks :

    [​IMG]


    TP10 Printer :

    [​IMG]


    Rear of CoCo 2:

    [​IMG]

    Looking inside the cartridge slot :

    [​IMG]

    The game :

    [​IMG]

    [​IMG]


    Original Tandy cover :

    [​IMG]

    I know absolutely nothing about these machines so it will be interesting to get them up and running and have a play with them. I'm hoping the software images are available such that I can make a cartridge and play with some of the titles.

    From my brief reading so far it seems there are a couple of ROM sockets on the mainboard, so I may be able to have a play around with those as well :)
     
    Last edited: Oct 31, 2019
  2. power

    power Member

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    I remember when these things were in Tandy stores, from memory the software library was very average.

    Be interested to see what you decide to do with them.
     
  3. Agg

    Agg Lord of the Pings

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    What's the protocol with something as old as this? Do you just fire it up and see how you go? Or do you strip/inspect/prepare it to avoid damaging it?
     
  4. OP
    OP
    aXLe

    aXLe Member

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    Well, I normally like to open them up and do a visual on the power supply first.

    In this case it turned out to be a linear power supply (step down transformer at top left, feeding into a bridge rectifier and voltage regulators etc) with a couple of filter caps that actually looked ok visually. I didn't go to the effort of testing them in this case before turning it on, but it would be a good idea to measure the ESR and capacitance of the large filter caps as well.

    In the case of switching supplies there are usually AC caps across the mains and these don't age well - worth replacing before plugging in so as they don't let out the magic smoke. Nothing like that here though.

    [​IMG]


    Tidy little PCB with most chips soldered directly down to reduce cost.

    Powering it on was a little bit of an anti-climax though as there is no power indicator light on the unit, nor is there a speaker in the case!

    I was met with the click of a relay and that was it. I now need to tune the TV into it (only has an RF output) and see/hear it fire up. I didn't have time to probe around the board for various voltage etc to ensure the power supply was ok.

    Some more shots of the internals :

    [​IMG]


    ROM chip is to the left of the cartridge slot connector (far right) - the ROM is directly soldered in. 6809 CPU is the the left of that (the relay I was talking about is above the CPU and seems to be isolating the cassette interface), followed by a 6821 PIA (peripheral interface adapter), and 67331P which seems to be some form of Tandy custom PIA, and appears to be handling the keyboard interface. RF modulator can be seen beside the transformer (metal shielding over and around it) :

    [​IMG]

    Immediately below the keyboard connector (which uses a flat flex cable) are the 2 chips responsible for generating the video output.

    [​IMG]

    Pretty sure these CHS labelled chips are the RAM which are socketed. Also some type of memory expansion headers on the board. The large chip to the right of the ram is likely providing glue logic (hanging everything together), with a bunch of jumper blocks above it:

    [​IMG]

    Some of the PAL video generation circuitry :


    [​IMG]

    Most of the chips in this machine are dated some time in 1984, so that gives a fair indication of the manufacture date.

    Next post I hope to get some video out the RF connector (have to work out how to tune into it - yay for still having analog tuners in my TV!), plus I'd like to dig up a schematic to get a better idea of how it all hangs together
    :)
     
    Last edited: Oct 31, 2019
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  5. /invariance\

    /invariance\ Member

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    Some very nice examples there aXLe.

    This was our first computer when I was in high school, but we only had the printer, cassette player for storage and a few game cartridges. I punched in a few programs from the supplied books and CoCo mags, but was more into riding my pushbike in the local bush.

    Do I vaguely recall to tune the TV to VHF 0 or 1 depending in the switch on the back?

    Now get peeking and pokeing!
     
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  6. OP
    OP
    aXLe

    aXLe Member

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    I had a chance today to hook them up to the TV - didn't have to tune anything at all - they just worked which was kind of cool :)

    Interestingly there is still no sound when you turn it on :

    [​IMG]

    The software that I have is a kids game and it's very basic, but it did at least give me the first sounds that I've heard from the machine :

    [​IMG]

    [​IMG]

    It seemed that the cartridge would be a good candidate to be opened up and the ROM swapped out, as there are plenty of ROM files online for download. Not quite that easy though - the things are made as cheap as possible with the ROM potted on the pcb (ROM die is under that black blob on the PCB - bonding wires would go from die to pads under the blob) :

    [​IMG]

    Still, most tracks have vias from the edge connector, so I think I can easily cut the tracks to the potted rom and add my own - worth a go to see some of the better games that came out on this platform (I'll do some googling for the best of CoCo 2 games and select one or two once I've prepped the cartridge).
     
    Last edited: Oct 31, 2019
  7. OP
    OP
    aXLe

    aXLe Member

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    Hacked up the cartridge so I can use an eprom - really only using the cartridge for the edge connector and housing :

    [​IMG]

    [​IMG]

    Toggle switch allows selection of 2 16KB images in a 32KB rom (27C256 which I have lots of) - I'll basically stack 2 game images in a ROM and be able to switch between them :

    [​IMG]

    I'm not concerned about hacking up the cartridge - far handier as a platform for playing with some ROM images.

    Hope it works! Need to now test continuity on each pin and burn a ROM.
     
    Last edited: Oct 31, 2019
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  8. OP
    OP
    aXLe

    aXLe Member

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    After some reading on the train this morning it seems many of the better games were available on tape or disk only. There are definitely some goods ones on the games carts though so I will start there.

    I found a good list here :

    http://www.icepeople.net/coco/reviews.html
     
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  9. OP
    OP
    aXLe

    aXLe Member

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    Work last night was interrupted by Better Call Saul, but back into it tonight.

    First up I popped some EPROM's in the eraser...

    [​IMG]

    ... and while they were cooking I tested continuity on my hacked cartridge to ensure all pins were connected to where they were supposed to be, and no shorts. Then I took ROM images for 2 games (Downland and DoubleBack) and inserted them into a 32KB hex file - the first at address 0x0000 and the latter from address 0x4000, and burned them to an EPROM fresh out of the oven.

    [​IMG]

    Took it all down to the TV and set it up - it works!

    Switch in position 1:

    [​IMG]

    Position 2 :

    [​IMG]

    Good thing I have joysticks as I couldn't get past the front screen (select 1 or 2 players) without one plugged in.

    The graphics are very basic. I really need to study the game list as I was expecting more :

    [​IMG]

    Anyway, the hacked cartridge was a success and I have a bunch of games (found a 20MB .zip file which has a huge amount of the available software including cassette, disk, and ROM titles), but I've only made the one EPROM so far. I ordered some cheap ZIF (zero insertion force) sockets from eBay so I'll plug one of those into my socket when it turns up - will make it easy to swap EPROMs without damaging the hacked cartridge trying to lever them out.

    I also need to look into the spec of this machine some more - maybe upgrade it to 64k and try some of the 64k titles.

    EDIT : this is what it should look like - I hadn't tried this particular TV before so perhaps it needs to be tuned/tweaked as it's not displaying the colours nor the full screen area.
    [​IMG]

    The TV I used previously worked fine with the Kindercomp game so I'll have to try it on that one...
     
    Last edited: Oct 31, 2019
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  10. wombat_58

    wombat_58 Member

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    'it's not displaying the colours'

    I had Cocos back in the day and if memory serves the issue seen is that your tv is not correctly supporting ntsc. I recall (perhaps incorrectly) that the high res 2 colour mode of the Coco would work as a 4 colour on an ntsc tv but when output to pal you would only get the base 2 colours, black/white or green/dark green.

    At the time I had a monitor that supported all tv modes and it needed to be set to ntsc to display the high res mode in 4 colours.

    Hopefully I have remembered correctly but it is a long time ago :)

    Cheers
     
  11. OP
    OP
    aXLe

    aXLe Member

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    Interesting - could well be the case.This is actually a PAL unit, and I was reading that the 4 colour "high res" mode was actually a hack - the unit only officially supports 2 colours in that mode as you said, but clever programmers worked out by lighting alternating pixels they could get blue and orange out of it. Thus why most of the better looking games are in white, black, blue and orange :)

    Artifact colors
    The 256×192 two color graphics mode uses four colors due to a quirk in the NTSC television system (see composite artifact colors). It is not possible to reliably display 256 dots across the screen due to the limitations of the NTSC signal and the phase relationship between the VDG clock and colorburst frequency.

    In the first colorset, where green and black dots are available, alternating columns of green and black are not distinct and appear as a muddy green color. However, when one switches to the white and black colorset, instead of a muddy gray as expected, the result is either orange or blue. Reversing the order of the alternating dots will give the opposite color. In effect this mode becomes a 128×192 4 color graphics mode where black, orange, blue, and white are available (the Apple II created color graphics by exploiting a similar effect).

    Most CoCo games used this mode as the colors available are more useful than the ones provided in the hardware 4 color modes. Unfortunately the VDG internally can power up on either the rising or falling edge of the clock, so the bit patterns that represent orange and blue are not predictable. Most CoCo games would start up with a title screen and invited the user to press the reset button until the colors were correct. The CoCo 3 fixed the clock-edge problem so it was always the same; a user would hold the F1 key during reset to choose the other color set.


    I'll try messing with the input mode on the TV and see if I can get 4 colours in high res mode, but maybe on a PAL system this won't be possible? Worth a shot! Interesting about the reset thing too - last night while messing around with the cartridge I briefly saw a colour image but after reset again it was lost - could be the TV being too smart for its own good as well and switching modes on me.
     
    Last edited: Apr 21, 2016
  12. wombat_58

    wombat_58 Member

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    My first coco was an ntsc unit and the 4 colour hack worked. Once I switched to a pal unit I don't recall seeing 4 colours again, so may not be possible on pal units.

    Good luck with it.

    Cheers
     
  13. OP
    OP
    aXLe

    aXLe Member

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    I had another brief look at the colour situation this evening - it was displaying the full picture etc, but no colour. Must be a PAL vs NTSC thing - I'll have to do some reading on it.

    Kind of ironic that I have an NTSC BBC I want to convert to PAL, and now a PAL CoCo 2 that I'd rather was NTSC!

    The games are not bad actually - played Downland for a bit. One machine has an oddity with the audio - I'm thinking a dodgy cap on the board but will have to have a probe around and see what I find. I'll check out the RAM at the same time.

    Also I just found out about an application called DriveWire which allows emulation of disk drives using the PC as a server. A special DriveWire DOS ROM would be booted via the cartridge, and a serial connection between the CoCo and PC. The PC runs the DriveWire software which emulates multiple disk drives and allows you to use the disk images on the CoCo - cool!

    http://www.cocopedia.com/wiki/index.php/Getting_Started_with_DriveWire

    Another thing to try some time to test some of the disk only games :)
     
    Last edited: Apr 21, 2016
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  14. OP
    OP
    aXLe

    aXLe Member

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    Found this in regard to the artifact colours :

    Users in PAL countries saw green and purple stripes instead of solid red and blue colors.

    That's exactly what I'm seeing, so looks like no colour for me. Ah well.

    Found some good info on the large logic chip to the right of the RAM - this is the S.A.M chip which does indeed provide glue logic plus a number of other functions :

    • Clock generation and synchronization for the 6809E MPU and 6847 VDG
    • Up to 64K Dynamic Random Access Memory (DRAM) control and refresh
    • Device selection based on MPU memory address to determine if the MPU access is to DRAM, ROM, PIA, etc.
    • Duplication of the VDG address counter to "feed" the VDG the data it is expecting
    • The SAM was designed to replace numerous small LS/TTL chips into one integrated package. Its main purpose is to control the DRAM but, as outlined above, it integrates several other functions as well. It is connected to a crystal at 4 times the television colorburst frequency (14.31818*MHz for NTSC countries). This is divided by 4 internally and is fed to the VDG for its own internal timing (3.579545*MHz for NTSC). The SAM also divides the master clock by 16 (or 8 in certain cases) for the two phase MPU clock; in NTSC this is .89*MHz (or 1.8*MHz if divided by 8).
    • Switching the SAM into 1.8*MHz operation gives the CPU the time ordinarily used by the VDG and refresh. As such, the display shows garbage; this mode was seldom used. However, an unusual mode available by the SAM is called the Address Dependent mode, where ROM reads (since they do not use the DRAM) occur at 1.8*MHz but regular RAM access occurs at .89*MHz. In effect, since the BASIC interpreter runs from ROM, putting the machine in this mode would nearly double the performance of a BASIC program while maintaining video display and DRAM refresh. Of course, this would throw off the software timing loops and I/O operations would be affected. Despite this, however, the "high speed POKE" was used by many CoCo BASIC programs even though it overclocked the hardware in the CoCo, which was only rated for 1*MHz operation.
    • The SAM has no connection to the MPU data bus. As such, it is programmed in a curious manner; its 16-bit configuration register is spread across 32 memory addresses (FFC0-FFDF). Writing even bytes sets that register bit to 0, Writing to odd bytes sets it to 1.
    • Due to limitations in 40-pin packaging, the SAM contains a duplicate of the VDG's internal 12-bit address counter. Normally this counter's settings are set to duplicate the VDG's display mode. However, this is not required and results in the creation of some new display modes not possible when the VDG is used in a system alone. Instead of the VDG requesting data from RAM by itself, the VDG is "fed" data by the SAM's internal copy of the VDG address counter. This process is called "Interleaved Direct Memory Access" (IDMA) by Motorola and ensures that the processor and VDG always have full access to this shared memory resource with no wait states or contention.
    • There are two versions of the SAM. The early one is labeled MC6883 and/or SN74LS783; the later version is labeled SN74LS785. There are some minor timing differences, but the major difference is the support of an 8-bit refresh counter in the 785 version. This allowed for use of inexpensive 16K by 4-bit and certain 64K by 1-bit DRAMs. Some third party bank-switching memory upgrades that used 256K DRAMs needed this 8-bit refresh counter to work.

    Oh - and the unit I have does in fact have 64KB ram installed :) The warranty sticker was intact, so I assume it left the factory like that even though the model designator identifies it as a 16KB machine. There is a link on the mainboard that is marked 64K and it has been connected, plus they are 4464 dram chips in there (4bit x 64K).
     
  15. mjunek

    mjunek Member

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    Just from what I remember of the old systems and how they did the colour, alot of it was exploiting the way a CRT also renders the pixels with the scanning etc.
    You may find that on a flat-screen, the lack of an actual raster prevents this occurring, and therefore you get the "original" colour, rather than the "intended" colour.

    I may be thinking of something else however - so take it with a grain of salt :)
     
  16. apap100

    apap100 Member

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    Awesome

    This is really awesome! My first PC was a 1986 CoCo 3, although I later acquired a couple of CoCo2's. CoCo 3 was almost identical on outside but it had better graphics and faster 68b09e CPU (1.79Mhz vs 0.890Mhz). I remember having to desolder the CPU so I could install a Hitachi 63c09C (installed socket of course) and replacing the GIME chip with the 1987 version that added hardware horizontal scrolling. I also shelled out on a Disto 4 in 1 cartridge so i could connect a SCSI HDD + Floppy Drives.
    Damn makes me fee old though :)
     
  17. supercruiser

    supercruiser Member

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    T

    Awesome... I lived on my Trash 80... loved the thing.

    Yeah the NTSC to PAL thing was pretty ordinary with the colour issue..

    I would have killed for a multi cartridge doova... Had a disk drive which made life a hell of a lot easier..

    Kids have got it too good these days.

    Enjoy!
    Supercruiser
     
  18. Draebor

    Draebor Member

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    Nice looking old coco :)

    one game you should try to get running on it is dungeons of daggorath, great little dungeon crawler... https://en.wikipedia.org/wiki/Dungeons_of_Daggorath

    spent most of my childhood playing games on a coco2, stuff like pooyan, sailorman, zaxxon and cashman
     
  19. OP
    OP
    aXLe

    aXLe Member

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    Thanks - yes I've heard good things about that game - its on the list to burn to a rom when I get back to playing with the CoCo.

    At present I'm just finishing re-capping multiple Mac SE logic boards, power supplies, and analog boards (over $400 worth of caps alone!), and when I get through with that I now have a C64 to repair and get running plus a 1084S monitor too that also needs repair. Maybe after that I'll get another chance to take a look at the CoCo, or perhaps when I want to take a break from the other stuff :)
     
  20. dred0r

    dred0r Member

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    Aaahhh i still have my Coco2 :) Wonder if it still powers up.

    I have the voice cart somewhere as well, which did some basic synth voice output from memory. If the weather is bareable tommorrow I'll see if i can find it.

    Most of the games of the time were off tape. Not sure if any of those survived.
     

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