Is the Original Tandy Color Computer Worth your Gaming Time?

Courtesy of Wikipedia

Tandy Corporation's first home computer was the TRS-80, released in 1977. The TRS-80 was fairly affordably priced for its time but limited to monochrome text and extremely blocky semigraphics. 1979 saw the release of the TRS-80 Model II, a business machine with an 8" floppy disk drive incompatible with the previous computer or the budget of a middle-class family. With color computer systems like the Atari 400 and TI-99/4 being released in the late 1970s, Tandy realized that if it wanted to have any chance of capturing the growing home market for personal computers, it would need to offer a low-cost model with colorful graphics and a family-friendly appeal. Fortunately it had an ace up its sleeve in the form of the thousands of Radio Shack company and franchise stores dispersed across the United States and Canada that could sell a lower cost computer. That computer turned out to be the TRS-80 Color Computer, released in 1980. Having recently acquired one, let me go over some of its features and quirks.

The Tandy Color Computer originally came with a pair manuals, the "TRS-80 Color Computer Operation Manual" and "Getting Started with Color BASIC." If the Extended Color BASIC upgrade was applied to the system as shipped from the factory, then the machine will come with a "Getting Started with Extended Color BASIC" manual instead. It also originally came with an RF switchbox (twin-lead) and an RCA cable to connect to that switchbox.

Design - Memory

The Tandy Color Computer was originally released in 4KiB and 16KiB models, with the former retailing at $399.99 and the latter for $599.99 (with Extended Color BASIC). Like the TRS-80 Model I and TRS-80 Model II, the CoCo was advertised and sold through Radio Shack. Except for some peripherals shared in common, the CoCo was utterly incompatible with the Model I or the Model II despite "TRS-80" being part of its name. Early 4KiB and 16KiB models could be distinguished by a raised badge on the top of the case. Eventually the 4KiB model was discontinued and a 32KiB model was introduced. The CoCo could be officially upgraded to 64KiB internally.

The CoCo is not an original design, it is an implementation of a reference design for a home computer from Motorola. The five main chips which perform virtually all the functions of the CoCo are Motorola designs, the 6809 CPU (running at 0.895MHz), the 6847 Video Display Generator (VDG), the 6883 Synchronous Address Multiplexer (SAM) and a pair of 6821 Peripheral Interface Adapters (PIA). 

The memory map for the system is very simple. RAM occupies the area between $0000-$0FFF (4 KiB), $0000-$3FFF (16 KiB) or $0000-$7FFF (32 KiB). Video memory is assigned to this area. ROM occupies the remaining 32KiB - 256 bytes (internal ROM from $8000-BFFF, cartridge ROM from $C000-$FEFF). When the system wishes to access 64 KiB of RAM, the ROMs are banked out. This is how the Apple IIe and its successors support 128KiB of RAM. 

I/O is memory mapped. The CPU can access the SAM and the PIAs at locations between $FF00-$FFFF. There is no direct access to the VDG, the CPU can only address the SAM to affect the video display.

Color BASIC Program Test

Design - Text, Semigraphics, Graphics, Monochrome & Color

The CoCo can display a number of text, semigraphics and graphics modes and up to nine direct colors. The direct colors it can display are black, green, yellow, blue, red, buff, cyan, magenta & orange. There is no white, only buff, output from the Color Computer. "Buff" is the color's name as given in the official VDG datasheet and it has a chroma component. Most of the time it is shown either as as a grayish or beige color.

The text mode can display 32x16 characters with 8x12 text cells, only uppercase characters are available. You can designate lowercase characters in BASIC by pressing Shift + 0, these characters will appear as inverse text and will be treated as such by a printer. 64 text glyphs are built into the Color BASIC ROM. The text mode usually shows as black text on a green background but can also show an orange background instead. Later revisions of the CoCo 2 use a newer revision of the VDG, the 6847T1,  that supports lowercase glyphs.

Semigraphics modes can assign multiple colors to text blocks with increasingly greater resolution. The official effective resolutions of these modes are 64x32 and 64x48.  64x64, 64x96, 64x192 can also be achieved semi-officially but were seldom used and removed with the CoCo 3. Any of the nine colors can be assigned in semigraphics modes but the 64x48 mode has restrictions. Text mode glyphs can be accessed in any character cell in semigraphics modes. The border is always black in text and semigraphics modes.

Pure graphics modes come in two forms. There are four monochrome graphics modes supporting resolutions of 128x64, 128x96, 128x192 or 256x192 and have green or buff pixels on black. The border color is either green or buff depending on the pixel color chosen.

Four colors are available in four color graphics modes with resolutions of 64x64, 128x64, 128x96 or 128x192. You can chose from one of two palettes of four colors in these modes. Palette 0 is green, yellow, blue & red, Palette 1 is buff, cyan, magenta & orange. The border color is green and buff, respectively.

The 256x192 "monochrome" graphics mode supports artifact colors on NTSC systems, the colors will usually be some variant of violet/orange and green/blue (Popcorn has been adjusted below to show either color set). Which color pair is predominant is random at startup based on the VDP phase, so some games will tell you to press the reset switch until you get the intended colors (see Dungeons of Daggorath's two potential images below).

While the graphics hardware had no support for sprites or hardware scrolling, the video hardware could generate interrupts during horizontal blank (every line, 63.5usec) and vertical blank (every frame, 16.66ms). These are the only source of timers for the system outside peripheral upgrades.

Design - Sound

Sound can be output by a 6-bit DAC, an MC14050B. The DAC is driven by the CPU so the production of complicated music and sound effects takes CPU cycles away from other tasks. The system can also accept sound input from either the cassette or the cartridge port. There is also a single-bit toggle, also CPU driven, for basic sound output like the Apple II or ZX Spectrum. Only one sound source can be heard at a time.

Design - Keyboard

The keyboard has 53 keys and uses chicklet-style keys. These keys are spread out more than a keyboard designed for true typists, I imagine Tandy had overlays printed for the keyboard in mind but I only know of the one for Color Cubes. The keys on my keyboard use individual rubber domes over sheets of mylar membrane and the larger keys have stabilizers, I have felt worse rubber domes (IBM PCjr.'s keyboard). I do not believe the CoCo 2 or 3 used anything better for switches despite the more conventional keycap shapes. 

The first CoCo 1s not only had a badge indicating the amount of RAM in the machine but a keyboard with an extra black border around it. (These machines also have the badge with the computer's name to the left instead of the center).

The final CoCo 1s had a white enclosure with the "melted keyboard' used in the earlier CoCo 2s. These are the least common of the CoCo 1s.

Ports Courtesy of Wikipedia

Design - Ports

The rear of the system has a hardwired AC cable (convenient), a power switch, a reset switch, two joystick ports, an RS-232 serial port, a cassette port, an RF output and a Channel 3/4 switch. Early CoCo 1s had the names of the ports printed above the ports in black text which may wear off. Later CoCo 1s had the names imprinted into the plastic. The right side of the system has a 40-pin cartridge port with a spring-loaded door.

The cartridge port can support ROM cartridges (Tandy used the term "Program Paks") of 2, 4, 8 or 16KiB without bankswitching but was the only way the CoCo could support a wide variety of peripherals, including Floppy Disk Drive Adapters. There is a Multi-Pak interface which can allow you to select from up to four cartridge devices.

There are four DIN ports on the back of the system, two 6-pin joystick ports, a 5-pin cassette port and a 4-pin RS-232 serial port. These ports are oriented differently so you cannot plug the wrong peripheral in, something Tandy must have learned from the TRS-80 Model 1, which had three identically oriented 5-pin ports for cassette, video and power.

Peripherals

The standard Tandy Joystick for the CoCo are single-button non-self centering joysticks with a 6-bit responsiveness range. They have a black body and red button and were sold in pairs. The (usually) silver handle can screw off if you keep twisting it counterclockwise. Different games consider either the left joystick or the right joystick to be "player one", so keeping a joystick plugged into each port is a good idea. You can insert a Tandy Deluxe Joystick into a CoCo 1 or 2 but the second button is not wired to anything inside and thus is unusable. (The Tandy Deluxe Joystick was introduced just before the Tandy 1000.) There also exist mice for the CoCo in one button and two button forms but these are potentiometer-based and function like joysticks.

There are several floppy disk drive controllers for the CoCo, but they need enclosures to house and power the disk drives. The FD-501is the only one which has a gold plated cartridge edge connector for a more reliable connection. The first floppy drive controller for the CoCo is only compatible with the CoCo 1 unless a Multi-Pak Interface is used. PC disk drives should be compatible with the interface but you will need to find some way to power them externally.

CoCo floppy drive controllers, with the exception of the final one (FD-502) only support reading off a single side of a disk drive. The standard disk format is 35 tracks, 18 sectors and 256 bytes per sector. As track 17 is reserved for the file allocation table, the standard capacity of a formatted CoCo disk is 156,672 bytes.

Many of the printers sold by Tandy have a serial connector like the Color Computer, that is the port to which this RS-232 serial port is intended to connect. The TRS-80 MC-10 also has this port. It can also be connected to Tandy's acoustic couplers and some of its external modems. The RS-232 is bit-banged, meaning it is driven by the CPU directly, instead of using a dedicated UART chip. When serial port communication is happening, nothing else is.

The cassette port requires the special Tandy cassette cable, which would have been sold with the CTR-41 & 80A and the CCR-81-83. This cable can be used with the TRS-80 Model I, III & 4, MC-10 and the IBM PC Model 5150. Some CoCo software will need to control the start and stopping of tape playback, so a cassette recorder with an REM jack will be necessary for these titles. When the cassette deck is connected you will need to press the Play button before the remote signal will do anything. The CoCo manuals state to plug in the data out plug into the AUX port on the recorder, not the MIC port, so it is preferable to get a recorder with both an AUX and MIC port. I have used a Radio Shack CTR-121 with good results. The cassette format supported by BASIC has a 1500 baud rate, so it is rather speedy, especially compared to the TRS-80 Model I.

Frogger

Upgrades

Upgrading a 4KiB CoCo to a 16KiB CoCo is a simple matter of swapping out the chips for 4116 DRAMs. All CoCo 1 & 2s can be upgraded to 64KiB of RAM with 4164 DRAMs, the newer your CoCo 1's mainboard, the easier it will be to upgrade. CoCo 1s with the final board revision will require no soldering. The details are available here.

All CoCo 1 & 2s come with Color BASIC on an 8KiB ROM chip at $A000-$CFFF. All can be upgraded with Extended Color BASIC, which comes in an 8KiB ROM chip at $8000-$9FFF which plugs into a socket on the mainboard alongside the Color BASIC ROM. Some CoCo 1 systems were sold with Extended Color BASIC but most were not. The socket is intended for a 24-pin chip, EPROMs are 28-pins in 8KiB sizes, so you will need an adapter. The BASIC splash screen text will change from "Color BASIC" to "Extended Color BASIC" and the version number will be displayed. Extended Color BASIC is required to support floppy disk drives and some programs but few games. The floppy disk drive controller will add and show "Disk Extended Color BASIC" to the system.

The CoCo 1 is RF output only but can be modified for composite video and separate audio output for improved picture and sound quality.

Opening the CoCo 1 is very easy, a long shaft screwdriver will get into the recessed screw holes. There are seven screws, the front two screws are shorter than the other five. One screw might be under a tamper-proof warranty, but good luck getting anything out of Radio Shack these days. The top cover comes right off but be careful about the cartridge door flap and its spring. Later CoCo 1 mainboards will require removing the mainboard to dislodge the RF shield over the RAM chips. Be very sure of the connections from the transformer board to the terminal lugs on the mainboard, you do not want to swap +5v and +12v when you reinsert the mainboard!

Most of the chips are socketed in CoCo 1s, making repairs and upgrades very easy to accomplish. You can swap out the 6809E CPU for a more capable and efficient 6309 CPU, but unless a program supports the "native" mode of the 6309, you will not see any speed benefit to this upgrade. The speed benefits would only be seen by homebrew software and most of the best homebrew is for the CoCo 3. The upgrade in my opinion makes more sense for a CoCo 3.

Skiing

DAC, Joystick & Tandy 1000s

It is well known that Tandy CoCo joysticks are compatible with the Tandy 1000. Inside each joystick is a pair of 100Kohm potentiometers. Each potentiometer is wired to +5v and GND, which functions as a voltage divider on the output terminal. The resulting voltage is sent to the system where it is compared with a value sent by the CPU to the 6-bit DAC. This gives the joystick its 0-63 value range. When the values are equal the state of a pin on the PIA will change. 

In the Tandy 1000, a similar design was used to maintain compatibility with Tandy CoCo joysticks, but there was one change to make the circuit compatible with the IBM PC-method of reading joysticks. The IBM PC relies on the time it takes to charge a capacitor to give an analog voltage value. The Tandy 1000's comparator is connected to a joystick potentiometer's voltage-divided output and a triggered circuit which ramps up from 0-5v in 1.12ms. The output of the comparator changes when the to voltage sources match.

When Tandy integrated their joystick reading function into the PSSJ chip used in the later Tandy 1000s, it took design inspiration from both the CoCo and the previous 1000s. The sound and joystick functions of this chip rely on the 8-bit DAC integrated into the chip. In joystick reading mode, the DAC is triggered to ramp up from 0-255 over a period of 1.7ms. When the voltage level of the DAC matches that of the voltage divider from the joystick, the signal will change.

In either the Tandy CoCo or the later Tandy 1000s, you cannot use the DAC to output sound and read the joystick at the same time. The CoCo can interleave sound output and joystick reading by carefully interleaving DAC writes intended for each purpose. This is not feasible on the Tandy 1000 DAC because of the amount of time it takes for the voltage ramp for the joystick reading which is excessively difficult with the DMA-driven method of digital audio output games used. The CoCo also has a 1-bit speaker toggle like an Apple II or ZX Spectrum, so the system need not be completely silent when a joystick is being used. The SN76496 and the PC Speaker functionality do not rely on the DAC, so some audio can be heard from the late Tandy 1000s when joysticks are enabled.

Running Games

Tandy CoCo games can come on cartridge, cassette or disk. Most early cartridge games can work on a 4KiB CoCo but some will be able to take advantage of a 16KiB system for better gameplay. Later cartridge games will probably not work on a 4KiB system. Tandy's cartridges are cheap with thin PCBs, usually no gold on the contact pins and an easy to misalign spring loaded contact protector.

Cassette games require at least a 16KiB system but there are cassettes which require a 32KiB system and/or Extended Color BASIC. Some disk games will run on a 32KiB system but many others require a 64KiB system. 

Most games require a joystick but also have some need for keyboard input to select game options. Polaris is a Missile Command clone which supports the three bases of the Atari arcade cabinet by using the X, Y, Z buttons to fire missiles and the joystick to position the firing cursor. Bustout is a Breakout clone which annoyingly requires you to press a keyboard key every time you wish to serve the next ball.

Several games support the Speech/Sound Cartridge, including Pitfall II, Ghana Gwana and Pegasus and the Phantom Riders. The Speech/Sound Cartridge only works properly (without modding) with a CoCo 1 & 2, it was not designed for the higher speed of the CoCo 3. Some SSC games may be on tape but all are on disk, necessitating the use of the Multi-Pak Interface.

Cartridge games are easy enough to load, but you will likely need to take isopropyl alcohol to clean their pins if they've been sitting unused for a while . Cassette games are loaded with either CLOAD and RUN or CLOADM and EXEC. Disk based games are usually booted with LOAD "*" or DOS, the CoCo does not automatically boot disks. To see the directory of a disk enter DIR 0 or DIR 1 with the number of the drive you wish to check. Check the manual for specific commands if these do not work.

Some games had CoCo 1 & 2 and CoCo 3 versions on them. Arkanoid, Mind-Roll, Silpheed, Sokoban and Tetris fall into this category.

Mickey's Alpine Adventure

The Power of the Cassette Port

The original CoCos might not seem particularly interesting without a disk drive because many of the best games came out on disk or cartridge. Tandy as far as I could tell never released game software for more than one medium, so if a game came out on cartridge it was only available for purchase as a cartridge. Cartridges are small and can be fit on one side of a tape. Moreover, due to the bankswitching capability of a 64KiB CoCo, the cartridge's data can be copied from cassette to RAM in the location where ROM normally would sit. Possibly due to the efforts of the hobbyist community almost all cartridge games have been ported to cassette tape. They tend to take a minute to load, but it beats building a cartridge collection.

Using the cable one can play back a wav file from the go-to site for Tandy Color Computer documentation and software, TRS-80 Color Compute Archive. Cartridge to cassette conversions should be a simple matter of CLOADM followed by EXEC. Some might need CLOAD followed by RUN.

Many cassette exclusive games may not be a single load. I would suggest using Audacity to play the audio. If your game uses the cassette remote jack to start and stop the tape playback, like the educational titles, then you may want to record the wav file to tape. Otherwise you will have the tricky time of synchronizing the pausing of the track to the clicks of the relay inside the CoCo.

One useful but perhaps often forgotten feature is that BASIC for most systems saves data to tape as files. That data can be loaded without having to hone in on a reference on the counter dial of the tape deck. This is very important for a tape like Card Games which comes with six programs on the tape. If you wish to play a game which is not the first game on the tape, such as Blackjack, you would type in CLOAD "BLACKJAK" and the tape would play until it found the Blackjack game. You can list the filenames of all the programs on a tape by typing in SKIPF "[insert anything other than a known file name]"

A few cartridge games, Dungeons of Daggorath, Color Cubes and Roman Checkers support saving the game in progress to cassette tape. The only other home computer system with cartridge games that support saving to cassette I know of is the MSX. Metal Gear on that system saves to tape.

Because cassettes were designed to store audio and were repurposed for storing data, the Disney educational titles such as Mickey's Alpine Adventure, store prerecorded music, voice acting and speech in between data portions of the tape. Atari did something like this with its cassette tape software for its 8-bit home computers but it generally only plays speech and music while the program loads, not in between loads.

Disk software was more complicated and took up a lot more space, no ports to cassette appear to be available. To run those games it appears buying a CoCo SDC would be the best solution. The CoCo SDC also can simulate ROM cartridges like a flash drive. If I obtain one I would review it.

Popcorn Coco 2 Artifact Colors

Popcorn Coco 1 Artifact Colors

Hard Drive Support

It was very rare for 8-bit home computers to have official hard drive support and almost unheard of for games for those systems to support installation to a hard drive officially. The drives cost far more than the computers themselves, one generally used an 8-bit computer because they were on a budget that could not afford the more capable hardware. The Tandy Color Computer is one of those rare exceptions, Tandy was selling a Hard Disk Controller, Catalog # 26-3145, as early as 1985. This controller, which plugged into a cartridge slot (through a Multi-Pak interface) could support hard drives of 10, 15 and 35MiB. A floppy disk drive and OS-9 2.0 were also required to use these hard drives. The cost of the smallest hard drive and the interface cartridge could be greater than the cost of the system and the floppy disk drive and controller cartridge combined. There are at least five CoCo games that support installation to a hard drive but only one, The Interbank Incident, runs on a CoCo 1 or 2.

Descendants & Cousins

The Tandy Coco 1's direct successor, the CoCo 2, is no more capable than the CoCo 1 when fully upgraded. Its only benefits are the nicer keyboard and the smaller case. I have read that composite video mods may be more difficult on the CoCo 2 than the CoCo 1. The Motorola system reference design was adapted for the Dragon 32 and 64 computers made in Britain. These computers support PAL and come with 32KiB and 64KiB of RAM, respectively. The Dragon 64 has an RS-232 serial port, the Dragon 32 lacks it but both have a printer port. 

Some of the software written for those machines should be runnable on the CoCos but the keyboard matrix between the Dragon and CoCo computers is not compatible. There are also official CoCo machines with PAL capability but the Dragon computers have more PAL software. PAL machines cannot display artifact color, so they run the highest resolution games in black & buff or green.

The Tandy Color Computer 3 was the last member of the line and the first to receive significant upgrades over the prior models. The CPU ran twice as fast, the system came with 128KiB standard and could be upgradeable to 512KiB and the keyboard had four extra keys. The CoCo 3 added composite video and RGB video output. Lowercase letters in text mode was finally standard. The second joystick button of the Deluxe Joysticks was supported. A new extension of BASIC, the Super Extended Color BASIC, was included.

The most significant upgrade to the CoCo 3 was the graphics capabilities. The VDP and SAM were replaced by the GIME (Graphics Interrupt Memory Enhancer). Multiple additional graphics resolutions were supported, including 320x192x4, 320x192x16, 640x192x2, and 640x192x4. These could select from a palette of 64 colors. 40, 64 and 80-column text modes with more text cell attributes and colors were available. Composite colors do not necessarily look identical to RGB colors, so some games allowed you to indicate which kind of monitor you had. 

The Tandy Micro Color Computer MC-10 was something of red-headed stepchild of the CoCo. This computer had a similar CPU (6803), 4KiB of RAM (expandable to 20KiB via its expansion port), an 8KiB BASIC ROM, the same VDP (6847 but no SAM) and a 48-key chicklet keyboard. The same cassette and serial ports were used but the system was not compatible with CoCo software outside certain type-in BASIC programs. It was a failure and pulled from the market in a year.

The TRS-80 Videotex uses the same enclosure, keyboard and most of the same chips and ports as the CoCo 1. The difference between a Videotex and a CoCo 1 is that Videotext has a built-in modem capable of 300 baud communications. It has a data LED where the RAM badge would be on a standard CoCo 1. This was not particularly successful and the basic CoCo could have good modem support with an RS-232 interface cartridge and a proper UART. There is a Videotex cartridge which uses the RS-232 interface and provides the terminal software to connect online.  

The AgVision was built by Tandy and sold by Elanco to farmers and agricultural businesses. Other than the blue color of the enclosure and the unique system badge it uses the same enclosure and internals as the Videotex. The TDP System 100 is another rebadged CoCo 1 with a very similar but off-white enclosure and a keyboard that was also found on the later CoCo 2s (full-height keycaps). It was sold by RCA in partnership with Tandy through a partnership company called "Tandy Data Products" (TDP). The mainboard inside the TDP System 100 is identical to the later mainboard in the CoCo 1.

Final Thoughts

The CoCo is a better system than you might think. It has a lot of games and many of them are freely accessible through the cassette port. I liked Popcorn, which is an Avalanche clone. Polaris is a good Missile Command clone and more faithful than the Atari conversion. It works even better with a mouse. Some people like Dino Wars but you need two players. Canyon Climber is a pretty good exclusive game that reminded me of Jumpman mixed with Conan: Hall of Volta. Zaxxon has a good cassette port. Donkey King is a rather impressive unofficial port of Donkey Kong with good DAC sound effects.

Some games are not so great. Poltergeist is rather tedious. Project Nebula's monochromatic green screen is dull. Bustout has weird physics for a a Breakout clone. Space Assault plays too fast for Space Invaders. Fortunately the library is easy to explore as I have described above.

Of course no CoCo overview can be complete without a mention of Dungeons of Daggorath. This game was an early wireframe dungeon crawler and packs a lot into its 8KiB cartridge space. There are quite a few monsters and items to find. The heart beat as health mechanic is pretty unique and the ability to save a game to tape was very forward-thinking in 1983. I will warn you that the game can be very unforgiving and the need to type in keyboard commands (with a less than responsive keyboard input routine) will give you fits!

The Tandy Color Computer is a well-documented system, all the software and manuals you could ever need can be found on the TRS-80 Color Computer Archive.