From Wikipedia, the free encyclopedia.
Written by Mike Pepe.
The Radio Shack TRS-80 color computer (also called
Tandy Color Computer, or CoCo) was a home computer
based around the Motorola 6809E processor and part of the TRS-80
line.
Origin and history
Power-on screen of a CoCo 2
The TRS-80 Color Computer, often referred to as CoCo by its
users, started out as a joint venture between Fort Worth based
Tandy Corporation and Austin based Motorola, Inc. to develop a
low cost home computer in 1977.
The initial goal of this project, called "Green Thumb", was to
create a low cost VideoTex terminal for farmers, ranchers, and
others in the agricultural industry. This terminal would connect
to a phone line and an ordinary color TV and allow the user
access to near real-time information useful to their day to day
operations on the farm.
Motorola's MC6847 Video Display Generator (VDG) chip was
released and about the same time as the joint venture started.
(It has been speculated that the VDG was actually designed for
this project!) At the core of the prototype "Green Thumb"
terminal, the MC6847 along with the MC6808 Microprocessor Unit (MPU)
made the prototype a reality by about 1978. Unfortunately the
prototype contains too many chips to be commercially viable.
Motorola solves this problem by integrating all the functions of
the many smaller chips into one chip, the MC6883 Synchronous
Address Multiplexer (SAM). By that time, in late 1979, the new
and powerful Motorola MC6809 processor is released, and together
the SAM, VDG, and 6809 are combined and the AgVision terminal is
born.
The AgVision terminal is also sold through Radio Shack stores as
the VideoTex terminal around 1980. Internal differences, if any,
are unclear as not many AgVision terminals survive to this day.
With much of the engineering work already done, the VideoTex
terminal contains all the basic components for a general purpose
home computer. The internal modem is removed, I/O ports for
cassette storage, serial I/O and joysticks are provided. An
expansion connector is added to the right side of the case for
future enhancements and program cartridges ("Program Paks"), and
a RAM button (a sticker indicating the amount of installed
memory in the machine) covers up the hole where the Modem's LED
"DATA" indicator was. On July 31, 1981, Tandy announces (along
with 2 other computers for 1981) the TRS-80 Color Computer.
Sharing the same case, keyboard, and layout as the AgVision/VideoTex
terminals at first glance it would be hard to tell the TRS-80
Color Computer from its predecessors!
The inital model, which was catalog number 26-3001, shipped with
4K of Dynamic Random Access Memory (DRAM) and an 8k Microsoft
BASIC interpreter on an internal ROM. Its price was $399. Within
a few months, Radio Shack stores across the US and Canada begin
receiving and selling the new computer.
A cartridge-based system
The CoCo differed from the Z80-based Models I/II/III/4 and 4p
by having a different processor (Motorola 6809e) and cartridge
slot rather like the popular Atari 2600 VCS system.
Consequently, many games and applications (including, in fact, a
disk system) were released in cartridge form. Like the Model I,
the Coco came with Standard and Extended BASIC (analogous to the
Level I and II BASIC). A Disk Controller added Disk Extended
BASIC (DECB). These BASIC Languages were licensed from
Microsoft. It is interesting to note that DECB wasn't a true DOS
(disk operating system), but added commands to the BASIC
programming language. These commands directly accessed the disk
drive controller.
Besides Tandy's licensed Disk BASIC from Microsoft,
additional operating systems were available for the CoCo line.
These included the TSC FLEX operating system (distributed for
the CoCo by Frank Hogg) and Microware's OS-9 operating system.
Both systems turned the CoCo into a much more powerful system,
and in the case of OS-9, made it multi-user/multi-tasking.
Tandy also released a Multi-Pak Interface which allowed up to
4 cartridges to be mounted at the same time, a Voice Synthesiser,
300 Baud Modem Pak, and other accessories. The CoCo was the
first Tandy computer to have a mouse available for it.
The Dragon clone
A British clone of the CoCo was called the Dragon 32/64. An
American company Tano, attempted to import these units into the
U.S. but met with no success. The Dragon was a much improved
unit with RGB Video (rather than the TV Output of the CoCo and
CoCo 2, and much like the later CoCo 3), a Parallel Printer port
(the CoCos only printed through a slower Serial port), and a
better keyboard.
In 1983, a version of the Dragon was licensed for manufacture
for the North American market by Tano Corporation of New
Orleans, Louisiana.
Description of different versions
There were three versions of the Color Computer:
Color Computer - Large Case (1980-1983)
The original version of the Color Computer shipped in a large
silver-gray case with a calculator-like "chiclet" keyboard, and
was available with a memory sizes of 4K (26-3001), 16K
(26-3002), or 32K (26-3003). Some versions shipped with standard
Microsoft Color Basic or (optionally) Extended Color Basic if at
least 16k of memory was installed. It used a regular TV for
display, and TV-out was the only available connection to a
display device.
The early versions of the CoCo 1 had a black keyboard
surround, the TRS-80 nameplate was above the keyboard to the
left side, and a RAM badge ("button") was affixed on the top and
right side of the case. Later versions deleted the black
keyboard surround and RAM button, and moved the TRS-80 nameplate
to the midline of the case.
Initial D-board versions of the CoCo were upgraded to 32k by
means of piggybacking two banks of 16k memory chips and adding a
few jumper wires. The E motherboard version that followed
deleted the 4K RAM option and were upgraded to 32k with
"half-bad" 64k memory chips. These boards have jumpers marked
HIGH/LOW to determine which half of the memory chip was good. An
initial cost-cutting measure, this we also transparent to the
BASIC programmer, since only 32k of memory was available anyway.
As production yields improved and costs went down, many (perhaps
most) 32k CoCo 1's actually had perfectly good 64k memory chips
in them, and many utilities and programs did begin to take
advantage of the "hidden" 32k.
Even later versions of the CoCo 1 dropped the 32k memory
option entirely and were available in 16k or 64k versions. All
versions that shipped with standard Color BASIC could be
upgraded to Extended BASIC by simply plugging in a ROM in an
empty socket provided on the motherboard.
Toward the end of the CoCo 1 production run, some models
shipped in a white case with a modified keyboard, often referred
to as the "melted" kayboard, with bigger keycaps but a similar
rubbery feel. At about the same time, another white-cased "CoCo",
the TDP-100, was marketed through Tandy Data Products (TDP) and
sold through a different distribution channel. Except for the
nameplate and case, the TDP-100 was completely identical to the
CoCo 1. The TDP-100 had ventilation slots that ran the entire
length of the case, rather than only on the sides. This
ventilation scheme was carried over to the CoCo 2.
A number of peripherals were available: tape cassette
storage, serial printers, a 5.25 inch floppy disk drive, a pen
and graphics tablet called the "X-Pad", speech and sound
generators, and joysticks.
Color Computer 2 - Small Case (1983-1986)
During the CoCo 1 production run, much of the discrete
support circuitry had been re-engineered into a handful of
custom integrated circuits, leaving much of the circuit board
area of the CoCo 1, by the end, empty space. To cut production
costs, the case was shortened by about 25% and a new, smaller
power supply and motherboard was designed. Physically the
"melted" keyboard and TDP-100 style ventilation slots were
carried over. Aside from the new look and the deletion of the 12
volt power supply to the expansion connector, the computer was
essentially 100% compatible with the previous generation. The
deletion of the 12 volt power supply crippled some peripherals,
such as the original floppy disk controller, which then needed
to be upgraded, installed in a Multi-Pak interface, or supplied
with external power somehow.
Upgraded BASIC ROMs were also available to add a few minor
features and correct some bugs. Also a redesgned 5 volt only
disk controller was introduced with its own new Disk BASIC ROM
adding the DOS command, which was used to boot the OS-9
operating system by Microware. (64k memory required)
Later in the production run, the "melted" keyboard was phased
out and a new full-travel typewriter style keyboard was finally
available. Production was also started in Korea during the CoCo
2's lifespan, and many owners of the Korean-built systems
referred to them as "KoKos". To add to the confusion, production
in the USA and Korea happened in parallel using the same part
numbers. Very few, if any, differences exist between the USA
built and Korean built CoCo 2 machines.
The final significant change in the life of the CoCo 2 was in
the models 26-3134B, 26-3136B, and 26-3127B (16k standard, 16k
extended, and 64k extended respectively) In these models, the
nameplate no longer reads "TRS-80 Color Computer 2" and instead
says "Tandy Color Computer 2". Internally this model was
redesigned to use the enhanced VDG, the MC6847T1. This enhanced
VDG allowed the use of lower case characters and the ability to
change the text screen border color. Unfortunately, both these
features are not used for compatibility reasons and are not
enabled in BASIC.
Color Computer 3 (1986-1991)
On July 30, 1986, Tandy introduced the Tandy Color Computer
3. This new model of the Color Computer line was meant to
compete with the Amiga and Atari ST systems. Based on the faster
68B09E, and with improved graphics, the CoCo 3 was meant to be
more of a gamers system. It came with 128K of RAM Standard, and
could be upgraded to 512K.
The CoCo 3 was compatible with most of the CoCo 2's
peripherals. Most older software ran on it. Taking the place of
the graphics and memory hardware in the CoCo 1 and 2 was an ASIC
called the "GIME" chip. It could display graphics at 640x225
with 4 colors, and at 320x225 could display 16 colors. The
colors were selectable out of a palette of 64 colors (though by
changing the selected colors during refresh the CoCo could
display all 64 at the same time). The GIME provided RF,
composite video and analog RGB output, allowing direct
attachment to RGB analog monitors. The GIME also handled memory
mapping (in 8K blocks, which some developers considered
outrageously large for a 64 kB address space) and RAM refresh
functions.
Previous versions of the CoCo ROM had been programmed by
Microsoft. For the CoCo 3, Microware extended the Extended Color
BASIC to support the new display modes. This was done by
providing a CoCo 2 ROM and a special startup ROM which loaded it
into RAM and patched it. While this worked reasonably well, the
result contained more bugs than previous ROMs.
Microware provided a version of the OS-9 Level 2 operating
system shortly after launch. This OS featured multitasking,
windowing, and a more extensive development environment that
included a bundled copy of BASIC09. C and Pascal compilers were
available. (Various members of the CoCo OS-9 community enhanced
OS-9 Level 2 for the CoCo 3 at Tandy's request, but Tandy
stopped production of the CoCo 3 before the upgrade was
officially released. Most of the improvements made it into
NitrOS-9, a major rewrite of OS-9/6809 Level 2 for the CoCo 3 to
take advantage of the added features and speed of the Hitachi
6309.)
The 6809 in the CoCo 1 and 2 ran at 0.895 MHz; the CoCo 3
runs at that frequency by default, but is software controllable
to run at twice that rate; OS-9 takes advantage of that
capability. (Some models of CoCo 1 and 2 were also capable of
running at this higher speed, but this was not recommended and
could even damage some of them.) Those are one fourth and one
half of the color burst frequency used in NTSC color television.
Synchronizing the CPU clock to the color burst was common in
home computers and video game consoles of the time; even the
original IBM PC ran at 4/3 color burst frequency. This technique
was no doubt convenient in lowering part count, but it limited
how designers of the computer's successors could adjust the
clock rate.
Tandy took many other shortcuts in CoCo design, eating CPU
cycles to cut the part count. The most notorious were probably
the "bit banger" serial port and the "high-res mouse interface,"
which put the CPU through a busy wait loop while a capacitor
discharged to figure out the position of the mouse, so that
unless you were actively using the mouse, you learned to move it
to the upper left hand corner of the screen.
A popular accessory was a high-resolution joystick adapter
(designed by software whiz Steve Bjork). It was first used by a
software package named Color Max 3 which was a clone of MacPaint
but added support for color graphics. This was a very desirable
product for CoCo owners and, interestingly enough, the
prototypes of the Macintosh Computer were built using the same
Motorola 6809 Processor.
Third-party companies such as DISTO and Cloud-9 have done
considerably more with the CoCo than Tandy perhaps thought
possible. For example, one can with third-party hardware attach
IDE and SCSI drives to the CoCo, as well as upgrade the memory
up to 2 MB of RAM. The CoCo still has a small but active user
community.
Hardware Design and Integrated Circuits
Internally the CoCo 1 and CoCo 2 models are functionally
identical. The core of the system is virtually identical to the
reference design included in the Motorola MC6883 data sheet and
consists of 5 Large Scale Integrated (LSI) chips:
- the MC6809E Microprocessor Unit (MPU)
- the MC6883/SN74LS783/SN74LS785 Synchronous Address
Multiplexor (SAM)
- the MC6847 Video Display Generator (VDG)
- two Peripheral Interface Adapters, either MC6821 or
MC6822 chips
The SAM
The SAM is a multifunction device that performs the following
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. It
is generally connected to a crystal at 4 times the television
colorburst frequency (14.31818MHz for NTSC countries) This is
divided by 4 internally and is fed to the VDG for its own
internal timing. (3.579545MHz 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 .89MHz (or 1.8MHz if divided by 8)
Switching the SAM into 1.8MHz operation gives the CPU the time
ordinarily used by the VDG and refresh. As such, the display
shows garbage- as such this mode is seldom used. However, an
unusal mode available by the SAM is called the Address Dependant
mode, where ROM reads (since they do not use the DRAM) occur at
1.8MHz but regular RAM access occurs at .89MHz. In effect, since
the BASIC interpreter runs from ROM, putting the machine in this
mode would nearly double the performance of your 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
1MHz 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.
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 64k DRAMs and some
third party bank-switching memory upgrades that used 256k DRAMs.
The VDG
Character set display of 6847T1 VDG
The MC6847 is a relatively rudimentary display generator
compared to the display chips of some of the CoCo's 8 bit
rivals. It is capable of displaying text and graphics contained
within a roughly square display matrix 256 dots wide by 192
lines high. It is capable of displaying 9 colors: black, green,
yellow, blue, red, buff (almost-but-not-quite white), cyan,
magenta, and orange. The low display resolution is a necessity
of using television sets as display monitors. Making the display
wider risked cutting off characters due to overscan. Compressing
more dots into the display window would almost assuredly make
them unreadable.
Alphanumeric/Semigraphics display
The CoCo is physically wired such that its default
alphanumeric display is actually "Semigraphics 4" mode.
In alphanumeric mode, each character is a 5 dot wide by 7 dot
high character in a box 8 dots wide and 12 lines high. This
display mode consumes 512 bytes of memory and is a 32 character
wide screen with 16 lines. The internal ROM character generator
only holds 64 characters, so no lower case characters are
provided. Lower case is instead "simulated" by inverting the
color of the character.
Semigraphics is a hybrid display mode where alphanumerics and
chunky block graphics can be mixed together on the same screen.
If the 8th bit of the character is set, it is a semigraphics
character. If cleared, it is an alphanumeric. When the 8th bit
is set, the next three bits determine the color and last 4 bits
determine which "quadrant" of the character box is either the
selected color or black. This is the only mode where it is
possible (without sneaky tricks) to display all 9 colors on the
screen simultaneously. If used to only display semigraphics, the
screen becomes a 64x32 9 color graphics mode. The CoCo features
several BASIC commands to manage this screen as a low-res
graphics display.
The alphaumeric display has two colorsets. The one used by
default on the CoCo has black characters on a green background.
The alternate has black characters on an orange background. The
colorset selection does not affect semigraphics characters. The
border in this mode is always black.
Another display mode the 6847 is capable of is Semigraphics
6, where two bits select a color and 6 bits determine which
1/6th of the character box is lit. In this mode only 4 colors
are possible but the Colorset bit of the VDG can select two
different groups of the 4 colors. This mode is not implemented
on the CoCo.
Additonal Semigraphics modes
By setting the SAM such that it believes it is displaying a
full graphics mode, but leaving the VDG in Alphanumeric/Semigraphics
4 mode, it is possible to subdivide the character box into
smaller pieces. This creates the "virtual" modes Semigraphics 8,
12, and 24. In these modes it was possible to mix bits and
pieces of different text characters as well as Semigraphics 4
characters. These modes were an interesting curiosity but not
widely used, as the Semigraphics 24 screen consumed 6144 bytes
of memory. These modes were not implemented on the CoCo 3.
Graphics display
There were several full graphics display modes, which were
divided into two categories: "resolution" graphics and "color"
graphics. In resolution modes, each dot was addressable as
either on or off. There are two colorsets available, the first
was black dots on a green background and green border, the
second, more commonly used one has white dots on a black
background with a white border. In color modes, each dot was two
bits, selecting one of four colors. Again the colorset input to
the VDG determined which colors were used. The first colorset
has a green border, and the colors green, yellow, red, and blue
were available. The second colorset has a white border and the
colors white, cyan, magenta and orange were available.
Resolution graphics have 8 dots per byte and are available in
128x64, 128x96, 128x192, and 256x192 densities. Color graphics
have 4 dots per byte and are available in 64x64, 128x64, 128x96,
and 128x192 densities. The maximum size of a graphics screen is
6144 bytes.
Aritifact Colors
There is a curious footnote to the 256x192 two color graphics
mode. Due to the limitations of the NTSC signal and the phase
relationship between the VDG clock and colorburst frequency, it
is not actually possible to display 256 dots across the screen
reliably. 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 you would expect, you will get either red or blue.
Reversing the order of the alternating dots will give the
opposite color. In effect this mode becomes a 128x192 4 color
graphics mode where black, red, blue, and white are available.
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 red 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! Note that on a
CoCo 3, using an analog RGB monitor, the black and white dot
patterns do not artifact- you'll need to use a TV or composite
monitor. Also the CoCo 3 eliminates the random artifact color
set. If the colors are wrong, hold down F1 while
booting/resetting and the alternate color phase will be
selected.
Lower Case and the 6847T1
The 6847 is capable of using an external character generator.
Several adapter boards were available as third party add-ons
that would allow the CoCo to display real lowercase characters.
Very late in the CoCo 2 production run, an enhanced VDG was
available. Called the 6847T1, it included a lower case character
generator and the ability to display a green/orange or black
border on the text screen. Its other changes were mainly to
reduce parts count by incorporating an internal data latch.
The PIAs
There are two PIA chips in all CoCo models. The PIAs are
dedicated mainly to I/O operations, such as driving the internal
6 bit digital to analog converter (DAC), reading the status of
the DAC's voltage comparitor, contolling the relay for the
cassette motor, reading the keyboard matrix, controlling the VDG
mode control pins, reading and writing to the RS232 serial I/O
port, controlling the internal analog multiplexors, and the
like.
The earliest CoCo models had two standard 6821 chips. Later,
due to changes in the keyboard design, it was found that the
6822 IIA (industrial interface adapter) was better suited to the
keyboard's impedance. It also provided better resistance to
electrostatic discharge due to its sturdier design (can handle
up to 18V without damage). Later the 6822 was discontinued by
Motorola but was produced for Tandy as an ASIC with a special
Tandy part number. Functionally and electrically the 6821 and
6822 are identical and one can put a 6821 in place of the 6822
if that part is bad.
Interface to external peripherals
Due to the CoCo's design, the MPU encounters no wait states
in normal operation. This means that precise software controlled
timing loops are easily implemented. This is important, since
the CoCo has no specialized hardware for any I/O. All I/O
operations, such as cassette reading and writing, Serial I/O,
scanning the keyboard, and reading the position of the joysticks
must be done entirely in software. This reduces hardware cost,
but reduces system performance as the MPU is unavailable during
these operations.
As an example, the CoCo cassette interface is perhaps one of the
fastest available (1500 bits/sec) but it does so by literally
playing software generated sine waves through its internal 6 bit
DAC. While this is happening, the CoCo cannot do anything else
as this uses all the CPU time. Similarly, to read data off the
cassette, the CoCo must count waveform crossings and thus cannot
do anything else until either an error occurs or the operation
ends.
CoCo 3 hardware changes
The hardware in the CoCo 1 and CoCo 2 models was functionally
the same. The only differences were in packaging and some
integration of some functions into small ASICs. The CoCo 3
radically changed this. A new VLSI ASIC, called (officially) the
Advanced Color Video Chip (ACVC) or (unofficially) the Graphics
Interrupt Memory Enhancer(GIME)integrated the functions of the
SAM and VDG while enhancing the capabilities of both. Aside from
the graphics enhancments outlined above, the CoCo 3 offered real
lower case, 40 and 80 column text display capability, and the
ability to run at 1.8MHz without loss of video display. As such
the processor was changed to the 68B09E and the PIA was changed
to the 68B21, which are 2MHz rated parts.
Hardware comparison to other 8-bit PCs
While the CoCo sported perhaps the most advanced 8-bit
processor of the time, that processing power came at a price
premium. Compared to the ubiqitous 6502, the 6809 was far more
expensive. Other manufacturers of the time instead invested
their time in custom video and sound chips for their computers,
making them far better suited as game consoles, which drove
sales. In contrast the CoCo has a very rudimentary unaccelerated
video display controller and no sound hardware. (although the 6
bit DAC is capable of wavetable sythesis and good quality sound)
For example, it is physically impossible for the CoCo to read
the position of its joysticks and play a sound at the same time.
This made many of its games far less sophisticated than the
competition and even the lowly Atari 2600. The hardware design
of the CoCo is both elegant in reducing cost and complexity but
also somewhat inflexible- a problem which plagued the platform
through most of its life.
Some of the hardware limitations were overcome with external
add-ons, particularly expansion cartridges. Some were made by
Tandy, some by other manufacturers. Examples are:
- RS232 program pak, which provided a real RS232 UART (the
6551A)
- The speech & sound pak, which provided a speech
synthesizer and a sound generator chip
- 80 column display adapter, which connects to an external
monochrome monitor
- 300 baud modem pak, which provides a modem in an
integrated package
- Advanced floppy and hard drive controllers
- And of course, the Multi-Pak interface, which permits 4
devices such as these to be attached to the system at the
same time.
Software comparison to other 8-bit PCs
In contrast to its hardware limitations, the CoCo had much
advanced software to its credit. This is helped along by its
powerful MPU. The integrated ROM BASIC, by Microsoft, was
powerful and simple to use, which encouraged the owner to write
their own programs. This was particularly true with the graphics
and sound commands in Extended Color Basic, which were much
simpler to use than the competition. The advanced MPU allowed a
true multiprogramming operating system, OS-9, to be run on the
CoCo. Many users today would be surprised to learn that OS-9
users on a 1MHz Color Computer could multitask and run multiple
programs at the same time nearly a decade before such features
were available on IBM-compatible PCs.
Despite the hardware limitations, the CPU power available to the
programmer did allow for some sophisticated games. Steve Bjork's
Zaxxon comes to mind as a fairly faithful (if somewhat quiet)
port of the arcade game. Dave Dies of Diecom also produced many
high quality games. The CoCo, due to its limited hardware, did
not attract many authorized arcade ports, but there were many
knock-offs made by third parties. Some examples include Ms. Maze
(Ms. Pac-Man), Draconian (Bosconian), and The King (Donkey
Kong). In some cases these knock-offs had new interesting twists
that improved gameplay.
The OS-9 divide
There is/was a major division of CoCo users into two groups:
those who used OS-9 and those who "used" DECB (Disk Extended
Color BASIC); the quotes are present because many if not most
non-OS-9 programs for the CoCo used DECB only as a loader and
for disk I/O, beating directly on the hardware for everything
else. That meant that not carrying on every wart and shortcut in
the original CoCo design would break non-OS-9 CoCo applications,
whereas with OS-9 one would need only rewrite device drivers.
This perceived requirement of total backwards compatibility
killed off at least one attempt to improve on the CoCo--Frank
Hogg's "Tomcat" TC-9 fizzled out while Chris Burke was
attempting to make it simulate all the details of CoCo
hardware--and probably killed them all; if there were an archive
of the CompuServe OS-9 SIG messages, Kevin Darling's cri de
coeur directed to DECB users with the subject line "You're
Killing the CoCo!" would be a useful link. Tandy threw away a
significant opportunity--one should recall that a 1.8 MHz 6809
processor readily outperformed the 4.77 MHz 8088 in the original
IBM PC, and people have run the Hitachi 6309 at 5 MHz.
CoCo 4
On October 26, 1990, Ed Juge of Tandy announced that the CoCo
3 would be dropped from its computer line. Without an apparent
successor, the announcement was disheartening to many loyal CoCo
fans. One CoCo owner went as far as to write a letter to
then-president of Radio Shack, Bernie Appel; to that
individual's surprise, Mr. Appel called him one day explaining
over the phone in detail the reasoning for discontinuing the
CoCo 3.
Even today, current and former CoCo owners agree that Tandy
did not take the CoCo very seriously, despite it having been
their best-selling computer for several years. They failed to
market the CoCo as the powerful and useful machine that it was,
and offered customers no hint at the massive third party
software/hardware market that grew to fill the void.
In spite of Tandy's apparent lack of concern for the CoCo
market, there were rumors of the existence of a prototype CoCo 4
at Tandy's Forth Worth headquarters. Several first hand accounts
of the prototype came from people like Mark Siegel of Tandy and
Ken Kaplan of Microware, yet there exists no known physical
evidence of such a machine.
A few third party hardware companies attempted to carry the
CoCo torch, but the lack of decent backwards compatibility to
the CoCo 3 failed to entice much of the CoCo community over to
the new machines. These new machines did help to pave the way
for OS-9/68K.
Tomcat
Frank Hogg Labs introduced the Tomcat TC-09 in June of 1990,
which was somewhat compatible with the CoCo 3, but was mostly
only able to run OS-9 software. A later version called the TC-70
(running on a Signetics 68070) had strong compatibility with the
MM/1, and also ran OS-9/68K.
MM/1
The Multi-Media One was introduced in July of 1990,
ran OS-9/68K on a 15 MHz Signetics 68070 processor with 3MB RAM,
and had a 640x208 graphics resolution as well as supporting a
640x416 interlaced mode. It included a SCSI interface, stereo
A/D and D/A conversion, an optional MIDI interface, and an
optional board to upgrade the CPU to a Motorola CPU32 family
processor. It is estimated that about 200 units were sold.
AT306
The AT306 (also known as the MM/1B) was a successor to the
MM/1 that contained a Motorola 68306 CPU and was designed to
allow the use of ISA bus cards.
Delmar System IV/Peripheral Technology PT68K-4
Peripheral Technology produced a 16 MHz Motorola 68000 system
called a PTK68K-4, which was sold as a kit or a complete
motherboard. Delmar sold complete systems based on the PT68K-4
and called the Delmar System IV. The PT68K-4 has the
footprint of an IBM PC, so it will fit in a normal PC case, and
it has seven 8-bit ISA slots. Video was provided by a standard
IBM style monochrome, CGA, EGA, or VGA video card and monitor,
but for high resolution graphics the software only supported
certain ET4000 video cards. It appears that most users of this
system used/uses OS-9, but there are several operating systems
for it, including REX (a FLEX-like OS), and SK*DOS. Dan
Farnsworth, who wrote REX, also wrote a BASIC interpreter that
was fairly compatible to DECB, but it was too little, too late
to be of interest to many CoCo users. There was also a card
available called an ALT86, which was basically an IBM XT
compatible computer on a card, which allowed the user to run DOS
programs on it. In fact, you could run both the 68000 and the
ALT86 card at the same time, if you didn't need access to the
ISA bus from the 68000 side of it.
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