TRS-80 Color Computer 2

The CoCo2 is a nice little 6809 machine featuring 32x16 characters video output,
a builtin keyboard, a ROM slot, RS-232, I/O ports, more than enough to have fun
with Collapse OS on it.

The most straightforward way to run Collapse OS on it is to build a custom ROM
cart. At first, you would think that you could cannibalize a ROM cart you have
laying around, but the ones I had had some kind of unmovable round plastic chip
on the PCB, so nowhere to place a AT28 on. I built my own from scratch.

Relevant Documents

* M6809 datasheet
* Service Manual - TRS-80 Color Computer 2 NTSC Version
* Color Computer ROM Cartridge Schematic

Gathering parts

* A Coco2. Mine is the 64K RAM version.
* A 40 pin male card edge connector. If you can get a version that has its pins
  pre-bent over 2 rows, you'll save yourself some work.
* A protoboard that is large enough to accomodate 20 pins, narrow enough to fit
  in the ROM card slot, long enough so that you can still comfortably hold it
  while fitting it in the slot.
* A AT28C64B EEPROM
* A socket for it.
* A disposable CoCo2 ROM cart helps when trying to visualize pin placement.

Building the cart

Then, it's only a matter of wiring the proper connector pin to the proper AT28
pin. The CoCo2 ROM cart pinout is this:

7: Q
8: CART/
9: 5V
10: D0
11: D1
12: D2
13: D3
14: D4
15: D5
16: D6
17: D7
18: no connect
19: A0
20: A1
21: A2
22: A3
23: A4
24: A5
25: A6
26: A7
27: A8
28: A9
29: A10
30: A11
31: A12
32: CTS/
33: GND
34: GND

When you hold the cart with the edge facing you, pin 1 is on the top pane, at
your left. Pin 40 is on the bottom pane, at your right. Pins 1-6, 18 and 35-40
are all no connects.

Q and CART/ are wired together and don't touch the AT28. Data and address lines
are connected to the same AT28 pin. CTS/ is wired to AT28's CE/.

On the AT28, you will want to hard-wire WE/ to 5V and OE/ to GND. If you want
your cart to accomodate bigger EEPROMs, you'll want to hard-wire A13 and A14.

Running Collapse OS

Once you have your cart, run "make" in arch/6809/coco2 and write os.bin onto
your AT28. Then stuff it on your cart, plug it in, and poof! Collapse OS.

ALL CAPS

The CoCo2 has 64 character glyphs builtin and Collapse OS piggy-backs on them.
In those 64 glyphs, there are no lowercase letters. However, every letter can
be displayed with a dark background. This is what we use to indicate a lower-
case letter.

Keyboard input is by default uppercased. Hold shift to type a lowercase.