COMIX-35 Home Computer

The COMIX-35 is an open-source clone of the 1980s COMX-35 8-bit home computer. It uses the RCA 1802 microprocessor which is strange and slow but holds the distinction of being the first CMOS microprocessor. The "35" is because the original computer has 32K of main memory + 3K of video RAM.

There are a number of differences between the COMIX-35 and the original:

• Single +5V power supply
• Uses SRAM; no stolen cycles for DRAM refresh
• Configurable PAL or NTSC by changing three jumpers and swapping a crystal
• Additional CPU crystal for a new turbo mode
• Mechanical key switches for your typing comfort
• Cassette input data light indicates incoming information

Design files

Here are the design files. The BOM includes Mouser part numbers for everything except the CDP1802, CDP1869, CDP1870, CDP1871, the trimmer capacitor (optional), and the expansion slot header (also optional). I recommend you purchase IC sockets for the four CDP parts, as well as for the ROM chip U15.

Schematic

Bill of Materials

Fab Files

The board is rather large, at 12 x 6.5 inches (304.8 x 165.1 mm), but it does include the keyboard. It is a 2-layer design so it should be relatively cheap to manufacture. A word of advice: pick out your keycap set first, then choose the PCB color to match it.

The keyboard keyswitches are easily substituted for lower-cost ones from other vendors. I recommend you purchase DSA-profile keycaps since some of the keys are located in rows that are not typical. I ended up buying the YMDK 125 DSA Dye Sub 9009 Retro PBT Full Keyset for MX. You can choose whatever you want, but make sure there is a 1 unit control key and a 1.25 unit shift key. The enter key needs to be 1.5 unit, so I just used the tab key for that instead. If you have any doubt, check the PCB layout which has the key cap unit sizes marked below each key.

I have not included part numbers for the 0.1" headers which usually come as a long strip that you break into the appropriately-sized sections.

Most other components may be safely substituted. U4 is a little tricky. It should be a 74HCU04 in order for the oscillators to operate, but in a pinch, a 74HC04 will also work. A 74LS04 or a part from another logic family will not work.

Assembly notes

Since the standard COMX-35 ROM configures the CDP1869 to use extra-tall characters, you need to tie CDP1869 pin 25 to CMA3. Do this by soldering a jumper wire from pin 1-2 on JP6. If you wish to write your own code to deal with short characters and more text pages, then install a 0.1" header and jumper instead.

The default configuration for PAL is as follows:

Location	PAL	NTSC
JP1	2-3	1-2
JP2	2-3	1-2
JP3	2-3	1-2
Y1	17.734 MHz	14.318MHz

The trimmer capacitor, C12, is optional. It allows you to adjust the color image for best clarity. If you don't use it, then just stuff C13 with a 22pF capacitor.

You can also configure your COMIX-35 for turbo mode which allows the CPU to run at much higher frequencies than the original computer, limited only by the maximum frequency of your particular CDP1802. Check the datasheet for the variation you have, or experiment. Turbo mode is enabled by jumpering JP4 to position 1-2. This enables the clock oscillator installed at Y3. To run at the standard speed, set JP4 to 2-3.

If you use a 28C256 or 27C256 ROM, jumper JP5 will allow you to select between the upper and lower 16K of ROM. This is useful if you want to use store two different ROM binaries, such as the standard COMX-35 ROM and your own custom program.

The COMX-35 ROM image is relatively easy to find so I will not provide it here.

Video adjustments

There are two trimmer resistors that adjust the video:

• RV3 adjusts the sync pulse amplitude
• RV4 sets the overall video amplitude

The simplest way to get it working is to use an oscilloscope to set the sync pulse amplitude to about 300mV and the video amplitude to about 700mV. If you don't have an oscilloscope, set RV4 to the maximum, set RV3 to the minimum, and then slowly increase RV3 until you get a stable picture. If the colors are not saturated enough, you can decrease RV4.

The C12 trimmer capacitor, if you installed it, adjusts the frequency of the colorburst by a small amount. Since the colorburst and the pixel clock are not sychronized, you'll get a "shimmering" effect on colored pixels. Adjust C12 until you can't see shimmering anymore.

Loading programs off a computer using the cassette port

The process I've had success with is this:

• Run the Emma 02 emulator. Turn off SB mode, turn off Turbo LOAD/SAVE. Make sure Auto LOAD/SAVE is turned on.
• Hit run. Press enter to go past the COMX screen and get to the BASIC prompt.
• Click the LOAD button and select the program you are interested in.
• From BASIC, type PSAVE.
• Name the file (it is actually a raw sound file, not WAV.)
• It will take a while but it will eventually save the cassette sound.
• Load the raw file in Audacity using File->Import->Raw Data. Select 22050 as the sampling rate, mono, signed 16 bit little endian.
• You may wish to save the program as a regular WAV file at this point.
• Connect an audio cable between the COMIX-35's EAR jack and your computer's speaker output.
• Make sure your computer has absolutely no audio processing going on at all. No echo, no equalizer, nothing at all. You may have to really crank up the volume.
• Hit play, and then adjust the input bias control until the data LED flickers.
• Stop, rewind, hit play, and then (on the COMIX-35) type "PLOAD" and hit enter. If everything works, PLOAD will return you back to the BASIC prompt.
• Type RUN and hit enter to start the program.

The cassette interface is a bit tricky to work due to various reasons. The input circuit uses a Schmitt-trigger buffer, so the DC bias of the incoming audio signal must be adjusted to the halfway point of that threshold. This is what the INPUT BIAS trimmer does.

You also need enough volume to be able to exceed the rising and falling thresholds.

Tips and tricks if things don't work right the first time:

• Does your computer really provide clean audio? You've turned off all effects processing? Audio managers, headphone bass boosters--all that must be turned off!
• Check your cables
• Check the volume level on the PC. If it is too low, it will not work
• Connect an oscilloscope to the cathode of D5 and make sure you see a clean waveform

Compatibility

Note that I have not tested the COMIX-35 with real COMX-35 accessories, such as the expansion chassis, RAM boards, disk controllers, etc. It should work with all these accessories except for the RAM expansion because it uses the RAS and CAS lines. Since the COMIX-35 does not have the DRAM refresh circuit, these signals don't exist on the expansion connector.

License

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License. See https://creativecommons.org/licenses/by-sa/4.0/.