HOMEBREW Computer 6809: RedBoard


This page describes a small homebrew computer 68B09P based processor.

Updated: 2015/03/13: EKMonitor.zip up to date, Version 1.3, Bug fix (Roland Leurs)


This project has been started in 2003 and is just a hobby. I always was interesting to build  myself from scratch a small computer. In order to avoid complexity, I chose to use two separated PCBs. In addition, the goal was to use classics existing components.

In 2012, I started to play to write a small 6809 Emulator with Xcode. When it was really usable, I wrote a small monitor for fun. At the end, when this one was running, I'm wondering why not to use it now on a real 6809 system ? Then, I remembered my old both boards done in 2003 and finally picked up them.

On february 1, 2013 the board CPU was working and 24 februray I/O board too.


This computer is made up of 2 boards:

Board 1: Motorola 68B09P, EEPROM 28C64 (8kB), Static RAM 62256 (32kB) and memory decoder built with a 74HCT138.

Board 2: 68B50 for RS-232 interface with a Max232, 4060 for independant clock generation and 68B31 for parallel I/O

To communicate with the system, the serial interface will be use. On Mac OSX the native screen command is already here: “screen /dev/tty.PL2303-000013FA 9600”. Monitor considers the serial device for communication whereas the 6821 is keep free for general purpose usage.

Memory Map

  1. $E000-$FFFF:EEPROM for Boot/Monitor 8kB

  2. $D000-$DFFF:I/O 6821, 6850 (Board#2] 4kB

  3. $C000-$CFFF:No hardware in this area 4kB

  4. $B000-$BFFF:No hardware in this area 4kB

  5. $A000-$AFFF:No hardware in this area 4kB

  6. $9000-$9FFF:No hardware in this area 4kB

  7. $8000-$8FFF:No hardware in this area* 4kB(Checked by the monitor)

  8. $0000-$7FFF:Static RAM 32kB


I/O Board Schematic


CPU Board Schematic


Monitor 6809

The following EKMonitor.zip contents the source and binary for the EEPROM. You can use it, modify it in accordance to what's you want to do. The binary produced is sized to fit the full EEPROM 8kB.


The following RedBoard6809.pdf contents the complete description of the hardware and explanations about the Monitor. Go to the download section below.

Mac OSX Assembler

The following i6809asm.zip archive contents the complete sources for XCode. Example of command line to assemble the source:

./i6809 -l -bin /user/myaccount/documents/xcode/Monitor/EK6809Monitor.asm


Schematic editor

I used the eagle software in free edition from Cadsoft: http://www.cadsoftusa.com/


I'm thinking about to add a new card in the $C000-$CFFF area with:

  1. A RTC chip, may be a Dallas DS1307 (I2C) or DS13887 (Like in Atari Falcon)

  2. Another memory decoder to split this page in 8x512 with a 74HCT138

  3. A connector to add an expansion board with slots and use the previous new ChipSelect decoding.


Download the complete RedBoard6809.zip which contents the hardware schematics, documentation and pictures.

The following compressed EKEmulator.zip file contents the full source code of my Emulator for the RedBoard.

CoCoBasic.zip is a source assembly of a Microsoft Basic running on 6809. A 16 KB Eprom is required but it run on my Emulator as Cartridge.


Gallery: The monitor


“screen /dev/tty.PL2303-000013FA 9600”: Monitor started and ready

Dump memroy from the start of EEPROM

Memsize, Version and Memory test

Tiny Basic started !

Laurent Favard, 2013

Gallery: The Homebrew 6809