MC6808 Bench Unit

mc6808 Bench Unit
Heathkit ET-3400-A Clone




I was first introduced to the ET-3400 in College.  It didn’t take me long to decide that I wanted one of my very own.  At one point during the course I thought it would be cool to build a clone as a final project.  As a result this little trainer has been a “test bench” in my shop for over twenty years now.  I have a photo copy of the original et-3400 manual which contained a listing of the ROM contents for the trainer.


I developed an interface bus that consists of two 16-pin ribbon cables.  One cable contains the address bus, the other contains the eight data bus lines, the six control bus lines and vcc/gnd.  I designed the system using multiple cards.  One card contains the mc6808 Microprocessor and TTL for the control bus.  Another card contains the Trainer ROM and TTL address decoding for the ROM.  Another card contains 1K of RAM and TTL address decoding for the RAM.  Another card contains the Decoding and interfacing TTL for the keypad.  Another card contains the address decoding and interfacing for the trainer LED displays. The last card contains the Address decoding and ZIF socket for a USER ROM.

I downloaded a cross complier from Motorola.  The compiler and manual are available for download from this site.  I also created a memory map document and some notes about the ROM routines.

Pictures, Circuit Schematics and data sheets are also available from this site.


Name Link
Schematic Page 1 6808_CPU_RAM_ROM
Schematic Page 2 Keyboard_Display_Decode
Schematic Page 3 6808_Display_driver
Cross Compiler 6808 Compiler
Cross Compiler Manual 6808 Compiler
Memory Map 6808 Compiler
ROM Routines 6808 Compiler