Some years ago I acquired a 6800 based Motorola D2 development kit. I later discovered the 6809 and it has become one of my favourite microprocessors. I started thinking about using a 6809 instead of the 6800 with the D2 kit. I compared the datasheets of the two microprocessors and I felt it would be possible as there wasn't a lot of differences between the two chips.

Using what I learned from the datasheets I wired up a small circuit board that would hold the 6809 CPU and which could be plugged in to the 6800 socket of the D2 kit.

I also rewrote the firmware of the D2 kit for use with the 6809 and enhanced it to support the extra internal registers of the 6809. The original firmware of the D2 kit in a 1k ROM chip. My rewrite almost fit in the same space but I wound up just a few bytes over 1k. It forced me to modify the D2 kit to use a 2k EPROM to hold my updated firmware. In order to make single stepping work on a D2 kit using a 6809 the time delayed NMI generator circuit needs to be modified as the 6809 takes more clock cycles to execute the RTI instruction.

I no longer have the D2 kit but I think I still have the original manual stored away in a box. I hope it will have notes about the adapter I built and a listing of the updated firmware. I remember my board used an OR gate built using two diodes and a resistor, and an inverter built using a transistor.

The schematics

The schematic for the 6800 to 6809 adapter was created after reading the 6800 and 6809 datasheets and partly from memory. I remembered the OR gate combined the E and Q signals to create VMA. I didn't remember which signal required the inverter. A study of the datasheets indicated it was the TSC signal. The 6802 to 6809 adapater was created after I saw a posting on a vintage computer website where someone was asking for such a device.

The links below are for PDF files of the schematics I created for the CPU adapter boards. I have not tested either circuit as I do not currently have a working system that uses either a 6800 or a 6802. I do have a Motorola D1 kit which has been heavily modified to use modern EPROM and RAM but it is not yet in a working state, as far as I know. If (when?) I get it working I will be able to test out the 6800 to 6809 adapter for myself.

Final thoughts

I have not tested either adapter circuit in a working system. If you are able to make a working adapter using my schematics, great. If one of the adapters does not work and you are able to make it work, please let me know what change was required and I will update the circuit diagram.