Dragon like 6809 Retro Computer

Details

The Dragon 32 and Dragon 64 computers, along with the CoCo 1 & 2 computers as based on the reference design provided in the Motorola MC6883 Synchronous Address Multiplexer (SAM). The design uses a 6809E CPU, 6847 VDG and 32 or 64K dynamic RAM.

The SAM creates the required clocks for the CPU, and interfaces both the VDG and CPU to the dynamic RAM.

This project aims to be largely compatible with the Dragon 32 (and maybe the Dragon 64) but will use SRAM instead of DRAM. This may actually make the circuit more complex since the SAM is designed for DRAM.

Components

1 × 6809E CPU

1 × SN74LS783 Microprocessors, Microcontrollers, DSPs / Microprocessors (MPUs)

1 × MC6847 Video Display Generator

1 × 27C512D ROM

1 × 62256P RAM

Project Logs

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More progress
Matt Callow • 06/04/2021 at 23:15 • 0 comments

I now have a working keyboard. I took an old cherry portable keyboard, which has mechanical keys and re-wired it to the dragon matrix. Wired to the keyboard PIA and it works!
Once I got the keyboard working, I was able to type in some basic programs and check out a few things. Initially I did not have the mode select pins for the VDG connected so was unable to change video modes. One odd thing I noticed was that I only has 16K of RAM reported. I suspected that this was to do with the RAM settings in the SAM. Looking at the ROM disassembly, I saw that it checks bit 2 from one of the PIAs to determine memory config. I wired this up, but it still did not work. Probably because it was now selected 32/64K dynamic RAM. I need 64K static RAM setting.
The Dragon lives!
Matt Callow • 05/26/2021 at 23:00 • 0 comments

I've added one PIA to the prototype and changed the ROM address decoding so that the FLASH ROM is mapped to ROM0 and ROM1. I flashed the ROM with the dragon 32 ROM image and turned on. After a few false starts (I still had some ROM address lines tied low, and I initially tried to use all 3 ROM select lines to enable the ROM) it works!
This is a big step forward. Not useful yet, since there is no keyboard but I'm happy to see it runing the Dragon ROM.
Also no graphics modes since the VDG pins are hardwired. Only monocrhome output for the moment.
Basic SAM and VDG System
Matt Callow • 05/24/2021 at 13:24 • 0 comments

Next step was to build a computer using the SAM, VDG and 6809E to test the RAM interfacing and better understand the SAM functions. This was also built on prototype boards and ironed out some problems with the RAM interfacing.
VDG Test
Matt Callow • 05/24/2021 at 13:22 • 0 comments

To test the 6847 VDG, I build the following circuit on a protoboard. This produced a stable video monochrome image on an LCD monitor. So far so good.
Test Board
Matt Callow • 05/24/2021 at 00:25 • 0 comments

I bought most of the chips from ebay. I'm sure the SAM and CPU are used. So in order to test them I built a simple computer with a serial port that runs basic. This is based on Grant Searle's design and uses firmware from Jeff Tranter.

I built this on a prototype board using point to point wiring. Not pretty but it works!

This design proved that the 6809E (and 6309E) are working