The 6809 machine

The first machine I had was a CompuKit UK101, a 6502 based machine. I wanted to build my own machine, and went for a 6809 based system.

I had a metal case in which I put a toroidal transformer, a rectifier and four voltage regulators: -12V, -5V, +5V and +12V.

I used double sized Euro cards and built a backplane that could support four of these cards.

THE FIRST BOARD

This consisted of the 6809 processor, 4kBytes EPROM, 4kBytes static RAM, an ACIA, a VIA chip and an AY-3-xxxx speech synthesiser chip. This was hooked up to an 8 digit 7 segment display and a keypad to enter hex values. I hand wrote the monitor software and used a friends machine to burn the EPROM.

THE SECOND BOARD

I built a 6545 video controller card with 4kBytes of dual ported static RAM. The 6809 has a symetric clock, so one half could be the video controller, the other the CPU. I initial had problems with the static RAM being too fast, as it picked up addresses that weren’t read on the video side resulting in the first character being displayed as a rapidly changing character. I solved this by shifting the clock until it loaded the address 250ns after the video bus supplied it and the issue went away.

I created my own character generator, 128 characters in the first part and blocky graphics in the second part.

THE THIRD BOARD

I designed a dynamic RAM board with 8 1 bit chips that gave me 64kBytes, and did the essential refresh in TTL counters. The 6809 had no support for dynamic RAM (unlike the Z80), so I had to design my own. It surprised me when it worked first time and was rock solid.

THE FOURTH BOARD

I got hold of data on a Western Digital chipset – two chips. I bought a double side 5¼” floppy disk drive and built the electronics around the chipset. I used hardware to drive the timing for the heads, motors etc. I eventually got it working and got 1.8MBytes out of a standard floppy and started running FLEX with it.

All these boards were wire wrapped. It did not last and the main CPU board died. By then, I’d moved on, and got interested in the BBC range of machines. I went from Acorn Electron, to BBC Master and finally the Acorn Archimes A410/1. I still have that in my garage… rotting away…

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