Dragon 32 / 64 6809 Computer

The Dragon, like the Radio Shack Color Computer, is an implementation of the Motorola MC6883 / 74LS783 Synchronous Address Multiplexer (SAM) datasheet, including the MC6847 Video Display Generator (VDG).

This is / was my Dragon (it found a new home), as received, minus the ROMs which I removed to dump (26, 27). I don't have a top cover and the "2" key was probably damaged and has been replaced with some funny switch. Me, I would have replaced the "break" key instead. These are "short-stem" Alps keyswitches.

I'm thinking it's an Issue 2A board maybe, for what it's worth

Dragon 32 Block Diagram. It's functionally close to the same.

OK, so how this works.

On its own, the MC6847 Video Display Generator (VDG) shares some display memory with the processor of your choice, as seen in the APF Imagination Machine (6800) and the TRS-80 MC-10 (6803) -- which both use Motorola processors, but that's not a requirement -- see the Vtech Laser 200 (Z80A), the NEC PC-6001 (NEC µPD780C-1, == Z80), and the Acorn Atom (6502, but that's close enough to Motorola for there to be case law). It also doesn't have to be an 8-bit CPU, see AN-815: Color Graphics for the MC68000 using the MC6847.

The VDG generates the video address, grabs the data when the processor is not looking, and makes a video signal from that.

This use seems to be discouraged. It's shown in Figure 23 (of 23!) in the MC6847 datasheet. Sort of as an afterthought. Whereas using it in conjunction with the SAM is... Figure 1 of 23. Get the hint, people (well, it's obvious both Dragon Data and Tandy did just that).

The trick is that the VDG generates sequential memory addresses, and if you (in this case, SAM) are synchronised to it, you can generate those same addresses for it, and it won't know the difference. Look at the Dragon schematic. Or the SAM appnote. The 6847 address lines go nowhere. The SAM looks at the horizontal sync line to synchronise with the VDG and then feeds it data from any "page" in memory, the VDG makes video from it, and you have a nice powerful system.

You can also use the SAM without the VDG, to refresh the DRAM and provide address decoding, see the Microbox and the Sardis (both 6809 machines).