Hardware Inverse Video Mod

[Serenarian]

I've done this to all my Dragons in the past. The usual static precautions apply, but it isn't too difficult ...

You will need:
1. 74LS02 Quad NOR gate
2. Single pole Double throw miniature toggle switch
3. 1/2 metre of fine insulated wire

And here's how's...
a) Bend up all the pins of the LS02 except pins 7 and 14.
b) Cut off pins 4,5,6,8,9,10,11,12,13 - leaving pins 1,2,3, and 7(gnd) and 14(Vcc) intact.
c) Bend pins 2 and 3 till they touch and solder together (pins 2 and 3 are the two inputs to one of the 4 gates on the chip - pin1 will be the NOT output).
d) Identify a convenient 14 pin 74LS series chip on the board that has pin 14=5V and pin 7=Gnd (74LS00,74LS02, 74LS04, 74LS08, 74LS32). On my D64 Rev3 it sits on IC9.
e) Piggyback your LS02 on top - soldering pins 7 and 14 onto the host chip.
f) Drill a small hole in the bottom casing (under the side alongside the keyboard is good) to mount the toggle switch.
g) Identify the 6847 chip (40 pins) and mark carefully pins 2 and 32.
h) Cut pin 32 as close to the board as possible and bend up the piece of leg attached to the chip util it is clear of the stump and track.
i) USING AN INSULATED/EARTHED SOLDERING IRON solder a wire from pin 32 of the 6847 to the middle pin of the toggle switch.
j) Solder a second wire from the either of the other toggle switch pins to pin 1 of the LS02.
k) Solder a wire from the remaining toggle switch to pins 2 and 3 of the LS02.
l) Finally, and carefully, solder a jumper wire from 6847 pin 2 (don't cut the pin at all) to pins 2and 3 of the LS02

The toggle now selects Black on Green or Green on Black.

Now I know that they recommend forcing the outputs of all unused gates High to save current, but I never bothered and had no problems.

[tjewell]

Just reviving a super old post! Has anyone here ever done this? I'm looking for a way to get green on black, especially under OS-9's 51 column mode.

[GlassTTY]

I'm not familiar, yet with the Dragon schematic, but I wonder if there is a spare gate in one of the existing chips on the board that can be used for this mod.

[tjewell]

Ah, no, hang on ... this is just acting on the data lines isn't it? Doesn't it just make the text modes use the inverse characters rather than the usual ones? Clever, but if I'm right, not what I'm after ...

[Serenarian]

As the person who posted the original, let me explain - and add some pictures perhaps
The mod simply uses an LS chip wired to act as a NOT gate to invert the text video output.
Any NOT gate will do - and you may find a spare one somewhere on the board.
But it is very easy to do a piggyback as only 2 pins are soldered to the chip below

1. This mod will only invert the text characters on the normal 32x16 screen. It does not affect the semigraphics characters.
2. It will NOT invert any text that uses the graphics PMODE pages. So the OS-9 Go51 text will still be unaffected, sorry.
3. It will invert the black on orange as well as the black on green text display.
4. It is completely safe - you can flick the switch back and forth without switching off or resetting the computer.

I've just taken the lid off two of my Dragon 64s to take the pictures. One is series 3 with the mod hiding under the Dragon Plus board. The naked one is a series 2A and both work perfectly

Peter Mooney