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Apple IIe RGB to CGA |
The 'A2E EXT. 80COL/RGB' card gives the Apple //e RGB output to an AppleColor M100 monitor. I got the card but the monitor seems unobtainable. I had read that a CGA monitor could be used and I had one of those. But I found it was not that easy. The CGA monitor requires the signal to be 60Hz, vsync also and positive pulses on hsync and vsync. My Apple IIe did output a 50Hz signal and a negative pulse composite sync. I changed the motherboard to the US NTSC version to get 60 Hz and I made this circuit:
The circuit is not very advanced, pretty straight out of the LM1881 datasheet except for the inversion.
What the circuit does is to filter out a vsync from the csync and adapt polarities of the signals. The LM1881 inputs the negative pulse csync and outputs a negative pulse vsync. Then the 74LS04 inverts the csync and vsync signals so they fit the positive pulse hsync and vsync signals of the CGA monitor
Originally I thought that vsync could be got from the //e motherboard but this is not so; the csync signal is generated in a 40pin ic and output already mixed.
As this is not a big cicuit it is kind of a waste to build it on a veroboard and
enclose it in a box so I thought; how about making it a lump on the cable? And,
yes it can be done. Here is how I made the lump:
Some pictures:
For it all to work the RGB card has to have the 5V jumper installed.
This is the jumper near the 74LS173 in the upper hindmost corner of the RGB card.
This jumper connects 5V from the Apple to pin 15 of the DB15 and is necessary for powering the interface.
Note: I have corrected this webpage. Unfortunately I made a mistake when I made the compact version and also this webpage, I used the wrong output of the LM1881 for vsync; it has to be 3 (not 1), then inverted as before. Unfortunately I didn't notice my mistake because the IBM5153 works happily with the inverted csynk signal on vsync. It was Bryan Parkof who discovered the problem when he looked at these signals with his oscilloscope. He has the one in the pictures. My other CGA monitor wants a real vsync signal.