Schematic/spin code that would allow a Propeller to use a CGA arcade monitor
codepro
Posts: 22
Does anyone have a schematic/spin code that would allow a Propeller to use a CGA arcade monitor as a display device?
The CGA arcade·monitor has the following characteristics:
input signals
Video: RGB analog, 1v to 5v peak-to-peak (adjustable with contrast control), 4.7k ohm input impedance, 40 usec to 50 usec active video.
horizontal scan
Frequency: 15.1 kKhz to 16.8 kHz
Linearity: +- 5%
Vertical Scan
Frequency: 47 Hz to 63 Hz
Linearity: +- 5%
The CGA arcade·monitor has the following characteristics:
input signals
Video: RGB analog, 1v to 5v peak-to-peak (adjustable with contrast control), 4.7k ohm input impedance, 40 usec to 50 usec active video.
horizontal scan
Frequency: 15.1 kKhz to 16.8 kHz
Linearity: +- 5%
Vertical Scan
Frequency: 47 Hz to 63 Hz
Linearity: +- 5%
Comments
you just need to adjust the resolution down to whatever the monitor can handle.
(I don't have any CGA monitors at the moment, so I can't test it)
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You could probably make a clever circuit that would let you derive 3 RGB plus a brightness signal from the "normal" VGA pins. It would be pretty straightforward. All you have to do is put a two input "OR" gate across each of the 2 bit DAC's (minus their resistors) to generate CGA RGB signals, and slap a 3 input OR gate across the pins that are normally connected to a 240 ohm resistor in the VGA DAC's, and this would give you a brightness signal. You could also do this with small signal diodes (called "wired OR"), but you would need pull down resistors - in the end, it would be easier to use a pair of real 7400 series OR gate chips.
But, since you have to modify the code anyway to get the resolution right, it might be easier to modify the VGA driver to also work with the 4 bit CGA colors. I don't know the VGA object well enough to say how you would have to set up the video registers.