I would like to be able to drive a 4x20 line LCD with a VGA output from an old computer. Is there a simple interface to do this?
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The short answer is no. VGA is an analog interface, LCDs use a digital interface. You would need a 3 channel ADC to start with, but even that’s not enough. It would be very difficult to extract alphanumeric characters for the LCD from that data. Plus the 4x20 LCD is so small compared to the VGA display that there’s no way it can hold all the information shown on VGA.
You would be FAR better off rigging up some kind of serial port interface to the PC with the VGA output and selectively sending whatever data is of interest to the LCD.
In all honesty, I believe this would be a fruitless exercise. The interface would be far from simple, and the resulting image on the 4x20 LCD display would most likely not be recognizable from the original VGA image.
VGA has a resolution of 640 x 480 and the LCD display would be 100 x 28, assuming 5 x 7 matrix for each character. Keep in mind the LCD display consists of a 4 rows of 20 characters each. Each character is formed by a 5 x 7 dot matrix, and the characters are separated by approximately a pixel width and 0.2 pixel height. So you are going from an aspect ratio of 4:3 to 25:7 or 12.5: 3.5, a significant difference.
In addition the pixels in the VGA display can vary in color and brightness, where the LCD display is only black and white, and are not individually adjustable in brightness.
And finally, the electronic interfaces to drive each type of display are significantly different both in hardware and software, and would require a very complex interface to even come close.