Get to Know An Old VGA Card

4/19 2019

Way to Digital World

Digital is all over the world, even though i fight to resist to use too much digitalis instrument. By discuss fixing an interesting SA, MS2601A , recently, It's obviously needed to know some digital stuff. And There is a VGA graphic Card used by this devices.

The Hart of this graphic accelerate card is Hitachi HD63484, ARTIC: Advanced CRT controller. This chip is some sort of dummy 'GPU'. It provide some accelerating graphic operations, anyway, I don't care too much about that yet. But I do interest how this beast dump it's video memory to the Screen.

VGA/S-Video

MS2601 use CPU control everything and display everything on screen, it's a standard compute with almost standard VGA card. VGA signal sent to it's own VGA CRT. S-video also available.

I don't know anything about instrument with a VGA CRT. I gonna make most out it by take this chance.

I notice the signal to the CRT drive part had H-sync and V-sync plus a Video signal. Ok the H-sync and V-sync is typical TV stuff, what a supprise, This is exactly how the VGA display works too!

I heard of the composite signal months ago, which just insert the H-Sync and V-sync signal to video signal.

The video signal is analog, corresponding to the CRT scan line, control it's brightness(by each color). MS2601A obviously only one video signal, so it's mono-color.

The Oscilloscope CRT dominating the SA back the days digital was weak, i like that, hence I don't know VGA and VAG controller .

If the video signal just want to display some black-white bars, this it is.

H-sync mark the starting of the scan line, CRT drive responsible to generating the saw-tooth waveform driven the CRT beam move from left to right( most of time i suppose). Video siganl then control the Z, hence the brightness of the beam, display ONE line's pixels of a video frame.

A little bit about HD63484

HD63484 use 4 bit for each pixel, and HD63484 managed 4 logical screen for you, which is make program much easier, i suppose.

I don't care how the HD63484 speed up drawing a line or something like that. But I do want to know how the pixies in memory dump to the CRT, and how the DAC works.

Video DAC

HD63484 response to generating H-sync and V-sync, but does not do DAC for you.

Simple way to convert memory bit to Video voltage is use the resistor. If only 1 bit for each color, the left circus does all the convert.

if R is high, then output is 5V, if R pull to zero, output Zero, if R is high-impedance state, output is middle. 220 ohm resistor make VGA signal level is correct while inseminated by 75 ohm.

MS2601A use HD63484 which 4 bit per pixel, and further more, 4 logical screen, so PAL14L4 does all the stuff, calculating the final brightness, and out put the bit to 14, 15, 16 each of the pin connected to a buffer and a resistor to generating the 16 level brightness voltage, this is where the Video signal come from!

bottom of this schematic, the S-video also generated!

What is PAL14L4

First of all, PAL, mean programmable Array Logic, which use fuse as interconnection, programed means remove specific link fuse.

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