8bit ISA video board version 1


The aim of this project is to build a videoboard that can be put in a personal computer. The software in the PC will control the framebuffer and some functions of the board itself. This is version 1 as i intend to build a more complex one, but this one is more as a test to get familiar with ISA busses and VGA signal generation.

Specifications

The board will have the following capabilites:
The board will be built out of the following:

Pictures

Here are some pictures of the card itself. And some of its operations.

Front side of the board. The pictures where taken at low light so you cant make out all of the details. The big PLCC84 chips are the XC9572 chips. Upper one is the clock generator and video driver. Lower one is the bus interface. The memories and some latches have not been placed. The upper white pin header is the boundary scan access port. I program the whole via a selfmade XilinX parallel cable 3. (Schematic is also on my site). The connector part has been sawed off a old modem board and reattached to a protoboard.


Underside of the Card, the blue wires are mostly the interface wiring to the analog output and the ISA bus. The small orange cubes are tantalum capacitors to decouple the chip. There are smaller ceramic capacitors of 100nf and 10nf on each power and ground pin to reduce ground bounce.

Here is the card installed in a old 80286 PC. I use this computer as a sacrificial computer so if anything blows on the board. I just put in a new 80286 mainboard. Got plenty of them lying around here that are still working. Altough i am stripping more and more of their logic chips :-) . The yellow LED is the power indicator.

Here is some of the output. The adress counters output has been XORed with eachother to make a colorful block pattern. Altough the LCD doesnt really show it too well. I test my card on a LCD screen becouse its safer. A multisync screen can be oversteered and may ruin the monitor if you feed it weird signals, some screens are more sensitive then others. But better not take any chance.


There 2 pictures show my VGA signal generator. This was the basis of my own VGA card, i started generating signals. There is a great site on VGA signal generation here. It contains a lot of other video modes as well. The generator is now also generating adress, read and chipenable signals to read out a flash bios rom. I use the rom as a sort of test output if the signals that it generates are correct. The rom is 70ns but i really dont care if its correct. (You should need 45ns or faster chips for the framebuffer). Here is the code for the signal generator.


This is what is stored in the flash rom BIOS chip, lots of colors and signals. So it works.

Description

This board is still under construction but here is some details.

The upper chip is the clock generator and drives RGB channels trough 3, 5 resistor networks. giving about 15 bits color. A SRAM chip will be attached to the CPLD and will be basic storage for the framebuffer.

The lower chip is the ISA bus interface. It maps a segment of memory for the framebuffer on adress D0000. and contains a register on adress 220 that controls some aspects of the card

Here is the Verilog code for the ISA interface and VGA generator, Version 0.0.1

When this board is finished, i am going to build a second version that is a full 16 bit board with a 24 bit interface. This board will be more enhanced becouse of more options like wider adresses and more memory. It will be put in a 486 computer so i can program the large adress range via DJGPP. But that project will get its own page.


Back

Last update at: 03-10-2005