Portotype v. 1.0 of the VGA display adapter is ready to go!. After some adjustments it will be fully functional and ready to be used in the computer.
Current project assumptions:
– mainboard and separate expansion modules
The motherboard includes:
– 32KB ROM
– 32KB RAM (simple decoder using 1xNAND, /CE signal for RAM can be disabled by jumper – a possibly expansion RAM could be used)
– keyboard interface using 74HCT574 + 74HCT245
– I/O port 2x8bit in & out with power supply (also 74HCT574 + 74HCT245). one output port could be used for 4-bit connection with HD44780, which gives us a complete system on one board.
– I/O decoder for 4xI/O devices (using 74LS138 )
– expansion bus: at least 5 slots. Suggested male 40-pin headers – all CPU signals but without dram refresh. Also 5V a 3.3V power supply. 40-goldpin headers are fairly functional choice for expansion cards, and also very cheap and easy to use (solder)
This setup is enough to run the motherboard as a standalone system with a possibility to enter and run programs in memory.