A portable CP/M machine

Some time ago I found FabGL, which is a fantastic library implementing a VGA controller and a PS/2 keyboard controller for the ESP32. It means you can use Arduino IDE to create a sketch that talks directly to a real VGA display and a real PS/2 keyboard. For VGA,  Red, Green and Blue signals and …

A dual-ported RAM for the MC6847

My CZ80 computer design requires a dual-ported RAM between the Z80 and the MC6847, to solve the memory contention problem. Now that the RAMs have arrived, I modified the MC6847 test circuit to try them out. The MC6847 was set to text mode. I removed the EEPROM from the circuit and replaced it with the …

CZ80: emulation

With a working MC6847 circuit, the CZ80 computer now has a viable video output option. But before tackling the hardware, I would like to test the concept in software, that is, build an emulated CZ80. I grabbed the NASCOM Basic Z80 source code I got from the internet and adapted it to my purposes: added …

MC6847 test circuit (part 4)

For this MC6847 graphic test, I chose mode CG1 (see table below). GM0, GM1 and GM2 must be 0, INV, INT/EXT and A/S are "don't care", and A/G must be 1. This mode uses two bits for pixel color, so one byte stores four pixels. The total space required to store one screen is then …

MC6847 test circuit (part 3)

The MC1372 modulator chip has finally arrived, so I can test the MC6847 in full color. The only problem is that it requires completely different clock and amplifier circuits. I decided to follow the schematics and information detailed on the MC6847 datasheet as much as possible, but had to replace some unusual resistor values with …