I write a book about old computers. Oh, sorry: I mean "vintage computers" from 80's. My primary goal is to show how one can build a 8bit computer from scratch. It means: how to set up a CPU, how to connect memories, how to connect peripheral chips, and why to connect...
This computer is a "demo construction": A computer, based on 8085 CPU, with 32 kB of (EEP)ROM and 32 kB of RAM, serial connection and parallel ports. All should be as simple as possible. And here is a result.
Wait a moment - why OMEN?
It is simple. OMEN is an abbreviation (of course) of Czech expression "Osmibitový Mikropočítač pro Elektronické Nadšence", which means "an 8bit Microcomputer for Electro Enthusiasts".
Of course - when I have connected the speaker and played a tune, the first tune I have implemented was... right, "The Omen" by Prodigy!
Monitor has four commands:
D is for memory dump
M is for memory change
G is for Go
: is for load a HEX file
EEPROM contains TINY BASIC at address 1000h, so enter G1000 to start BASIC.
As easy, just write a program in assembly language, compile it to the HEX and copy/paste to serial monitor. Then use G command (as GO) to run.
See this video:
OMEN Alpha, issue 2
The second version of PCB has some new features:
- System connector (24 pins) with data bus, A0-A2, /RD and /WR, seven /CS (decoded), system clock and an interrupt
- Selectable IRQ (RST7.5) for external interrupt request
- Switchable /WE for EEPROM to enable in-place programming
Parts placing on PCB
Here is parts placing image:
- JP1 is a serial port. Pins are (from bottom to up): GND, TxD, RxD (next to "JP1" label)
- JP2, JP3, JP4: Parallel ports from 8255. A square marks pin 1, see below to schematics. JP2 is port PA, JP3 is for PB, JP4 is for PC
- JP5 is a EEPROM write enable. Connect pins 1-2 to disable writing, connect 2-3 to enable (pin 1 is the leftmost, pin 3 is next to the label "JP5")
- JP7 is RST7.5 enabler. Close to disable interrupt, open to enable.
System connector (at the upper edge, square marks pin 1)
Square marks the "Vcc" pin.
I plan more OMENs, named Bravo, Charlie, Delta, Echo... Bravo is simillar SBC, but with 65C02 CPU. And other ones? Stay tuned!