Dirac: The simplest single-board computer I can face building
Dirac is a simple single-board Z80-based retrocomputer. It's the simplest one I can face building. I wanted to produce something that's a bit nicer than a pure minimal computer, so it has:
- Serial I/O
- A counter/timer to provide the clock for serial I/O and also allow regular timer interrupts
- Mass storage via SD card
- Banked RAM
The banked RAM is a bit excessive, but it always seemed a bit magical when I was small, and a 64KB address space is less than I really like.
What's the aim?
The aim is to get CP/M running, with associated programs, with a probably intermediate step of BASIC. The design is intended to be retro where convenient. So, it has separate DIP chips for Z80, RAM, EEPROM, CTC, SIO, and 74-series logic for the glue. I don't use a CPLD let alone an FPGA or high-power modern microcontroller to cheat.
However, the non-volatile storage is EEPROM rather than EPROM, and I'm just using a great big SRAM, rather than bothering with the pain of DRAM. Serial I/O uses a proper Z80 SIO, but one ports connects to a TTL serial to USB dongle, rather than RS-232 proper (the second port does real RS232). After a false start, I use an oscillator package, rather than a raw crystal. Mass storage is an SD card, via a break-out board. I see little reason to cause myself a lot of pain of using a more difficult and less available form of storage!
(Having said that, recreating old-school floppy drive control sounds like a fun project in itself...)
Why's it called Dirac?
Projects need names. "Z80-based single board computer" just isn't a snappy name, and doesn't distinguishing it from any other.
I grew up on a ZX Spectrum. It had colour graphics. This machine has no graphics at all. It has serial I/O, and that's basically it. Instead of a spectrum of I/O, it's got a Dirac delta function.
(Random fact: Dirac was a fellow of my old College, and his gown still hangs in the Master's Lodge. I believe it's chained in place, pesky undergrads.)
Why's it taken so long?
(I bought my first pile of components in early 2010.)
Children. They're lovely, but they don't mix well with soldering irons!
- schematics covers the hardware
- src contains the assembly source to program it with
- tools contains the tools to do the required translations to make workable images
What's the current state?
I tried creating a simple loader. It allows you to encode your binaries as hex, load them and execute them. With a recent fix, this is now working reasonably reliably. I can plug the thing in to USB, it'll power itself from the USB port, and I can talk to the monitor at 4800 8-N-1.
I have code to use the SD card interface in read-only mode, and have a basic CP/M port running.
It will run Infocom's Hitchhiker's Guide To The Galaxy.
Where did the CP/M parts come from?
src/cpm22.asm is a lightly-modified copy of CPM22.Z80, extracted from http://www.cpm.z80.de/download/cpm2-asm.zip
(I had disassembled a working CP/M image myself, but this comes with comments and produces the same results so... why not?)
I previously used images/appleiicpm.dsk, which is taken from http://schorn.ch/cpm/zip/cpm2.zip, as a relatively arbitrary easy-to-reverse-engineer disk image. I've switched to building my own disk images.