RC2014: 8 bit Z80 based computer

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Not quite there yet
I've been having some fun revisiting the past by putting together a Z80 based 8 bit computer from a commercial kit of parts.

In the picture, the oscilloscope is out and I'm trying to track down a fault with one of the boards which is preventing it from working properly.

It turned out that it was bad soldering on my part. I just knew this was going to be a problem, the main board that all the cards go into must have around 1,000 solder joints on it. I think 3 had to be re-done.

Some of the plug in cards had self-induced issues too, so it was a couple of evenings to build and a couple of evenings to visually inspect and test everything.

CP/M up and running
It's working now, the CP/M operating system has come up

Once all the gremlins were fixed, the CP/M operating system came up. This is a 1970s operating system for microcomputers and predates MS-DOS. It's said that some parts of MS-DOS (like the A> prompt) were modelled on CP/M. The bad news is there aren't many commands in CP/M so it doesn't do as much as a modern operating system. The good news is there aren't many commands in CP/M so not much to learn or remember!

The kit came with CP/M already installed on a Compact Flash card along with a selection of essential utilities so it worked right out of the box. I did some downloads of a Z80 assembler, C compiler, BASIC and Turbo Pascal, all of which worked perfectly.

Finished computer
The finished computer

The computer is now finished and working. It consists of a backplane board into which other cards are plugged in. From left to right, there's:
It runs off 5 volts and consumes only 1 watt of power. It can run a word processor, spreadsheet, database, and as previously mentioned, a selection of development tools.

Circuit diagrams, PCB layouts, Bills of Material and construction guides are all provided on the RC2014 website making it a pain free build.

The seller of these kits is extremely helpful, and there's a Google group to provide support. That's also very helpful and there are some quite talented people on there, some of whom design their own home-made computers and development tools.

It's been working for a week or so, my next task is going to be sorting out a case for it. Much as I like the "open architecture" look in the picture above, I just know I will break something if I leave it like that.

Here's some specs:

  • Power supply: Choice of USB to 5V 5.5mm power plug (cable supplied), room on the PCB for 7805 voltage regulator (not supplied) to allow higher inlet voltages like 9V. Option to power from FTDI cable by selecting a jumper (cable optional)
  • Processor: Z80 at 7.3728 MHz*
  • Clock source: Dual clock source each with a choice from: External / Manual clocking / Slow clock / Fast clocks between 0.3072 and 7.3728 MHz
  • RAM: 64k static RAM
  • ROM: 64k configurable in a single block between 1k and 32k in size mapped to address space starting at $0000. ROM in use will obscure the RAM at that address. ROM can be paged out by software commands releasing the full 64k RAM
  • Serial: Dual serial ports with FTDI connections. Optional jumpers to allow the FTDI cable and host USB to power the RC2014
  • Compact Flash: Supplied pre-configured with CP/M and utilities, partitioned into 16 x approx 8MB disk drives defined as A> to P>
  • An optional Raspberry Pi Zero terminal board can be ordered with the kit, with or without Raspberry Pi. This board allows a piggy-backed Raspberry Pi to act as an ANSI compliant terminal with HDMI or Phono/RCA outputs for video, and USB inputs to take a keyboard. This means an external PC or terminal is not required

* The reason a whacky clock speed of 7.3728 MHz is used instead of something more user friendly like 8 MHz, is that the Serial I/O card derives the transmission speed from the clock through dividing by 64. 7372800 / 64 = 115200

What you'll need to build the kit:

  • A soldering iron
  • Solder sucker or braided wick
  • Solder
  • Simple hand tools (sidecutters, pliers, screwdrivers etc.)

Optional:

  • A multimeter is invaluable for fault finding and are cheap these days. Everyone should have one, even £10.00 spent on eBay will get a half decent one 😁
  • An oscilloscope, if available, can really help with more stubborn problems
  • Illuminated magnifier - I can't stress how useful these are for finding bad solder joints

To make it operational, either:

  • An FTDI cable to link it to a PC running terminal software. This is a USB to serial cable but has different voltage levels to RS232. Buy one from RC2014, alternatively eBay or Amazon, but watch out for cheap and tacky stuff that may not work correctly. The link works at 115,200 bits per second, so cheap cables will make themselves known quickly...
  • PC or laptop with terminal software like PuTTY or Tera Term
Or:
  • The Raspberry Pi terminal board referenced in the Specs instead of the PC and FTDI cable
  • USB keyboard
  • HDMI or RCA/Phono compatible screen or monitor

Some handy links:

  • RC2014 - the company that provides the kits
  • RC2014 Google Group - I'm finding this very useful for support and advice
  • Tindie link - where you can buy the exact kit that I made above

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