SC114, v1.0, Description

SC114 is an easy to build Z80 Motherboard with RC2014 compatible expansion sockets.

SC114 Assembled 2.jpg

The aim of this design is to make entry into the RC2014 universe easier and more reliable, by using fewer components, less soldering, fewer configuration jumpers, and fewer circuit boards.

This motherboard can work in a modest way as a single board computer (SBC), yet can easily be expanded to run CP/M and more.

Here are the specs:

  • Z80 running at 7.3728 MHz
  • 128k bytes RAM (second 64k not easy to use, so best consider it to be 64k usable)
  • 32k bytes ROM, which can be paged out with the usual write to port 0x38
  • Simple bit-bang serial port to get you started
  • Four RC2014 standard bus sockets
  • Simple reset button (no power on reset or reset debounce)
  • LED for power and status indication
  • Runs the Small Computer Monitor, with ROM BASIC and the CP/M loader included
  • Printed circuit board size 102 mm x 102 mm
  • Comprehensive documentation

The idea is that you can build the board without even fitting those tedious RC2014 sockets, then test it with the simple bit-bang serial port. This only involves assembling the components shown above. That’s 5 chips + oscillator + 6 pin header + a few Rs and Cs + LED + reset button.

Looks like this takes about 175 solder joints. Sounds a lot, but way fewer than typical RC2014. Should be a good chance of such a simple circuit working the first time. If it doesn’t work then it is a relatively simple circuit to troubleshoot. The onboard LED can flash results of the self-test so a serial port problem will not look like a dead processor.

You can then add the four 40 pin RC2014 module sockets. So that’s a further 160 solder joints, but it is incremental assembly thus relatively easy to work out where any new fault is.

The next step would be to build and test a proper serial port, such as the official Z80 SIO module. Then if you want CP/M all you need to add a Compact Flash module and a Compact Flash card with the standard CP/M distribution installed. All this is supported by the ROM with no configuration required.

You can even add backplanes from the modular backplane range, although their functionality is limited by the single row standard 40 pin expansion socket on the motherboard.

The motherboard has relatively few components and is thus quite a cheap build, so if it does not prove expandable enough it is not a big deal to discard it, take the useful modules, and move to a more advanced system.

Suitable expansion options are described here.

Homebrew 8-bit retro computing