6-March-2019
I’ve built my own personal modular Z80 system, the Z80 Small Computer, or Z80sc for short.
Why? Well because I can. It was just too tempting.
Seriously though, I like designing boards and am happy to reinvent wheels. It is nice to create something from scratch. I intend using it mainly as a software development platform, so don’t expect to see it on Tindie. I may share the designs at some point, but I have a long way to before completing this project.
There is probably nothing significant here that could not be done using the RC2014 bus, the Z50Bus, or any other Z80 compatible bus system.
Having said that, there are a few things I particularly wanted in my system:
- Cards that are easy to insert and remove, but yet still stable. I find the RC2014 modules to be troublesome in this regard. My solution is to use similar low cost connectors, but with 2 rows or 20 pins. It is quite easy to put a finger under each end of the card and wiggle it out.
- Larger cards to allow more space between components or more functionality on a card.
- Good interrupt support. I want a system that supports a large number of interrupt sources, using Z80 mode 2 interrupts, with an interrupt manager card to support non-mode 2 devices.
Currently my system is made up from the following boards.
SC120 – Z80sc Backplane
This is similar to my modular backplanes for the RC2014.
SC121 – Z80sc Processor Card
This is very similar to my SC114 Z80 motherboard for RC2014, but built as a plug in card. It has 128k bytes of RAM, 32k bytes of ROM, a Z80 CPU, a clock, reset and bit-bang serial port. It can run in a modest way as a Single Board Computer (SBC).
SC122 – Z80sc Serial Card
This card contains two Z80 SIO chips, which provide a total of four serial ports. The Z80 CTC generates the baud rate clock for three of the serial ports, with the fourth serial port fixed at 115200 baud. The fourth channel of the CTC is used to generate a clock tick interrupt.
SC123 – Z80sc CompactFlash Card
This card adds a simple 8-bit IDE port, which can be used to connect a CompactFlash adapter, as illustrated below, or an IDE device via a short ribbon cable.
SC124 – Z80sc Digital I/O Card
This digital input/output card provides 8 input bits and 8 output bits, each with a status LED. It also has a 6 way DIP switch to select configuration options in the firmware. A second DIP switch sets the card’s I/O address, allowing many such cards to be included in the system.
Other Cards
I plan to build a number of other cards for the system. These include:
- Interrupt manager to generate Z80 mode 2 interrupts from devices that do not include native support for Z80 mode 2
- Synchronous serial card for I2C and SPI devices (I2C bus signals are included on the backplane)
- Analogue to Digital Converter (ADC)
- Digital to Analogue Converter (DAC)
- Flexible Z80 CTC card
- Flexible Z80 PIO card
- Flexible Z80 SIO card
- Z180 Processor
- Backplane extension section
Software
The system can run the Small Computer Monitor, ROM BASIC, and CP/M.
Small Computer Central 