SC131, v1.0, User Guide

SC131 is a Z180 based pocket-sized retro computer.

SC131 normally has Flash memory containing RomWBW in socket U1.

Documentation for RomWBW can be found here.

A short guide to getting started with RomWBW on SC131 can be found here.

Quick Guide to Jumpers

As a quick guide to getting started: Fit a jumper shunt in the position shown below.

And for those who are sensible enough to read User Guides…


The power required for this board is 5 V D.C. at 100 mA without an SD card or other accessories. With an SD card present the current requirement could rise to 300 mA.

SC131 is normally powered from the primary serial port (serial port A).

SC131 is typically connected to a computer or terminal with an FTDI style serial adapter. This adapter also provides power for the SC131. It is vital that the adapter is able to provide enough power without the voltage dropping significantly. If SC131’s red reset LED stays on it is likely due to the voltage supervisor (U7) holding the processor in reset because the voltage reaching the board is not adequate.

The board can also be powered from the serial port B. This requires a jumper shunt fitted to JP2 to connect power from serial port B.

WARNING: You should normally only connect one power source to the system at a time.

As power can flow either way, the jumper (JP2) also enables serial devices to be powered from SC131. If such a device is used in serial port B, fit a jumper shunt to JP2.

Write Protect Flash

Jumper JP1 enables the Flash chip to be write protected. It is unlikely the Flash chip will be corrupted in normal operation due to the software requirements to write to it. However, for peace of mind you can disable writing with a jumper shunt.

Fit the shunt in the “Rd” (read only) position shown below in green to disable writing (to write protect the Flash memory). Alternatively, fit the shunt in the “Wr” (write enabled) position shown below in red to enable writing.

Serial Ports

The motherboard has two asynchronous serial ports, with software selectable baud rate. These are 5 volt FTDI style ports.

The pin-out, below, describes signals with respect to SC131, so an output is a signal from the SC131 to a computer or terminal.

1Ground (GND)
2Request To Send (RTS) output
3Vcc (5V)
4Recieve Data (RxD) input
5Transmit Data (TxD) output
6Clear To Send (CTS) input

A typical FTDI style serial adapter is illustrated to the right.

This is fitted to the serial port by a 6-way Dupont cable.

The default serial ports settings are as follows:

Baud rate115200
Data bits8
Stop bits1
Flow controlHardware (recommended)

The default serial port connection is P3, Serial Port A.

SPI Port

SC131 has an SPI port, primarily for the connection of a Micro SD card adapter.

The signals are 5 volt levels, with the following pin-out.

1Chip select (active low)
3Master out, slave in
4Master in, slave out
5Vcc (5V)
6Ground (GND)

A typical micro SD card adapter is illustrated to the right.

This may be fitted directly to the circuit board (when installed in the recommended case) or by a 6-way cable (if required).

Memory Map

The physical memory map is simply 512k byte of Flash ROM at the bottom of memory and 512k bytes of RAM at the top of memory.

The Z180’s memory management unit deals with mapping chunks of this memory into the logical 64k memory map supported by the Z80/Z180 instruction set.

0x80000 to 0xFFFFFRAM (512k bytes)
0x00000 to 0x7FFFF Flash ROM (512k bytes)

Input / Output Ports

The SC131’s I/O port addresses are as follows.

0xC0 to 0xFFZ180’s internal I/O registers
Status LED port (write only)
Bit 2 = LED control (active low)
SPI device select (write only)
Bit 2 = SPI select (active low)

As SC131’s does not provide expansion through an accessible processor bus, there is no need for tight I/O address decoding. The address values in brackets, in the table above, are the binary addresses that each device can be accessed, where ‘X’ is either a high or a low bit. The hexadecimal addresses, not in brackets, are the addresses software should use.

The status LED is initially turned on but is otherwise free to use. The LED can be written to with output statements or instructions.

  • Turn the status LED off from BASIC:
    • OUT 14, 255
  • Turn the status LED on from BASIC:
    • OUT 14, 0

Note that the LED is turned on by writing a 0 (not a 1). This is because the LED output is active low. This is necessary to allow the LED to be turned on by a hardware reset. The reset signal clears the output latch to zero, thus zero is the on state.

Homebrew 8-bit retro computing