SC130, v1.0, Circuit Explained

The SC130, Z180 SBC/motherboard includes the following:

  • Z180 CPU running at 18.432 MHz
  • One 512k byte static RAM chip
  • One 512k byte Flash ROM chips
  • Two 5 volt FTDI style asynchronous serial ports
  • One 5 volt SPI / SD Card ports
  • Clock oscillator (18.432 MHz)
  • Power supply supervisor and a reset circuit
  • Two expansion sockets
  • Power input (5 volt, typically 100 mA)

C1 to C8

These capacitors provide power supply decoupling (or bypass). The fast switching in digital circuits creates spikes on the power supply lines which are suppressed with decoupling capacitors placed at key points on the circuit board.

The exact value of this component is not critical. The use of very cheap capacitors within the range of about 50 to 100 nF is acceptable.


This capacitor provides additional support for transient current demands.


This capacitor is required by the Voltage Supervisor and Reset device (DS1233-5+, U7) when an external reset switch and pull-up resistor are connected to the reset line. See DS1233-5+ datasheet for more details.

The exact value of this component is not critical. The datasheet for the DS1233 specifies a value from 0.5 to 10 nF, thus a very low cost component with a wide tolerance is acceptable.


Barrel style, 5 volt power input socket.


Screw terminal, 5 volt power input.


Jumper 1 allows Flash chip U1’s write enable input to be connected to either Vcc (5 volts) or the CPU’s write output (/WR).

When the Flash chip’s write enable input is connected to Vcc, the Flash chip will never get a write enable signal and thus the memory is protected against being changed. When connected to the CPU’s write output, it is possible to write to the Flash chip.

Writing to the Flash chip is unlikely to happen by accident due to the software requirements. However, for peace of mind it is generally best to disable writing with this jumper.


This is a 40-pin RC2014 bus socket, mounted vertically to allow expansion modules to be fitted.


This is an 40-pin RC2014 bus socket, mounted horizontally to allow a module backplane extension board, such as SC113, to be fitted. It can also be used to connect an expansion module horizontally.


This LED is used to indicate the presence of the 5 volt supply and also other status information. At reset the LED is turned on by hardware but after that it is software controlled.

P1 and P2

These connectors give access to the 5 volt SPI port. This port uses the Z180’s hardware clocked serial I/O for high speed interfacing.

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

Two sets of mounting holes are provided. This allows an additional connector to be fitted if required. For example, you my wish to fit a set of header pins which enable the SD card adapter to be placed away from the circuit using a 6-way female to female Dupont cable.

P3 and P4, and P7 and P8

These enable the motherboard’s Vcc (5 V) to be connected to serial ports A and B power pins. Typically, this allows the motherboard to be powered from an FTDI style serial adapter.

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

As power can flow either way, these jumpers (or switches) also enable serial devices to be powered from the motherboard. If such devices are used, fit a jumper shunt in the appropriate position.

Two sets of mounting holes are provided for each port. This allows an additional connector to be fitted if required.

P5 and P6, and P9 and P10

Serial ports A and B are connected via P5 or P16, and P9 and P10. These are FTDI style 5 volt serial ports. Port A includes RTS/CTS flow control signals, while port B does not.

1Ground (GND)
2Request To Send (RTS) output from Z180
3Vcc (5V)
4Recieve Data (RxD) input to Z180
5Transmit Data (TxD) output from Z180
6Clear To Send (CTS) input to Z180

Two sets of mounting holes are provided for each port. This allows an additional connector to be fitted if required.

P11 and P12

This set of holes provides access to the 5 volt supply and the reset signal, allowing an external reset switch to be connected or power to be supplied to, or taken from, the board.

One use of this feature might be to connect a voltage regulator daughter board so the system can be powered from, say, 12 volts.

R1 to R6

These provide current limiting between the Z180 system and the serial devices on serial ports A and B, providing protection for when one is powered and the other is not.

R7 to R9

The resistors provide pull up and pull down for the serial port inputs, thus holding them in known states when no device is connected.


This resistor pulls the reset signal up. When pulled low the motherboard is reset.

R11 and R12

These provide pull ups for the CPU’s interrupt input (R11) and the SPI master in signal (R12).


This is a current limiting resistor for the LED. 470 ohms should give reasonable brightness, but higher or lower values can be used if required.


This tactile button ground the reset signal, resetting the motherboard.


This are 512k byte Flash memory chip. This holds the board’s firmware, typically RomWBW.


This is a 512k byte static RAM chip. This is used to store the program and data being worked on at the current time.


The 74AHCT139 is the addresses decoder, providing chip enable signals for the three memory chips. It is a dual 2-to-4 line decoder. One decoder is used to select between the RAM and the Flash memory chips. The other forms part of the address decoding for input/output devices.

Memory: Inputs are memory request (MREQ) and A19. Memory request is low when memory is being accessed and A19 determines if it is Flash memory (A19 low) or RAM (A19 high).

Input/output: This provides two output device enable signals (active low). One of the output is low when a write is being performed to the input/output address 0x0C, the other when a write is being performed to the input/output address 0x0D. Address 0x0C is used to control the SD card’s enable signal, the other to control the status LED.


This is a 68-pin PLCC packaged Z8S180 CPU. It must be rated at a frequency at least as high as the clock signal PHI.

PHI is initially the on-board oscillator (X1) frequency divided by 2, as the Z180 turns on its clock divider on at reset. However, current firmware turns this off, so PHI is then equal to the on-board oscillator (X1) frequency.


These are 74HCT688 8-bit identity comparator. The output of this devices goes low when a write is being performed to an input/output device address 0000 11X0 binary. This signal is used to enable the input/output half of U3.


This is a dual D-type flip flop which is used to provide to software controlled output bits. One is the SD card enable signal, the other controls the status LED.


This is a DS1233-5+ voltage supervisor and reset device. It provides a clean, reliable reset for the Z180 system. The device pulls the system’s reset signal low when the supply voltage is less than about 4.5 volts. If your system does not run, check the supply voltage and the reset signal.

The reset line can also be pulled down by a reset button. The DS1233 senses this and pulls its own output low to provide a clean reset pulse. This eliminates the typical switch bounce that could lead to reliability problems.


This oscillator provides the CPU’s main clock. With existing firmware this oscillator is also used as a clock source for the serial ports. Typically this oscillator is 18.432 MHz. This is the frequency required when running the motherboard with current firmware..

Homebrew 8-bit retro computing