SC118, v1.1, Circuit Explained

The SC118, Z80 Processor card can be used in a modest way as a Single Board Computer (SBC) or as part of a modular computer based on the Z50Bus.

C1 to C7

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.


This capacitor is required by the Voltage Supervisor and Reset device (DS1233-5+, U6) when an external reset switch and pull-up resistor are connected to the reset line. See DS1233-5+ datasheet for more details.


This is the standard Z50Bus connector.


The card includes a simple bit-bang 5 volt, FTDI style, serial port. The connection to this is the 6-pin header, P2. You can either fit a straight or a right angled header. The angled connector is usually more convenient. If the card is to be supported by guides, as suggesting in the Z50Bus specification, then straight pins must be fitted.

This port is very basic, providing only a software generated 9600 baud, 8 data bit, 1 stop bit, no parity serial interface. It is designed to provide a very low cost ‘starter’ port to get the system up and running as easily and cheaply as possible. It is suitable for simple applications, but a more capable serial module should be added for sophisticated applications such as running CP/M.

The pin-out, below, describes signals with respect to the SC118 card, so an output is and signal from the card 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 (not connected)


A pair of simple ground pins are provided as a convenient place to attach test leads.


This resistor sets the LED current. You may change this value from about 330R to 1K depending on the type of LED you fit and your preference for brightness. A value of 470R will work fine with just about any LED, so if in doubt use this value.


R2 pulls up the Z80 reset input (/RESET). The reset line is pulled low to reset the system.


This resistor network provides pull-ups for bus signals that may otherwise be left floating, or are used with open collector signals.


This is the reset switch, which pulls down the backplane’s reset signal (/RESET) and thus resets the system. This simple motherboard does not have a power-on reset, so it will usually be necessary to press this button after the system is turned on.


This LED is a simple status indicator. It is normally on when the power is connected, but it can be controlled by software if required. At reset is will turn on, even if the processor is not running. It will then turn off and on again, indicating the processor is running. If a suitable serial port module is not found in any of the RC2014 sockets, the LED will turn off and on again to indicate the built-in serial port is being used. A log-on message is then sent to the terminal via the serial port indicated.


This integrated circuit is the Z80 Central Processing Unit (CPU). A minimum of an 8 MHz rated part is required, but a faster part can also be used. The CPU is the brains of the board. It is able to execute instructions stored in the read-only memory (ROM) chip U3 or programs downloaded or written to the random access memory (RAM) chip U2.
For further details see the Zilog CPU data sheet.


This is the motherboard’s Random Access Memory (RAM). Although this is a 128k byte RAM chip, the Z80 CPU can only ‘see’ 64k bytes at any given time. The second 64k bytes can be selected in software, but it is quite difficult to use. In practice, it is likely that only 64k will ever be used.


This is the motherboard’s Read Only Memory (ROM). The specific type listed is a Programmable Read Only Memory (PROM), meaning it can have its contents (Firmware) programmed in with a PROM programmer. The contents then remain fixed. The device specified above cannot be erased and re-programmed.

In order for a computer to do anything useful, it needs some program code to execute. Even reading a program from a disk drive requires program code to perform that read. Thus a computer needs some program code permanently available to execute when it is first switched on. This is the function of the ROM chip.

This card has been designed to have the Small Computer Monitor installed in the ROM chip, although any compatible code can be used instead. Currently, the recommended version of the Small Computer Monitor for this board is v1.0 configuration F1.


This forms part of the address decoding and RAM/ROM paging circuit. The integrated circuit contains three 3-input OR gates. Two of them are combined to partially decode the output port addresses used by the motherboard. The third decodes an active low ROM select signal. The partially decoded output port address is 00XXX0XX, expressed in binary with ‘X’ indicating a bit which can be in either state. This signal is further refined by integrated circuit U5.


This integrated circuit is an 8-bit addressable latch. It has eight outputs which can be individually turned on and off. The partially decoded port address signal from U4 is used to enable latching of data to this chip. The required output is determined by the address lines A3, A4 and A5, and the signal latched to that output is determined by data line D0. The output port addresses, expressed in binary, are thus:

Bits: 7654 3210HexadecimalFunction
00XX X0XXPartially decoded address enabled U5
0000 00XX0x00 to 0x03Not used
0000 10XX 0x08 to 0x0B Status LED (low = on)
0001 00XX 0x10 to 0x13 Not used
0001 10XX 0x18 to 0x1B Not used
0010 00XX 0x20 to 0x23 Serial port, request to send (RTS)
0010 10XX 0x28 to 0x2B Serial port, transmit data (TXD)
0011 00XX 0x30 to 0x33 RAM bottom 64k selected (active low)
0011 10XX 0x38 to 0x3B ROM selected (active low)


This is a DS1233-5+ voltage supervisor and reset device. It provides a clean, reliable reset for the 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 main 7.3728 MHz processor and bus clock.

Homebrew 8-bit retro computing