SC527 – Z80 Processor Card (Z50Bus)

SC527 is a Z80 processor card designed for the Z50Bus. It includes a Z80 CPU, 128k FLASH ROM, 128k RAM, clock oscillator and a simple serial port.



Kit SuppliersWebsiteShips from
Stephen C CousinsTindieUK
PCB SuppliersWebsiteShips from
Stephen C CousinsTindieUK
Assembled and TestedWebsiteShips from
Not available
Component Suppliers
See parts list

Parts List

PCB1SC527, v1.0, PCB
C1 to C88Capacitor, ceramic, 100 nF
C9 and C102Capacitor, ceramic, 22 pF
JP11Header, male, 1 row x 2 pin, angled
JP2, 3, 4,54Header, male, 1 row x 3 pin, straight
Jumper5Jumper shunt
LED11LED, green, 3mm, angled
P11Box header, 2 x 25 pin, angled, or
Header, male, 2 row x 25 pin, angled
P21Header, male, 1 row x 6 pin, angled
R1 1Resistor, 10k, 0.25W
R2 and R32Resistor, 1k, 0.25W
R41Resistor, 1M, 0.25W
RN11Resistor network, 8x10k, SIL, 9-pin
Spacer1Spacer, 10mm, M3, nylon
Screw (for spacer)1Machine screw, 6mm, M3
U11Z80 CPU, Z84C0008PEG, or
Z80 CPU, Z84C0010PEG
U21RAM 128k bytes, AS6C1008
U31FLASH 128k bytes, SST39SF010A
X11Oscillator 7.3728 MHz
(optional. not included in kit)
X21Crystal, 7.3728MHz
IC socket 40-pin
1Socket, DIP, 40-pin
IC socket 32-pin
U2 and U3
2Socket, DIP, 32-pin
IC socket 16-pin
1Socket, DIP, 16-pin
IC socket 14-pin
U5 and U6
2Socket, DIP, 14-pin

Printed Circuit Board

User Guide

SC527 only requires power and a reset signal to be a complete working computer. The recommended firmware is the Small Computer Monitor (SCM). With the addition of a Compact Flash interface and card this card is able to run CP/M 2.

At reset the output bits of integrated circuit U4 are all cleared to zero (low voltage). The FLASH ROM is paged into memory from 0x0000 to 0x7FFF (32k bytes) with RAM from 0x8000 to 0xFFFF (32k bytes). When input/output address 0x38 bit 0 is set to one (a high voltage) the ROM is paged out and RAM is paged in, giving a full 64k bytes of RAM from 0x0000 to 0xFFFF.

The 128k bytes of FLASH ROM is split into four 32k byte banks. The bank is selected with JP3 (FLASH address A15) and JP4 (FLASH address A16). By default these jumpers should be set to ‘0’ selecting the lowest 32k bytes. By setting jumpers to either 0 or 1, any of the four banks can be selected.

The 128k bytes of RAM is split into two 64k byte banks. The bank can be selected with output addresses 0x30 bit 0.

When using the Small Computer Monitor, reset should cause the status LED to flash off and on again, either once or twice.

  • One flash off and on indicates a serial module has been detected on the bus and is being used to connect to a terminal. Serial cards usually work at 115200 baud, 8 data bits, 1 stop bit, no parity and hardware flow control (RTS/CTS) enabled.
  • Two flashes off and on indicate a serial card has not been detected and the onboard serial port (P2) is being used to connect to a terminal. This port works at 9600 baud, 8 data bits, 1 stop bit, no parity and hardware flow control (RTS/CTS) enabled.
  • If the LED continues to flash, or turns off and stays off, the self-test has failed, most likely indicating the RAM is not working.

User software can then control the status LED, as required.

Input/output port functions
I/O Address ReadWrite
(0x08 to 0x0B)
n/aStatus LED (LED 1)
Low = ON, high = OFF
Output bit 0
(0x20 to 0x23)
n/aSerial RTS out
Output bit 0
(0x28 to 0x2B)
Serial RXD
Input bit 7
Serial TXD out
Output bit 0
(0x30 to 0x33)
n/aRAM bank select RA16
Output bit 0
(0x38 to 0x3B)
n/aMemory paging (lower 32k)
Low = ROM, high = RAM
Output bit 0
Jumper options
JP1Connect power to serial port P2
JP2Clock source select
X1 = Can oscillator X1, typically 7.3728 MHz
X2 = Crystal X2, typically 7.3728 MHz (default)
JP3FLASH ROM bank select, FA15
1 = High, 0 = Low (default)
JP4FLASH ROM bank select, FA16
1 = High, 0 = Low (default)
JP5FLASH write enable / write protect
Read only = Write protected (default)
Write = Read and write enabled

It is recommended that a jumper shunt be fitted in the “Read only” position of JP5 during normal use. It only needs to be in the “Write” position when the contents of the FLASH is to be updated. While the FLASH device requires a very specific sequence of events to protect it against accidental modification, it is best to play safe and use the jumper to ensure it can not be written to.

Assembly Guide

Below is the suggested order of assembly. A general guide to assembling circuit boards can be found here.

  • Resistors R1 to R4
    These can be fitted either way around
  • Sockets for U1 to U6
    Fit such that the notch in the socket matches the curve in the component outline on the PCB silkscreen
  • Header pins JP1 and P2
    This can be one strip with the pin between JP1 and P2 pulled out of the plastic
  • Decoupling capacitors C1 to C8
    These can be fitted either way around
  • Capacitors C9 to C10
    These can be fitted either way around
  • Resistor network RN1
    This must be fitted the correct way around as indicated on the PCB
  • Light emitting diode LED1
    Angled LED: These are mounted in a plastic holder and are supplied correctly orientated for this PCB
    Standard LED: Fit such that the short lead and/or small flat on the plastic is at the end marked with a line on the PCB silkscreen
  • Crystal X2
    These can be fitted either way around
  • Header pins JP2, JP3, JP4 and JP5
  • Bus header P1
    Make sure the pins are parallel to the PCB so that the board is vertical when it is fitted into a backplane socket
  • Insert the integrated circuits into their sockets
    Make sure the notch in the component is at the end indicated by the notch in the socket and the curve on the PCB silkscreen


  • This design is made with the permission of LiNC (designers of the Z50Bus).
  • This product is designed for hobby use and is not suitable for industrial, commercial, or safety-critical applications.
  • The product contains small parts and is not suitable for young children.

Homebrew 8-bit retro computing