Simple80, a simple Z80 SBC with no glue logic


This design was inspired by low-cost Z80 kit listed on eBay. The motivation is to build a CP/M-ready, RC2014-compatible Z80 SBC with part cost of about $20.



  • Z80 at 7.3728MHz
  • Dual serial port SIO
  • 128K RAM
  • 64K PROM or EEPROM
  • Three classic RC2014 expansion connectors
  • 100mm X 100mm pc board
  • No glue logic design

Theory of operation

This is a classical microprocessor design with CPU, I/O, RAM and ROM but with one unusual feature: To keep the part count at minimal, both RAM and ROM are chip selected when Z80 is accessing the memory space. To boot up, only ROM's output enable is asserted at reset; In this mode RAM is selected but write-only; the first routine in ROM firmware is to read its own code and write it back into the same location for the entire ROM program; this unusual operation does not affect the read-only ROM, but duplicate the ROM program into the write-only RAM. When the duplication operation is completed, the firmware enable the RAM's output enable and disable ROM's output enable so now the program is running in RAM.

Design Information


Schematic of Simple80 Motherboard

PC board Gerber files of Simple80 motherboard. The pc boards were made by JLCPCB

Bill of Materials for Simple80


Schematic of SimpleCF, a simple compact flash interface for Simple80

PC board Gerber files of SimpleCF, a simple compact flash interface for Simple80

Bill of Materials for SimpleCF


Simple80 Monitor, a basic monitor for Simple80. Use this monitor to initialize CF disk and load CP/M2.2 BIOS/CCP/BDOS.

SCMonitor for Simple80. This is Steve Cousin's SCMonitor ported to Simple80. Startrek program is included in this port. To run Startrek, type 'wbasic' followed by 'run'.

CP/M2.2 for Simple80. This is the BIOS/CCP/BDOS ported to Simple80.

XMODEM is the file transfer program to bring in all CP/M programs from PC to Z80SBC64. While in monitor, send XMODEM.HEX as Intel Hex file, type 'b2' to boot into CP/M2.2, then type 'save 17'. XMODEM.COM will be created as the first file on the CP/M disk. To invoke XMODEM to receive files, type 'xmodem filename /r/c/z1' and go to the terminal program to send file via xmodem.

CPM22DRI is system files for CP/M2.2. Unzip the file to CPM22DRI.ARJ then use XMODEM to transfer it to CP/M. Once transferred, use to decompress the files. CPM22DRI image is created using cpmtools. is the CP/M program that decompresses CPM3ALL.ARJ and CPM22DRI.ARJ above. The command is “unarj e filename

Instructions and Manuals

Getting Started with Simple80

builderpages/plasmo/simple80.txt · Last modified: 2020/04/07 23:31 by plasmo
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