80C85 board

80C85 board designed by Sergey Kiselev, constructed by me.

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80C85 processor
Runs Tiny BASIC and 8085 monitor.

I bagged nine OKI 80C85AH chips for just £8 on eBay. Rated 5 MHz clock according to the datasheet.

This project starts from a design by Sergey Kiselev. I intended to replace the 1-channel serial chip with a 16C552 which has 2 serial ports, internal baud-rate generation, and a parallel port, but that is not needed at the moment.


Monitor and BASIC binary ROM image.

octet-stream - 8.00 kB - 05/15/2021 at 14:07


Tiny BASIC for 8085

Zip Archive - 33.17 kB - 05/15/2021 at 14:04


Monitor software for 8085

Zip Archive - 54.35 kB - 05/15/2021 at 14:04


SPLD logic equations

Zip Archive - 7.66 kB - 05/15/2021 at 14:04


OKI MSM80C85AH datasheet (2003-05-16).pdf

OKI MSM80C85AH datasheet (2003-05-16)

Adobe Portable Document Format - 427.23 kB - 02/03/2021 at 23:56


View all 6 files

  • 1 × 80C85 Microprocessor, CMOS, 8-bit
  • 1 × RAM Power Management ICs / Power Supply Modules
  • 1 × ROM Power Management ICs / Power Supply Modules
  • 1 × 74ACT573 Electronic Components / Misc. Electronic Components

  • Project log

    Keith04/13/2020 at 16:48 0 comments

    I bagged nine OKI 80C85AH chips for just £8 on eBay. Rated 5 MHz clock on data sheet.

    Original intent was to base architecture on the Tandy Model 100, early laptop which ran for 20 hours on four AA cells. 

    Four NiMH cells = 600 to 2850 mAh at 50mA drain, x 1.2 volts = 0.72 to 3.42 Watt-hours.

    This laptop claims 10.5 hours from a 32.5 WH battery

    So a model 100 should run 9.5 to 45 times longer!

    The Model 100 uses an 81C55 for parallel I/O and a 6402 UART for serial I/O. I had six of the latter, but none of the former.

    The Model 200 uses an 82C51A for serial I/O. This is not the chip used in the IBM PC (i.e. the 8250).

    The 81C55 used the timer for baud rate control. If I use a UART with internal baud rate generation (the 8250) then I only need parallel I/O for the keyboard. A 74HCT574 and 74HCT245 should do. Later UARTs had 16-byte buffers (16C550) and printer ports (16C552).

    The tape interface can be safely abandoned. Solid state storage is widely available.

    Farnell sell the 4-channel, no LPT version in LQFP (0.5mm pitch):

    and the 2-channel 1-LPT version in PLCC68 for £7.30:

    Bought an MSM81C55 chip. I may not use it, but it will be there if I want to experiment.


    Bought a ready-made PCB:

    This will avoid a lot of wiring work. Since it is a ready-made computer already, I need to improve it.

    The 8251 is primitive and there is no user I/O for this board. So I shall use an 16C552 chip, which has two serial ports and a parallel I/O port.



    U1 80C85 CPU socket
    U3 ROM socket
    U4 32K RAM socket
    U5 GAL16V8 socket
    U6 74HCT573



    Fit links for 32K EEPROM
    Fit 5V inlet

    2020-06-06 weekend

    Program GAL 
    Program EEPROM
    Fit 10MHz crystal (near enough ideal 9.8304 MHz to work)
    Order null-modem adaptor for neater solution in future
    Create null-modem adaptor to get going this weekend
    Check board works with 8251 serial port and unmodified software

    Success, it runs Tiny BASIC, at 19200 baud. I will replace that with the dual monitor-BASIC ROM.

    To do:

    Replace 10 MHz crystal with 9.8304 MHz.
    Modify hardware and firmware to use 16C552 chip
    Build 16C552 board with serial and parallel ports.

View project log

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