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Minimal 8085 Single Board Computer - MiniMax8085


The MiniMax8085 project has been brewing for almost three years. Sometime in the spring of 2014 my kids and I visited local a surplus store, and my kids found some Intel 8000 series parts in the kids fill-your-bucket area. Among these ICs were there: an 8085 CPU, an 8155 RAM with I/O ports and timer, a couple of 8255 PPIs, an 8282 latch, and some 27C128 UV EPROMs. It looked like a good start for an 8085 based computer that can be used to teach my kids some computer design and programming basics.
I wanted the project to be simple to build, with a minimal number of components, but yet to be a fully functional single board computer (e.g. to include the memory and an input/output device, for example a UART) with an extension bus. I checked the Internet for existing homebrew 8085 computers and found Roman Fülek's NCB85 and NCB85v2 projects, and Glitch Works 8085 projects. While I liked these projects, they didn't quite meet my project design goals:
  • NCB85 was the closest, but it doesn't have an extension bus
  • NCB85v2 is a little bit too complicated for my needs
  • Glitch Works 8085 project didn't have an on-board UART
The resulting design includes 8085 CPU, 8251 USART, 32 KiB SRAM, 32 KiB or 16 KiB ROM. It uses a GAL16V8/ATF16V8 simple programmable logic device (SPLD) instead of discrete logic ICs for the address decode and the frequency divider for USART. It reduces the number of components, allows for tweaking the configuration by re-programming the SPLD, and provides an introduction to the programmable logic devices.


  • 80C85A or 8085A CPU, 3.072 MHz or 4.9152 MHz CPU clock frequency
  • 82C51A or 8251A USART for console connection. 9600 or 19200 bps
  • 32 KiB of battery backed SRAM
  • Up to 32 KiB of ROM. EEPROM, UV EPROM or Flash ROM memory is supported.
  • 40-pin extension bus connector, including most of 8085 signals and demultiplexed data and address buses

Hardware Documentation

Design Files

File downloads are at the bottom of this page.
  • Mini8085 - KiCad - 1.0.zip - KiCad schematic and PCB layout files
  • Mini8085 - Schematic - 1.0.pdf - Schematic in PDF format
  • Mini8085 - Board - 1.0.pdf - PCB layout in PDF format

Design Description

The MiniMax8085 is based on the Intel 8085A CPU (U1). The 74*573 octal latch (U6) is used to demultiplex the lower 8-bit of address and data bus.
The board uses the Intel 8251A USART (U2) for serial input / output, for example for connecting a console. The MAX232A RS-232 driver/receiver (U7) is used to convert TTL logic levels to RS-232 voltage.
The 62256 32 KiB SRAM (U4) is used for the system RAM. The RAM is battery backed using the DS1210 NVRAM controller (U8).
The board supports a variety of ROM options (U3): The 28C256 EEPROM; UV EPROMs, such as 27C256 (32 KiB), 27C128 (16 KiB), and 27C64 (8 KiB); or The 29C256 Flash ROM ICs. Normally the ROM is mapped to the CPU memory beginning from the address 0x0000, while the RAM is mapped beginning from the address 0x8000. The ROM and RAM addresses can be swapped by pulling /SWAP_MEM signal low. This can be used, for example, to implement a CP/M extension card. 
THe GAL16V8 simple programmable logic device (U5) implements all the required "glue" logic functions:
  • Address decode and chip select for the USART. The default USART I/O address is 0x08.
  • Clock divider for the USART, that divides the CPU clock by 10 - for 3.144 MHz CPU clock, or by 16 - for 4.9152 MHz CPU clock (SPLD needs to be re-programmed to change the divisor value)
  • Address decode and chip select for the ROM and SRAM. The address decode logic include additional /SWAP_MEM input, which by default is configured to swap address mapping of ROM and RAM.

The MiniMax8085 includes a 40-pin extension connector (P4), that features the 8085A address and data buses, and control signals. Please refer to the Jumpers and Connectors section below for the connector pinout and signals description.

Jumpers and Connectors

Jumpers JP1 and JP2 - ROM Configuration

These jumpers configure how pins 1 and 27 of the ROM IC (U3) are connected. The jumper settings are printed on the bottom side of the PCB.

 ROM Type Jumper JP1 Position - ROM Pin 1 Connection JP2 Position - ROM Pin 27 Connection
 27C128, 27C64, 27128, 2764 2-3 - pin 1 connected to VCC 1-2 - pin 27 connected to VCC
 27C256 2-3 - pin 1 connected to VCC 2-JP1 pin 1 (jumper installed vertically), pin 27 connected to A14
 28C256 1-2 - pin 1 connected to A14 2-3 - pin 27 connected to /WR
 29C256 2-JP2 pin 3 (jumper installed vertically), pin 1 connected to /WR 2-JP1 pin 1 (jumper installed vertically), pin 27 connected to A14

Jumper JP3 - USART Clock Frequency

JP3 sets the USART input clock frequency

Jumper Position Description
 1-2 307.2 kHz - 19200 bps (default) or 2400 bps, depending on USART configuration
 2-3 153.6 kHz - 9600 bps (default) or 1200 bps, depending on USART configuration

Connector P1 - POWER

Connect regulated +5V power supply to this connector.

 Pin Description
 tip (the inner contact) Positive terminal - +5V
 barrel / sleeve Negative terminal - ground

Connector P2 - SIO

P2 exposes the 8085A CPU serial input and output lines.

 Pin Description
 1 Ground
 2 SOD / Serial Output Data
 3 SID / Serial Input Data

Connector P3 - SERIAL

P3 is the serial port connector. It is normally used for connecting a console or terminal. P3 uses pinout similar to that of IBM AT serial port (with some signals missing). Use a null modem cable to connect to a PC.

Pin Number Signal Name Description and Notes
 1 DCD Carrier Detect; Not used - Not connected on the SBC
 2 RX Receive Data; Input to SBC
 3 TX Transmit Data; Output from SBC
 4 DTR Data Terminal Ready; Not used - Not connected on the SBC
 5 Signal Ground Connected to the SBC GND signal
 6 DSR Data Set Ready; Not used - Not connected on the SBC
 7 RTS Request to Send; Output from SBC
 8 CTS Clear to Send; Input to SBC
 9 RI Ring Indicator; Not used - Not connected on the SBC
 Shield DE9 Shield Connected to the SBC GND signal

Connector P4 - EXTENSION

P4 exposes the 8085A CPU address and data buses, and control signals.

 Pin Signal Description Pin Signal Description Pin Signal Description
 1 TRAP Non-maskable interrupt input 14 A12 Address bus - bit 12 output 27 /WR Write control output
 2 3.073 MHz CPU clock output 15 A15 Address bus - bit 15 output 28 RESET Reset output
 3 RST5_5 RST5.5 interrupt input 16 A14 Address bus - bit 14 output 29 A1 Address bus - bit 1 output
 4 INTR Interrupt request input 17 AD1 Address/data bus - bit 1 bi-directional 30 A0 Address bus - bit 0 output
 5 RST7_5 RST7.5 interrupt input 18 AD0 Address/data bus - bit 0 bi-directional 31 A3 Address bus - bit 3 output
 6 /INTA Interrupt acknowledge output 19 AD3 Address/data bus - bit 3 bi-directional 32 A2 Address bus - bit 2 output
 7 ALE Address latch enable output 20 AD2 Address/data bus - bit 2 bi-directional 33 A5 Address bus - bit 5 output
 8 /SWAP_MEM Swap ROM & RAM addresses input 21 AD5 Address/data bus - bit 5 bi-directional 34 A4 Address bus - bit 4 output
 9 A9 Address bus - bit 9 output 22 AD4 Address/data bus - bit 4 bi-directional 35 A7 Address bus - bit 7 output
 10 A8 Address bus - bit 8 output 23 AD7 Address/data bus - bit 7 bi-directional 36 A6 Address bus - bit 6 output
 11 A11 Address bus - bit 11 output 24 AD6 Address/data bus - bit 6 bi-directional 37, 38 VCC Power - 5V
 12 A10 Address bus - bit 10 output 25 /RD Read control output 39, 40 GND Power - Ground
 13 A13 Address bus - bit 13 output 26 IO/M I/O / memory cycle status output   

Connector P5 - VBAT

P5 is used to connect the battery for the SRAM backup. A 3-volt lithium cell, or 2-3 alkaline batteries connected in series can be used for this purpose.

 Pin Description
 1 Battery voltage, 3V - 5V
 2 Ground

Bill of Materials

Link to the project on Mouser.com - View and order all components except of the 8085A CPU, the 8251A USART, and the PCB.
Link to the project on OSHPark.com - View and order the MiniMAX8085 PCB.

 Component Type Reference Description Quantity Possible Sources and Notes
 PCB  MiniMax8085 PCB Version 1.0 1 Order from OSH Park
 Capacitor C1 - C11 0.1 uF ceramic, 5.08 mm lead spacing 11 Mouser 810-FK28X7R1H104K, 80-C323C104K5R; Jameco 25523
  C12 1 uF ceramic, 5.08 mm lead spacing 1 Mouser 810-FK24X5R1C105K
  C13 10 uF ceramic, 5.08 mm lead spacing 1 Mouser 810-FK24X5R1C106K
 Note: Can be replaced with tantalum or electrolytic capacitors 
  C14, C15 33 pF ceramic, 5.08 mm lead spacing  2 Mouser 810-FK28C0G1H330J
 Note: Install only when using CMOS 80C85A CPU. Do not install when using NMOS 8085A
 Connector P1 DC jack 1 Mouser 806-KLDX-0202-A; Jameco 101178
  P2, JP1 - JP3 3x1 pin header 4 Mouser 649-69190-103HLF, 649-78229-103HLF; Jameco 109576
  P3 DE9M, right angle PCB mount 1 Mouser 806-K22X-E9P-N, 806-K22X-E9P-N-99 (teal color), 571-7478404; Jameco 104943, 614459, 614432
  P4 20x2 pin socket  1 Mouser 517-929852-01-20-RB
 Note: Optional, used for extension bus
  P5 2 pin header with friction lock 1 Mouser 571-6404562, 571-3-641126-2, 571-3-641215-2; Jameco 232266, 613931;
 Note: Optional, used for SRAM battery backup
 Corresponding female connector: Mouser 571-770602-2; Jameco 234798
 Contacts (2 contacts needed): Mouser 571-770666-1; Jameco 234923, 736501;
 It is recommended to use a polarized header to avoid incorrect power polarity.
 Diode D1 1N4148 1 Mouser 512-1N4148, 771-1N4148-T/R; Jameco 36038, 179215
 LED D2 LED Indicator, Dialight 566-0206F 1 Mouser 645-566-0206F
 Note: Can be replaced with a standard 3 mm or 5 mm LED
 Resistor R1 33 Ohm, 1/4 W 1 Mouser 291-33-RC
  R2 470 Ohm, 1/4 W 1 Mouser 291-470-RC; Jameco 690785
  R3 100 kOhm, 1/4 W 1 Mouser 291-100K-RC
 Resistor Array RR1, RR2 4.7 kOhm, 6 pin, bussed resistor array 2 Mouser 264-4.7K-RC, 652-4306R-1LF-4.7K
 Switch SW1 Tactile switch, right angle  1 Mouser 653-B3F-3100, 653-B3F-3152, 706-95C06A2RAT, 706-95C06C2RAT, 706-95C06F2RAT
 Note: Switches can have different actuator length. You might want to select the switch according your own preferences, for example the enclosure type you want to use, and whatever you want the reset button to stick out of the enclosure or to stay hidden inside.
Jameco 1953575
 Crystal Y1 6.144 MHz quartz resonator for 3 MHz CPU; 9.8304 MHz quartz resonator for 5 MHz CPU 1 Mouser 520-61-18-4XEN (6.144 MHz), 520-HCU983-20 (9.8304 MHz); Unicorn Electronics 6.144 MHz or 9.8304 MHz according to desired CPU clock frequency.
 IC U1 80C85A, 8085A-2, or 8085A CPU 1 Jameco 52071 (8085A-2, 5 MHz), 52062 (8085A, 3 MHz); Unicorn Electronics 80C85, 8085A-2, or 8085A; eBay (search for 80C85 or 8085 CPUs, or IM1821VM85A / ИМ1821ВМ85А for Soviet 80C85 clone)
 Note: Use CMOS part - 80C85A for lower power consumption
  U2 82C51A, or 8251A  USART 1 Jameco 52652 (8251A), Unicorn Electronics 82C51 or 8051; eBay (search for 82C51 or 8251 USART, or 580VV51 / 580ВВ51 for Soviet 8251 clone)
 Note: Use CMOS part - 82C51A for lower power consumption
  U3 28C256 EEPROM, 27C256 UV EPROM, 27C128 UV EPROM, 27128 UV EPROM, 27C64 UV EPROM, 2764 UV EPROM 1 Mouser 556-AT28C25615PU; Jameco 74843 (28C256 EEPROM), 39714 (27C256 UV EPROM); Unicorn Electronics 28C256-15 (EEPROM), 27C256 or 27C128 (UV EPROM)
 Note: EEPROM parts are recommended. UV lamp needed to erase UV EPROMs
  U4 62256 SRAM 1 Mouser 913-AS6C62256-55PCN; Jameco 391822; Unicorn Electronics 62256LP-70 (or any other 62256)
  U5 GAL16V8, ATF16V8 1 Mouser 556-AF16V8B15PU, Jameco 37452; Unicorn Electronics 16V8 (10 ns, 12 ns, or 15 ns versions would work. Make sure to contact the seller and ask specifically for Lattice GAL16V8 part. iCT PEEL16V8 parts are not supported by MiniPro TL866CS programmer)
  U6 74HC573, 74ALS573, 74F573 1 Mouser 595-SN74HC573AN, 595-SN74HC573ANE4, 595-CD74HC573E, 595-SN74ALS573CN, 595-SN74F573N; Jameco 46076 (74HC573); Unicorn Electronics 74HC573, 74ALS573, 74F573; eBay (search for 74HC573, 74ALS573, 74F573, or KR1533IR33 / КР1533ИР33 for Soviet 74ALS573 clone)
  U7 MAX202,  MAX232A 1 Mouser 595-TRS202ECN, 701-SP202ECP-L, 700-MAX202CPE, 700-MAX232ACP; Jameco 1800552, 142535; Unicorn Electronics MAX202
  U8 DS1210 1 Mouser 700-DS1210; Jameco 114198; Unicorn Electronics DS1210
 Note: Optional, used for SRAM battery backup. If not populated - install two wire jumpers instead as shown on the PCB silk screen
 IC Socket U1 40 pin 600 mil DIP socket 1 Mouser 517-4840-6000-CP, 649-DILB40P223TLF; Jameco 41111
  U2 - U4 28 pin 600 mil DIP socket 3 Mouser 517-4828-6000-CP, 649-DILB28P223TLF; Jameco 40301
  U5, U6 20 pin 300 mil DIP socket 2 Mouser 517-4820-3000-CP, 649-DILB20P-223TLF; Jameco 38608
  U7 16 pin 300 mil DIP socket 1 Mouser 517-4816-3000-CP, 649-DILB16P-223TLF; Jameco 37373
  U8 8 pin 300 mil DIP socket 1 Mouser 517-4808-3000-CP, 649-DILB8P223TLF; Jameco 51571

Replacement Notes

  • CMOS parts are preferred for lower power consumption.
  • 80C85 (CPU) - Any type in DIP-40 package will do. Use 6.144 MHz crystal for 3 MHz parts. Such parts usually don't have any numeric suffix in the part name. For example Intel P8085AH is a 3 MHz part, while Intel P8085AH-2 is a 5 MHz part.
    • CMOS parts: MSM80C85A or M80C85A (OKI), CA80C85B (Calmos, Tundra), IM1821VM85A (Soviet/Russian Clone)
    • NMOS parts: P8085A (Intel, AMD), P8085AH, P8085AH-2, P8085AH-1 (Intel, newer HMOS process, 3 MHz, 5 MHz, and 6 MHz parts respectively), M5L8085AP (Mitsubishi), TMP8085AP (Toshiba), and so on.
  • 80C51A (USART) - Any type in DIP-28 package.
  • 28C256 (EEPROM, EPROM) - The board support a wide variety of ROMs. Note that JP1 and JP2 jumpers need to be set according to the ROM type.
    • 28C256 EEPROM: Atmel/Microchip AT28C256, Xicor X28C256
    • CMOS UV EPROMs: 27C256 UV EPROMs (32 KiB), 27C128 UV EPROMs (16 KiB), 27C64 UV EPROMs (8 KiB)
    • NMOS UV EPROMs: 27256 UV EPROMs (32 KiB), 27128 UV EPROMs (16 KiB), 2764 UV EPROMs (8 KiB)
    • 62256-like NVRAM (32 KiB)
    • AT29C256 Flash ROM
  • 62256 (SRAM) - A low-power CMOS SRAM 32KiB, in DIP-28 wide (600 mils) packages. Here are some common parts:
    • In production: AS6C62256 (Alliance Memory), CY62256N (Cypress)
    • Old stock: HM62256LP (Hitachi), HY62256 (Hyundai), KM62256 (Samsung), UM62256 (UMC)
  • GAL16V8 (SPLD): Needs to be in DIP-20 package
    • In production:  Atmel/Microchip ATF16V8
    • Old stock: Lattice GAL16V8 (any modification)
    • iCT PEEL16V8 might work, but it is not supported by common MiniPro TL866CS programmer
  • 74HC573 (Octal D Latch with 3-STATE Outputs) - Can be replaced with any 74-series family that works with 5V power supply voltage. 
    • CMOS parts: 74HC573, 74ACT573, 74HCT573, 74AHCT573, 74AC573, 74ACT573
    • TTL parts: 74ALS573, 74F573, 74LS573
  • MAX232A (RS-232 Drivers/Receivers) - There is a variety of compatible ICs available from various manufacturers. Note that the original MAX232 part as well as some compatible ICs require 1 uF capacitors for charge pumps (C7-C11).
    • Maxim: MAX232A, MAX202, MAX232
    • Texas Instruments: TRS202, MAX232
    • Exar: SP232
    • Analog Devices: ADM202, ADM232
    • Intersil: HIN202, HIN232
  • LED D2 - Can be replaced by a 3 mm LED with pins bent at 90 degrees.



MiniMax8085 uses MON85 Software Debug Monitor For the 8085/8080 written by Dave Dunfield. Specifically it was tested with the improved by Roman Borik version 1.2 (download at the bottom of the page). This version adds support for undocumented 8085 instructions and flags.


The famous Tiny BASIC 2.0 written by Li-Chen Wang and modified by Roger Rauskolb, runs on MiniMax8085 with very little modifications: The 8251 USART I/O address needs to be updated, and its initialization parameters need to be slightly updated - to use 1 stop bit instead of two, and to enable CTS/RTS flow control. Also the RAM addresses need to be updated to start from 8000h.
For your enjoyment the original Tiny BASIC article from the December 1976 issue of Interface Age magazine - TinyBASIC-2.0.pdf, the source files and the ROM image - tinybasic-2.0-mini85.zipare provided at the bottom of this page.

Test Programs

Minimal Test Code - Blink an LED 

The code below blinks an LED connected to 8085's SOD output.
The code is borrowed from Glitch Works 8085 SBC project: http://www.glitchwrks.com/2010/09/02/8085-sbc

;Assembly                                 Address    Opcode

;Flash a LED on SOD

;Load C000h to SP
START:          LXI     H,0C000h        ; 0000       21 00 C0
                SPHL                    ; 0003       F9

FLASH:          MVI     A,0C0h          ; 0004       3E C0
                SIM                     ; 0006       30
                CALL    DELAY           ; 0007       CD 13 00
                MVI     A,40h           ; 000A       3E 40
                SIM                     ; 000C       30
                CALL    DELAY           ; 000D       CD 13 00
                JMP     FLASH           ; 0010       C3 04 00

;Delay, return to HL when done.
DELAY:          MVI     A, 0FFh         ; 0013       3E FF
                MOV     B,A             ; 0015       47
PT1:            DCR     A               ; 0016       3D
PT2:            DCR     B               ; 0017       05
                JNZ     PT2             ; 0018       C2 17 00
                CPI     00h             ; 001B       FE 00
                JNZ     PT1             ; 001D       C2 16 00
                RET                     ; 0020       C9

USART Test Code

This code prints "8085" to the serial port, and then echoes received characters.

; USART registers
USART_DATA     EQU     08h
USART_CMD      EQU     09h

START:         LXI     H,0C000h
               CALL    USART_INIT

; write a banner
               MVI     A,38h           ; '8'
               MOV     C,A
               CALL    USART_OUT
               MVI     A,30h           ; '0'
               MOV     C,A
               CALL    USART_OUT
               MVI     A,38h           ; '8'
               MOV     C,A
               CALL    USART_OUT
               MVI     A,35h           ; '5'
               MOV     C,A
               CALL    USART_OUT
               MVI     A,0Dh           ; CR
               MOV     C,A
               CALL    USART_OUT
               MVI     A,0Ah           ; LF
               MOV     C,A
               CALL    USART_OUT

               CALL    USART_IN
               MOV     C,A
               CALL    USART_OUT
               JMP     LOOP

USART_INIT:    MVI     A,00h
; Set USART to command mode - configure sync operation, write two dummy sync characters
               OUT     USART_CMD
               OUT     USART_CMD
               OUT     USART_CMD
; Issue reset command
               MVI     A,40h
               OUT     USART_CMD
; Write mode instruction - 1 stop bit, no parity, 8 bits, divide clock by 16
               MVI     A,4Eh
               OUT     USART_CMD
; Write command instruction - activate RTS, reset error flags, enable RX, activate DTR, enable TX
               MVI     A,37h
               OUT     USART_CMD
; Clear the data register
               IN      USART_DATA

; Read character from USART
USART_IN:      IN      USART_CMD       ; Read USART status
               ANI     2               ; Test RxRdy bit
               JZ      USART_IN        ; Wait for the data
               IN      USART_DATA      ; Read character

; Write character to USART
               ANI     1               ; Test TxRdy
               JZ      USART_OUT       ; Wait until USART is ready to transmit
               MOV     A,C
               OUT     USART_DATA      ; Write character


Sergey Kiselev,
Mar 24, 2017, 10:23 AM
Sergey Kiselev,
Mar 24, 2017, 10:28 AM
Sergey Kiselev,
Mar 8, 2017, 1:50 PM
Sergey Kiselev,
Mar 7, 2017, 2:36 PM
Sergey Kiselev,
Mar 8, 2017, 1:50 PM
Mini8085 - KiCad - 1.0.zip
Sergey Kiselev,
Mar 7, 2017, 2:36 PM
Sergey Kiselev,
Mar 7, 2017, 2:36 PM
Sergey Kiselev,
Apr 24, 2017, 7:52 PM
Sergey Kiselev,
Apr 13, 2017, 8:06 AM
Sergey Kiselev,
Apr 7, 2017, 7:46 AM
Sergey Kiselev,
Apr 13, 2017, 8:05 AM