Z80-MBC2: a 4 ICs homebrew Z80 computer

Homemade 8MHz Z80 SBC, 128kB banked RAM, RTC, SD (HD emulation), Basic and Forth interpreter, CP/M 2.2 and 3, UCSD Pascal, Fuzix and more...

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The Z80-MBC2 is an easy to build Z80 SBC (Single Board Computer).It is the "evolution" of the Z80-MBC (, with a SD as "disk emulator" and with a 128KB banked RAM for CP/M 3 (but it can run CP/M 2.2, QP/M 2.71, UCSD Pascal, Collapse OS and Fuzix too).

It has an optional on board 16x GPIO expander, and uses common cheap add-on modules for the SD and the RTC options. It has an "Arduino heart" using an Atmega32A as EEPROM and "universal" I/O emulator (so a "legacy" EPROM programmer is not needed).

It is a complete development "ecosystem", and using the iLoad boot mode it is possible cross-compile, load and execute on the target an Assembler or C program (using the SDCC compiler) with a single command (like in the Arduino IDE).


The needed ICs for the "base system" are:

  • Z80 CPU CMOS (Z84C00) 8Mhz or greater
  • Atmega32A
  • TC551001-70 (128kB RAM)
  • 74HC00

If you want the 16x GPIO expansion (GPE option) add a MCP23017 too.

The schematic and the BOM are attached in the Files section. The MCU Atmega32A is used as universal I/O subsystem, as Eeprom, and as reset and 4/8MHz clock generator for the Z80 CPU.
Inside the Atmega32A it is flashed an Arduino bootloader taken from here, and it is possible to use the Board Manager of the Arduino IDE to "import" it.

Flash the Arduino bootloader at first (with the method you prefer), next you can upload the IOS "sketch" (the I/O Subsystem that interacts with the Z80 bus and "virtualizes" the EEPROM and all the peripherals seen by the Z80 CPU) using Arduino IDE.

You can use the on board ICSP port J3 (also called ISP port) to write the bootloader, but remember to disconnect any other connector when using it. Also both SD and RTC modules (if present) must be removed from the board when the ICSP port is in use.

As clock source for the Z80 CPU it is used the 16MHz Atmega32A oscillator, so the "external 16MHZ osc." bootloader variant must be chosen when flashing the bootloader from the Arduino IDE!.

The 74HC00 is used as RS flipflop to stop the Z80 CPU during I/O operation, giving the needed time to the Atmega32A to interact with the Z80 bus, and as part of the MMU.

Note that only the CMOS version of the Z80 CPU can be used here. This because only CMOS version, under given condition that are respected in this schematic, has logical levels compatibles with Atmega32A and 74HC00.


You should use a  Z80 CMOS speed grade of at least 8MHz for full speed, but setting the clock speed at 4MHz you can use a 4MHz Z80 CMOS version too (or you can try to overclock it at 8MHz...). The 74HC00 can be substituted with a 74HCT00 if you already have one. The RAM chip TC551001-70 can be substituted with any suitable 128kB SRAM).

Please note that the USER led  * must * be blue or white (or pink... I've some pink leds that seems to have a Vf like blue one. May be I'll do a board with them...) just to be sure that V(forward) is >= 2.7V (otherwise the USER key may not work as expected).

The J4 connector (AUX_P) is not currently supported and is not populated by default.

The three solder jumpers (SJ1-3) on the bottom side are not currently supported and must  be left opened (as stated in the schematic).


It is possible to choose to populate on the PCB a GPIO port expander (U5) to add 16 bidirectional GPIO pins. The GPE option (see the schematic) can be used with the SPP Adapter board (see the paragraph: SPP (STANDARD PARALLEL PORT) ADAPTER BOARD).


The  SERIAL port (J2, see schematic) can be connected with a TTL-RS232 adapter, or with a serial-USB adapter.
I've used a serial-USB adapter that acts also as power source for the Z80-MBC, and has the DTR signal for the "autoreset" driven from the Arduino IDE. For a terminal that has a serial TTL port no adapter is needed.

Of course to upload a "sketch" from Arduino IDE you need to use a serial-USB adapter connected to the SERIAL port.

Note that the RTS and CTS pins of the SERIAL port are not currently supported and must be left not connected (as the NC pin!).

The 3V3 pin of the serial-USB adapter must be left disconnected  (if present).

You should use those Serial-USB adapters that have the DTR pin on the connector. It is suggested to have also the CTS/RTS signals available for future upgrades.

Please note that all the pin names of J2 on the PCB are referred to the serial-USB adapter, so all the signals as TX and RX are relative to the serial-USB adapter side (in other words TX and RX are already "inverted". See the schematic).

Here a suggested serial-USB adapter based on a CP2102 (very common on ebay):


Read more »

The sketch for the IOS (with the needed libraries). Unzip into a folder and open the .ino file (with Arduino IDE). IOS must be uploaded into the Atmega32A flash. Adds support for Fuzix OS and the SPP Adapter board (more info in the changelog inside the .ino file).

Zip Archive - 43.75 kB - 09/24/2023 at 11:59



The sketch for the IOS in executable format (.HEX) with the bootloader. This executable file is intended for use with a programmer as the Atmel Ice or AVRISPmkII or others (Fuse bits: High Byte 0xD6, Low Byte 0xAF, Lock Byte 0xCF)

x-hex - 59.90 kB - 09/24/2023 at 12:04


The content of the microSD needed to run CP/M 2.2, CP/M 3.0, QP/M 2.71, UCSD Pascal, Collapse OS and Fuzix with IOS S220718-R290823 (More info in the Changelog.txt file inside)

Zip Archive - 5.79 MB - 09/24/2023 at 12:02


SPP Adapter board -

All the documentation needed to build the SSP (Standard Parallel Port) Adapter board (A240721-R270921) including schematic, PCB assembling guide, Gerber files for PCB production. PCB is 55mm x 60mm 2-layers.

Zip Archive - 747.10 kB - 09/24/2023 at 15:10



The famous game Super Startrek kindly debugged by a RetroBrew Computer Forum user. Play with Caps-Lock activated! Now the animation works...

bas - 20.18 kB - 08/07/2018 at 16:32


View all 12 files

  • 1 × See the file "A040618 BOM v2.ods" in the FILES section.

  • How use the ICSP port with the USBasp programmer under linux to burn the bootloader

    Just4Fun07/27/2018 at 15:57 0 comments

    A cheap and easy way to burn the Arduino bootloader is to use an USBasp programmer that is commonly available:

    The USBasp is also capable to give the power to the "target" using the VCC pin, but remember to check that the JP1 jumper is set to provide 5V to the target (as shown in the photo).

    Please note that the pinout of the USBasp is a little different from the "standard" ICSP (os ISP) pinout:

    In the previous picture it is possible see that pins 4 (TXD) and 6 (RXD) are not at GND as expected  by the standard ICSP port, and pin 3 is not NC.

    See the following picture showing the standard 10 pin ICSP pinout:

    So you must consider this when connecting the USBasp to the 6 pins ICSP port (J3) on the Z80-MBC2 (see the schematic):

    To avoid problems I suggest to use as GND pin 10 of the USBasp connector, and connect the other pins (VCC, MISO, MOSI,SCK, RST) accordingly.

    An handy way to connect the USBasp to the 6 pin ICSP port (J3) of the Z80-MBC2 could be to use a commonly available "10pin to 6pin" adapter like this:

    but I suggest not to use it "as is" because its internal connections are done for a "standard" ICSP port, and we have seen that the USBasp connector differs from the standard one.
    The schematic of the adapter shows that isn't compatible "as is" with the UABasp connector:

    To use it is a good idea isolate the pins 4, 5 and 6 cutting the trace on the PCB of the adapter that connects those pins together, and then check with a tester.
    In the following photo are shown the three cuts (thin red lines inside the green "circle") to do:


    To easily burn the bootloader follow these "quick and dirty" steps (tested on a linux Mint OS with Arduino IDE 1.8.5):

    STEP 1: Connect the 10 pins connector of the USBasp programmer to the 6 pins ICSP port (J3) of the Z80-MBC2 (using wires or a modified adapter as discussed before);

    STEP 2: Verify carefully that any other connector of the Z80-MBC2 is not used, and verify that both the SD and RTC modules (if present) are removed from the board;.

    STEP 3: Only at this point connect the USB side of the USBasp programmer to an USB port of your workstation;

    STEP 4: Open a "terminal" window on your workstation and go to the directory where there are the Arduino IDE executables, and get the root privileges with the command:

    sudo su

    then run the Arduino IDE with the command:


    STEP 5: Because Arduino IDE is running as the root user it is necessary re-install the "core" for the Atmega32. Open the Board Manager as you already did (anyway  the guide is here). Note that you must do this step only the first time you execute the Arduino IDE as root;

    STEP 6: Now from the Tools menu of Arduino IDE select "Atmega32" as "Board", "16 MHz external" as "Clock", and "USBasp" as "Programmer". Then you can burn the right bootloader (without playing with the FUSE setting) selecting "Burn Bootloader" from the same "Tools" menu.

    All done!

View project log

Enjoy this project?



DarS wrote 09/28/2020 at 20:29 point

Thanks for a plenty of joy with Z80-MBC2 !  I just assembled my copy and spent few evenings trying to understand the details. A kind of a user guide was crafted with all the information I could gather from different sources. If you find it useful, feel free to use/share:

  Are you sure? yes | no

ermanno wrote 09/04/2020 at 18:08 point

To format the sd card I have to use fat or fat32 and what size of the allocation unit?

Or it doesn't matter.

  Are you sure? yes | no

Just4Fun wrote 09/05/2020 at 08:42 point

You can use both FAT or FAT32. The micro-SD can be a legacy one (max 2 GB if I remember well) or a SDHC. Nothing more. No others special setting are needed.

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Vinicius Monteiro wrote 08/19/2020 at 00:13 point

Hey so i'm kinda noob in those stuffs (besides having a technical degree in eletronics, I never really had time to mess around with them in my spare time) and I have like a very, very limited knowledge in Arduino. How can I import a bootloader to the ATMega? Thanks everyone, and mainly for the chap who post the project.

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Peter Tabatt wrote 08/04/2020 at 15:51 point


an other day an other question. I use the hex-file S220718-R280819_IOS-Z80-MBC2... on the Atmega 32 and after starting the Z80-MBC2 i got the boot-menu. So far so good! I explored the menu-entries and after choosing Forth, I run into Forth on every reboot. How can I get back to the boot-menu at startup?

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villaromba wrote 08/04/2020 at 16:38 point

Easiest way is to press both RESET & USER keys, release the RESET key holding the USER key down until the IOS led starts to blink, or you see the menu on the screen.

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Peter Tabatt wrote 07/30/2020 at 17:11 point

What a cool project! It's like time-travelling for me, I feel backsituated to my time as student. My first homecomputer had a Z80-clone but no CP/M. Z80-MBC2 is a very cool project! I assembled two and they are up and running.

Now I have one question: How can I put some programs into the disk-images on the sd-card?

Is there a tool to fill the disk-images on a PC? Under Windows?

  Are you sure? yes | no

villaromba wrote 07/31/2020 at 21:45 point

If you scroll down on the 'Logs' menu you'll find  CPMtools GUI and instructions. on how to use it. (You can of course use XMODEM  to transfer files over to disk directly.)

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Jonathan Guthrie wrote 07/29/2020 at 16:30 point

Thank you for this most excellent project!

I built this recently.  It works fine for me, and now I'm trying to figure out what I want to do with it.

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Sigmar Roßmann wrote 07/13/2020 at 12:04 point


I am trying to compile the S220718-R260119_IOS-Z80-MBC2.ino in the Arduino studio.
If I uploaded the HEX file with the Atmel studio on the Z80-MBC2, everything works.
But the serial interface does not print any characters.

Does somebody has any idea.

Excuse my English.



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Sigmar Roßmann wrote 07/13/2020 at 12:32 point

already solved

I had to change the HardwareSerial.cpp in one place.

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Sigmar Roßmann wrote 07/06/2020 at 16:34 point

Hello everybody,

can someone explain to me what in the Arduino * ino

"void serialEvent ()
// Set INT_ to ACTIVE if there are received chars from serial to read and if the interrupt generation is enabled
{ if ((Serial.available ()) && Z80IntEnFlag) digitalWrite (INT_, LOW);
} "

Interupt on Z80 is triggered?



  Are you sure? yes | no

Just4Fun wrote 07/07/2020 at 13:43 point

As explained in the comment it triggers the INT line when a char is ready to be read by the Z80 CPU and the INT trigger flag is enabled in the IOS firmware.

This mode is used only by the stand-alone ROM Basic.

(Not sure to have understood the question...)

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Sigmar Roßmann wrote 07/11/2020 at 16:14 point

Thanks that was exactly the right answer.

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ermanno wrote 06/15/2020 at 20:12 point

just4fun you're right I confused it with the pinout of the bc 557 which is inverted.

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ermanno wrote 06/15/2020 at 17:20 point

The transistors 2n2907 if inserted as in the serigraphy would have the collector connected to vcc, instead it is the emitter that must be connected to vcc, therefore the flat part of the transistor goes towards the Z80 and not towards the LEDs. However inserted in one way or another they work equally. How should these transistors be inserted ...

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Just4Fun wrote 06/15/2020 at 17:38 point

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Stephan Kraus wrote 04/12/2020 at 20:57 point


Thanks for the great project!

I am wondering if there is the chance to add a second serial port to hook up a printer or paper tape reader?

Best regards


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Sebastiano Grimaldi wrote 03/29/2020 at 17:51 point

I've just checked the SD card board with an Arduino uno board. I've correctly write and read file in the SD. I understand that the SD card board is working! Thanks a lot to give me some idea for additional debug. 

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Just4Fun wrote 03/29/2020 at 18:38 point

If you press and release the RESET key, D11 must do a single very fast blink (also if the SD module is not present) just before the IOS SD error is "printed" on the serial port. If you don't see it there must be an assembling error/fault.

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Sebastiano Grimaldi wrote 03/31/2020 at 18:17 point

Thanks for the suggestion, I've fixed the problem. Now my MBC2 is perfectly running! Best regards, Sebastiano 

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Sebastiano Grimaldi wrote 03/29/2020 at 13:43 point

Very compliment for this fantastic project!

I build MBC2 everything it's working fine except the SD card!

Using the lite IOS I run BASIC and FORTRAN then the system is working.

Loading on the ATMEGA32 the IOS with SD management I got the following error:

Z80-MBC2 - A040618
IOS - I/O Subsystem - S220718-R280819

IOS: Z80 clock set at 8MHz
IOS: Found RTC DS3231 Module (29/03/20 15:34:36)
IOS: RTC DS3231 temperature sensor: 19C
IOS: CP/M Autoexec is OFF

IOS: Select boot mode or system parameters:

 0: No change (3)
 1: Basic
 2: Forth
 3: Load OS from Disk Set 1
 4: Autoboot
 5: iLoad
 6: Change Z80 clock speed (->4MHz)
 7: Toggle CP/M Autoexec (->ON)
 8: Change Disk Set 1
 9: Change RTC time/date

Enter your choice >1  Ok

IOS: SD error 2 (NOT_READY on MOUNT operation)
IOS: Check SD and press a key to repeat

There is not any activity on the LED D11, I've cheched all the route trace, no error.

Any help, please?

  Are you sure? yes | no

Just4Fun wrote 03/29/2020 at 14:21 point

That error says that there isn't any SD card inserted into the SD module...

BTW: Are you sure to have inserted the SD card module correctly (as shown in the photos)?

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Sebastiano Grimaldi wrote 03/29/2020 at 17:05 point

Thanks for fast response! The sd card is correctly installed with the proper contents as per your zip archive. Best regards, Sebastiano

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m.uhlmann.zwickau wrote 06/10/2020 at 06:18 point

I have also this error some times. After reconnect the sd-card or the board it works.

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mikeh32217 wrote 03/25/2020 at 07:02 point

Awesome project, great fun and perfect for the era of the shutin!

Thanks for the effort!

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Edgar Salgado wrote 06/09/2020 at 19:32 point

Hello! Any chance you are willing to sell me a spare PCB? :)

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O.E wrote 02/16/2020 at 17:47 point


After i setud circuit i connected to computer using FTDI programmer leds blinking correctly but terminal is not  open in computer when  pressing buttons.

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Just4Fun wrote 02/17/2020 at 11:18 point


have you set the sw terminal at 115200 8N1 (No HW handshaking)...? Are you sure to have connected RX/TX correctly...? An other "source" of common problems is a wrong fuse setting when flashing the bootloader first time...

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wojtek.p wrote 01/13/2020 at 12:20 point


Does anyone have 3D models (step format) for Z80-MBC2, uCom and UTerm? I want to design a 3D printing case and it would be helpful. If anyone has, please send to



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wenij wrote 01/12/2020 at 09:55 point


have any one knows how to build the Z80-MBC2_figforth130.asm source code?  which tools set to build?



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WestfW wrote 01/16/2020 at 23:41 point

For the MBC1, I used the TASM3.2 cross assembler for DOS/Win, which you can apparently download from:

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Tony Nicholson wrote 10/12/2019 at 11:32 point

This is a fantastic little board.  I'm now starting to tinker with things a bit and I'll be attempting to combine the CP/M 3 bios modules with Simeon Cran's ZPR3 replacement BDOS routines (obtainable from the Tesseract RCPM+ archives - volume 93 at ).

Before doing this I thought I'd attempt to reconstruct the supplied CPM3-128.SYS system from sources and I'm not having much success - in particular with all the choices when running GENCPM.  Would you be able to share the build procedure and corresponding GENCPM.DAT file?  Thanks.

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HerveGZ wrote 07/12/2019 at 17:08 point

Dear All

Thanks again for this wonderfull Z80 board. And sorry to start a question as beginner, but

Is there a link to a startup guide global of this board? (something detailled explain)

Even I use it yet, I  have some problems to understand the I2C (protocol I know):
In examples there are out 1,opcode and out 1, value in general is so. But as I want to add my own I2C systems (from Grove or Seeed firms) I do not understand the relation between those out 0,xxx and out 1,yyy and the I2C address (Each item has a unique one ) of my own components I2C.

Can someone thanks  to enligh me on I2C how to mbasic can i drive any component I2C added please?



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Just4Fun wrote 07/12/2019 at 17:31 point

Hi, opcodes are used to access to the virtual I/O devices. Read the comments in the .ino sketch about the virtual I/O engine to understand how they work (there are large comment parts in the code that are a sort of manual).

About  accessing your I2C device, this requires  changes in the FW. Some users in the FB users group have already done this to access an I2C LCD module.

You have to create new opcodes to access new I2C HW.

  Are you sure? yes | no

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