Z80-MBC2: 4ICs homemade Z80 computer

8MHz Z80, 128kB banked RAM, RTC, Disk on SD, Basic and Forth interpreters, CP/M 2.2 and 3, cross Assembler and C (SDCC) toolchains

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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 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.

... Read more »

The sketch for the IOS (with the needed libraries). Unzip into a folder and open the .ino file (with Arduino IDE). IOS is required for CP/M 2.2 and QP/M 2.71 (the SD module is mandatory).

Zip Archive - 35.52 kB - 09/17/2018 at 20:17


The content of the microSD needed to run CP/M 2.2 and QP/M 2.71 with IOS S220718-R100918

Zip Archive - 1.60 MB - 09/17/2018 at 20:15



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



Instructions for STARTREKV2.BAS

bas - 6.80 kB - 08/04/2018 at 21:48


Examples with Basic.txt

Some examples showing the new "Virtual I/O Engine" with the stand-alone 8K Basic interpreter.

plain - 4.07 kB - 07/31/2018 at 20:57


View all 10 files

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

  • QP/M 2.71 up and running (with IOS multi-boot management...)!

    Just4Fun6 days ago 0 comments

    With the last IOS revision and the corresponding SD files "pack" there is an interesting alternative to CP/M developed by MICROCode Consulting that supports also file timestamping, and it is 100% CP/M 2.2 "compatible".
    MICROCode Consulting has released the original installation files and all the documentation in their site with the "restricted usage" condition, that means free for non-commercial use and for personal use only.

    To enable timestamping (see upper screenshot) you need to install the optional RTC module.
    I suggest to read the QP/M documentation for the various commands (see the Downloads section in their site).


    The QP/M uses for the batch file the .QSB extension. So the AUTOEXEC file is now renamed AUTOEXEC.QSB. To enable the AUTOEXEC execution after the cold boot change the corresponding state to ON from the usual IOS boot selection menu. In the drive A: there is an example of AUTOEXEC.QSB file ready to run.


    Now the IOS has a new entry (8) in the boot menu to manage the OS multi-boot configuration:

    Each OS is associated with a set of virtual disks called "Disk Set", and changing the "Change Disk Set..." entry (8) will switch all the virtual disks of his "environment".

  • CP/M 2.2 up and running on the Z80-MBC2!

    Just4Fun09/08/2018 at 18:24 0 comments

    The new IOS is out, and the CP/M 2.2 OS with it, and 16 disks are available (from A: to P:), each 8Mbyte large.

    With IOS (not for IOS Lite) the SD module is mandatory to run not only CP/M 2.2 but also for the stand-alone Basic and Forth interpreters. You need a microSD card (FAT16 and FAT32 are both supported) to store the content of the SD zip file, retaining the directory structure.

    Pay attention on how the modules are inserted because their positions are fixed and absolutely not swappable (see the upper photo).

    To add, extract or delete files inside a virtual disk (virtual disks filenames on SD are "DS0Nxx.DSK", where "xx" is the disk number) it is possible use the cpmtools or cpmtoolsGUI utilities, with the diskdefs file in the \cpmtools directory of the SD zipped file.

    I suggest to use  cpmtoolsGUI (only for Windows) because is very easy.

    Unzip it into a folder and put the diskdefs file in the same folder.
    Select "z80mbc2-d0" only for disk 0, and "z80mbc2-d1" for the others (disk 1 - 15):

    NOTE: use cpmtoolsGUI only to add, extract or delete files inside a virtual disk. Not try to create new virtual disks files with cpmtools or cpmtoolsGUI because further processing is required for a valid virtual disk file.

  • 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 you cannot 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 all 3 project logs

Enjoy this project?



villaromba wrote 6 days ago point

QP/M  - Love the progression of this project., particularly the ease of getting software onto the SD card via CPMTOOLS guii - a good find!!!

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john wrote 09/11/2018 at 23:54 point

What are you guys using for terminal emulation settings?  Specifically, have you gotten backspace to work?  

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Just4Fun wrote 09/12/2018 at 09:48 point

Using Tera Term it works out of the box (if I remember well)...

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john wrote 09/12/2018 at 23:38 point

Thanks - I tried TeraTerm and it works fine.  Was using Putty and had to change the keyboard to send ctrl-H instead of ctrl-? under keyboard settings.  That makes things much easier!

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Jeff Davis wrote 09/09/2018 at 08:37 point

Just4Fun: On the BOM sheet, RN1 and RN2 are listed as a SIP8 (8 pins), but the board has 9 pins (8 resistor + 1 common 5v). Should the sheet say SIP9 (which has 9 pins, 8 resistors and 1 common)?

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Just4Fun wrote 09/09/2018 at 10:44 point

Yes is SIP9 (which has 9 pins, 8 resistors and 1 common). I'll change it in the BOM...

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Jeff Davis wrote 09/05/2018 at 06:48 point

Just4Fun: I just ordered 10 PCBs (in black) from pcbway, thanks for setting up that shared project to make it easy to order. When I ordered, buying 10 boards was the same price as 5, with a little extra shipping cost.

I'm looking at ordering parts now, and I see on the BOM that C1,C2,C5,C6,C8,C9,C12 are all ".1uF", while C10 is "0.1uF". Are those all 0.1uF, or is one of the values listed wrong?

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Just4Fun wrote 09/05/2018 at 07:36 point

No, it's the same value... (.1uF = 0.1uF). Because it is on an other sheet, I've forgotten to follow the same "convention" of the other sheet.... :)

Don't miss the Twitter "channel" with "some" preview and "spoilers":

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Norihiro Kumagai wrote 08/30/2018 at 02:54 point

Just for your information,  in Japan, building Z80-MBC2 becomes quite popular (among oldboys, like me:-),  a kind vendor "orangepico" in Japan start to provide  PCB's, so barriar on getting PCB in Japan becomes quite low.  I am quite sorry all of the tweets are written in Japanese, but almost all tweets includes their pictures with short messages, I hope you could feel our delightful moments.

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Just4Fun wrote 08/30/2018 at 18:58 point

Thanks you very much for you kind message... :)

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Cyril V. wrote 08/30/2018 at 00:16 point

I just got my PCBs, they look really good (the solder mask is black).

I got couple to spare. I think selling 2 for $5 plus shipping, PM if interested.

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villaromba wrote 08/28/2018 at 13:38 point

The message you get - is that after selecting it or does it just default to that?  OR Do you get full menu if you hold user key dow, then hold reset for a moment - release it and then release user key? If so can you select anything at all from keyboard?

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Peabody1929 wrote 08/28/2018 at 19:21 point

After pressing just reset I get:

Z80-MBC2 - A040618
IOS-LITE - I/O Subsystem - S220618

IOS: Z80 clock set at 4MHz

IOS: Loading boot program... Done
IOS: Z80 is running from now

After pressing reset and user and release I get:

Z80-MBC2 - A040618
IOS-LITE - I/O Subsystem - S220618

IOS: Z80 clock set at 4MHz

IOS: Select boot mode or system parameters:

 0: No change (->1)
 1: Basic
 2: Forth
 3: iLoad
 4: Change Z80 clock speed (4/8MHz)

Enter your choice >

After pressing "1" at the prompt I get:

Enter your choice >1  Ok
DEBUG: BootImageSize = 7603
DEBUG: BootStrAddr = 0
IOS: Loading boot program... Done
IOS: Z80 is running from now

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Just4Fun wrote 08/28/2018 at 20:46 point

Which method have you followed to flash the bootloader?

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Peabody1929 wrote 08/27/2018 at 18:30 point

I finished building my board.  It starts up, outputs messages to the serial monitor but seems to stop after the message: 

"IOS: Z80 is running from now".  

The Z80 clock is set at 4MHz

The boot mode is 1: Basic.

What should I do next to debug the issue?  

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Just4Fun wrote 08/27/2018 at 19:49 point

Which method have you followed to flash the bootloader?

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Peabody1929 wrote 08/31/2018 at 02:54 point

I have tried two methods of flashing the bootloader.  Each time used a different ATmega 32 part.  Both times the USB interface was a USBTinyISP V2.0.  First time it was connected via a 10 pin cable to a ATmega32 dev board with a 40 pin ZIF, a 10 pin socket and all the pins brought out to headers.  The second time was using a 6 pin cable connected to the ISP connector on the MBC2 board.  In the Arduino IDE v1.8.5 selected the programmer as USBTinyISP (MIghty Core) and click Burn Bootloader.  There were no error messages and the flash was successful.

Unfortunately this made no difference in behavior.

One difference is that I am using a Z84C00020PEC.  This is a 20MHz Z80.  Is there a possibility of a narrow clock pulse glitch that a 8MHz part would never see?

Finally, I think the ATmega32 is operating correctly.  I plugged in a MCP23017 and a new message appeared: "IOS: Found GPE Option".

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Just4Fun wrote 08/31/2018 at 07:31 point

I'm using the same Z80 (Z84C00020PEC) on Z80-MBCs and Z80-MBC2s without problems...

And the method you use to flash the bootloader seems correct to me (I use the USBasp, but the USBTinyISP should be ok because you are using the Arduino IDE "facility" to burn the bootloader, so should be not possible to have fuse bits configuration errors because they are managed by the IDE...).

Try to change a different Atmega32A, just to be sure that your part is not faulty.

After that it seems that the problem can be the CPU or the SRAM, because you arrive at the point when the program should be loaded into SRAM and the Z80 starts to execute it.

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Peabody1929 wrote 09/01/2018 at 01:02 point

IT WORKS!!!!  I replaced every chip except the SRAM.  Following your suggestion, I replaced the SRAM.  The board now works perfectly; BASIC comes up and runs.   I am a happy camper.  Thank you for the help!


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Just4Fun wrote 09/01/2018 at 07:47 point

Great! Can you tell me the exact part name  of the faulty SRAM?

BTW: Thinking to add a SRAM test....

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Peabody1929 wrote 09/01/2018 at 18:32 point

The faulty SRAM part number is: HM628128ALP-7  The replacement part is: HM628128LP-12.  The replacement part is an older generation and slower part.

If you like, I can send you the failing part by mail.

P.S.  I have some HM628128BLP-7 parts as well.  Perhaps in the next build I will give these a try.

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Just4Fun wrote 09/03/2018 at 09:40 point

Thanks, but it is enough if you could just keep it. So if I have the time to write a memory test it is possible to use the faulty SRAM for testing...

BTW: may be that this can interest you: :)

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Julian wrote 08/26/2018 at 20:24 point

Thinking of using this design as a basis for experimentation -- do you know if it's likely to run OK on breadboard, or is the PCB essential?

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Just4Fun wrote 08/27/2018 at 15:44 point

Yes it can be done. Just the quartz oscillator could be "critical" on the breadboard, but if you use the internal oscillator (8MHz) you shouldn't find any problem.

You can give a look to the previous Z80-MBC on breadboard too (

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villaromba wrote 08/11/2018 at 07:51 point

Built up my board yesterday - no output!! until I realised I had clock fusebits inverted!! Reprogrammed AVR (TL866) and then it worked immediately, pleased all hardware/soldering was 100%!!. Put my AVR on a ZIF, just fits nicely. RTC should be here over weekend. I already have a Pololu SD so hoping that will be ok when ready to fit. Thanks again J4F for another excellent design and look forward to where it might develop.

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Just4Fun wrote 08/11/2018 at 08:02 point


I think this may interest you... :) :

PS: Check carefully that the Pololu SD has the 3.3V regulator and level shifters on board, and how to connect it as it is different from that one I've used here.

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villaromba wrote 08/11/2018 at 10:39 point

Great!!! - it works with the Pololu SD card, with level shifters (all I had) just needed to make small adapter to ensure pin compatibility. 

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villaromba wrote 08/12/2018 at 15:42 point

What will be the longer term plan for CP/M files on the SD Card e.g will it be able to read files from Peter Schorns Z80 simulaior (  or Z80 pack (  OR ???

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Just4Fun wrote 08/12/2018 at 16:34 point

The first link is interesting as i didn't know anything about it. (the second is well known).

About the SD,  I'll use "disk files" each equivalent to a disk of 512 tracks of 32 sectors. A sector is 512 bytes large, as the SD physical sector. So any SW that can "understand" this can read/write files on it.

E. G. using cpmtools with the correct parameters will work (I hope...) :)

I've to manage a "deblocking" algorithm for CP/M2.2, because it can use natively only 128 bytes sectors (CP/M 3 can use 512 bytes sectors natively, as far I know...), and this will take "some" time...

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Just4Fun wrote 08/05/2018 at 11:57 point

For those interested I've set up a twitter account to follow the "day by day" developments (and others things too)...:

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Norihiro Kumagai wrote 08/05/2018 at 07:23 point

I have one comment, some N-MOS CPU also work well, at least, NEC upd780-1 works well on both Z80-MBC, and Z80-MBC2.  If someone have one or more N-MOS Z80, It might work on these boards.

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Norihiro Kumagai wrote 08/05/2018 at 07:19 point

I built a PCB and see the BASIC prompt and run a Blink BASIC program.  Quite good.  Thank you very much. If you are interested in it, please refer my blog  or my tweet (@tendai22plus).

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Just4Fun wrote 08/05/2018 at 09:27 point

You have a very good blog. It's really a pleasure read it!

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Just4Fun wrote 08/02/2018 at 17:02 point

HI all, and thanks for your comments!

Here IOS booting from SD:

Now starting to play with CP/M 2.2...

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Mark Jungwirth wrote 07/31/2018 at 17:05 point

I just ordered 5 PCBs from

Is anyone planning a component order that others could tag along on?

Awesome work, J4F!!!

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john wrote 07/31/2018 at 19:28 point

Hey Mark - I've got the parts from the original board that I'm planning to reuse, but  if you've got an extra board you'd be willing to part with, I'd be willing to buy one from you.

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Mark Jungwirth wrote 07/31/2018 at 20:01 point

John, not a problem.  I only paid for ePacket delivery so it might be a couple of weeks.  My cost is $3 and I should be able to send USPS first class for less than $1.

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Jeff Davis wrote 09/05/2018 at 06:51 point

I'm going to do a big parts order, if you haven't ordered yours yet. I also have bunch of parts in boxes I'll pull from and use, can send you whatever I have spare. I probably have a lot spare.

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john wrote 07/28/2018 at 20:52 point

Great job!!! Again!

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villaromba wrote 07/28/2018 at 19:51 point

Ordered pcb from EasyEDA. Looking forward to start building in a few weeks. It will be good to have the SD Card this time. Be so much easier to load up!!!

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Peabody1929 wrote 07/29/2018 at 04:01 point

Could you share how you uploaded the gerbers to EasyEDA?  I get "Bad File Format".

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villaromba wrote 07/29/2018 at 08:28 point

When I ordered (got transferred to JLCPCB) it asks for Gerber files - so I just uploaded the zip file from this site, kindly provided by Just4Fun. All went through ok without any problems. So maybe you should download zip file again and try a new upload to EasyEDA in case one or the other corrupted.

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nemfield wrote 07/28/2018 at 12:16 point

Excellent work. There is elegance in simplicity , and you nailed it. I'll be ordering boards asap. 

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Norihiro Kumagai wrote 07/28/2018 at 02:37 point

I ordered 10 PCBs with your garber data to fusionPCB.  I am quite looking forward to receiving them to start soldering.  Thank you for your kindly preparing.

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Just4Fun wrote 07/28/2018 at 06:26 point

Thanks! Happy soldering... :-)

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Jose Manuel Arenas wrote 07/27/2018 at 18:44 point

Very nice project. I love it.

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Just4Fun wrote 07/27/2018 at 19:22 point


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