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Background: I have many years of experience with AVR and ARM Cortex PLCs and I feel very comfortable using them in projects but I am not an EE or SE. So, if I make a mistake or misunderstand something, keep that in mind. Thanks.

I cracked open the case on my Maker Select v2.1 and I noticed that I have a 2x3 header that is labeled ISP. This is fairly standard for the AVR PLCs and the Melzi board that I have uses the ATMEGA1284P.

I have asked at numerous forums and nobody seems to really know the answer (the downside to RepRap--people use but don't fully understand).

I'm wondering if I can tweak the Repetier firmware (I'm using the stock version 0.91) to use the ISP headers as a standard SPI bus. It has the SCK, MOSI, and MISO pins but it doesn't have the SS pin.

What I want to do is put a port expander on the SPI bus and break out additional usable GPIOs.

So, is there a hardware limitation or any other reason why I wouldn't be able to repurpose the ISP headers into an SPI bus?

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In theory, you can; but, you may need those pins to attach an external programmer to bring your system back to life when playing around with the code.

Here are a couple of articles that could help if you still want to pursue that path:

Another option (the one I chose) is to just buy a RAMPS board set. I got one on ebay for $19 and it have lots of more options for IO. That way you can play and still go back and plug in your stock Melzi and print whenever you need it.

I got all this for $40 - boards, display, cables, power supply, and even shipping enter image description here

Here is a really nice detailed description of converting a Duplicator i3 from Melzi to RAMPS. The process would likely be very similar for your printer.

The biggest challenge will likely be setting up the firmware

BTW, what printer did you get?

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  • $\begingroup$ Fantastic answer! I ended up getting the Monoprice Maker Select v2.1 back in November. Couldn't beat the price and I saw it had a very large community of support. Made me really happy to see it's all parts that I've worked with and used before (Arduino and components). $\endgroup$ – Rincewind May 7 '17 at 21:35
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    $\begingroup$ I have a Tronxy X3. My printer has Repetier Firmware. I have found it on GitHub; but, that code base seems dormant. Marlin Firmware has a very active Open Source development (github.com/MarlinFirmware). I look forward to playing with the code and it looks like Marlin would be a better place to do that. It appears that either firmware will run the printer well. $\endgroup$ – markshancock May 8 '17 at 2:17
  • $\begingroup$ I like the simplicity of that design. No frills, no additional things to get in the way, just all that is needed to make it work. It looks like it has a rather large build space, too. I haven't seen too many people with a Tronxy--what do you think of it? $\endgroup$ – Rincewind May 8 '17 at 17:17
  • $\begingroup$ @Rincewind I like the X3; but, it is my first printer so I don't have much to compare it to. The one thing I can say for sure is that it would likely not be a good option for someone that doesn't like fiddling with things. It does have a few issues; but, there is a strong maker community around it so there are are fixes for all the issues. The reason I wanted is because I wanted to fiddle and the open design makes that easy to do. I am very happy with the printer and have not had a print fail yet. $\endgroup$ – markshancock May 12 '17 at 22:35
  • $\begingroup$ @markshancock, Repetier is the firmware shipped with the Maker Select (Wanhao rebranded). Repetier is FAR from dormant, and, frankly, has better bed-leveling. $\endgroup$ – Mark Ward Nov 5 '18 at 0:29

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