• USB Stick support
• SD-Card and other Memory Cards (USB Card Reader)
• FullHD(1920x1080) Display (HDMI)
• WebCam (RasPiCam or USB Cam)
• Ethernet (even two or three if you need that with an USB-Ethernet Adapter)
• Keyboard, Mouse, Touch( USB )
• ...

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matt.minuti
If you moved upfrom the 405 to the 407, which has a budgetary cost of about $0.50 more, you'd have the ability to add ethernet at a later time. The LAN8710A or 8720A PHY is pretty cheap, and I'm sure there's chinese equivalents for even less. All you need after that is a magjack and you're good to go. Since a lot of people have previously expressed interest in not needing an rpi or whatever and just plugging in, it might be worth keeping in mind. That said, those ESP8266 wifi modules might be cheaper and more convenient for a lot of people.

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matt.minuti
I'm very glad to see higher-voltage steppers - faster movement and less heat. The chip you found looks awesome. Is there a reason for having the steppers be modules rather than integrated into the same PCB? Heat? Ability to use less beefy stepper drivers if desired?

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matt.minuti
If the stepper modules are going to stick out perpendicular to the main board (which can be nice for heat flow), you might want to consider a card edge connector. Some of them are really cheap, like the FCI 10018784-10200TLF, which is only $0.16@1k, compared to the one you picked out which is almost twice as much. That one is rated for lower current, but it has nearly twice as many pins in roughly the same space, and your stepper board wouldn't need a separate connector, saving parts and assembly cost for each stepper board, especially if there's no through-hole parts.

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matt.minuti
How do you plan to talk to those stepper chips via SPI? It seems like if a module is plugged in, you can't talk to any later ones, which rules out reading the current in use on anything but stepper 0.

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matt.minuti
Thermocouple: looks like you picked the J-type chip, but the K-type connectors. I'd choose K, because they're cheaper and better suited to the temperature ranges we're looking at. Also, are you planning to always populate the thermocouple circuitry, or just have it there as an option (like my ethernet suggestion above)? I ask because that chip alone seems to be around $3-4 at quantity, and a proper thermocouple connector isn't insignificant either. The expansion port has SPI2 on it, and I get the impression most printers don't use thermocouples, so a thermocouple board could be an add-on. People who use two heads and thermocouples would need to add a second thermocouple anyways, so it would be good to think about that use case. Thermistor circuitry is cheap and tiny, so it makes sense to have a lot of channels available, like you do.

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matt.minuti
Maybe it's just because they're cool and under $0.10/ea at quantity, but you could consider the WS2812B in place of the red, green, and blue leds on PE10,12,15. It'd free up two pins which might make the routing easier. That said, there's something nice about dedicated indicators with silkscreen labels like "POWER."

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matt.minuti
I did end up making a design for an L6474-based stepper module, as a drop-in replacement for the standard pololu. Never got around to getting it made. Would it be helpful if I ordered some to verify, and share the design?

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VDX
... I'm using the Discovery boards for CNC-milling, laser-engraving and a XY-servo-scanner (not galvos!), but the firmware and used CAD/CAM-software isn't free or OS, so didn't post about here.

But the developer company is now changing/updating (could be because of me) for faster CPU's - the next one will be based on a BeagleBoneBlack ...

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arthurwolf
This looks like you are departing from G-code as the communication protocol, what is pushing you to do that ?