supporting more extruder/hotends

[Roland Mieslinger]

Hi,

I'm working on a design for a tool/hotend changer for CoreXY based printers, to make multi extrusion prints easier and/or to reduce setup time for single material prints by eg. keeping filaments loaded or have hotends with different nozzle sizes ready to use.

For more details see

- https://hackaday.io/project/26053-tool-switching-multi-extrusion *)

- https://www.youtube.com/watch?v=eOKCvtaMa08

The (rather big) prototype I'm building right now could support up to 7 tools and, using 2-in-1-out hotends, up to 14 extruders (ridiculous), but wiring up only 5 hotends would be challenging with current boards/firmware. In part, I guess this is due to the fact how the stepper motors and fan/heater MOSFET are connected to the micro controller. I'm aware of the "Chainable extruder board" proposal from v2 (https://docs.google.com/document/d/1yJnCG1fGhp64-zeYBpuORSVDRx_d7kL48Qw4BJoBYs4/edit#) and actually I'm aiming in the exact same direction, but with a much smaller footprint.

The current extruder/hotend connection requires a total for 5-7 GPIO pins for each hotend (3 pins for the stepper, 1 heater, 1 analog for the sensor and optionally one for the fan and filament sensor), which is a very wasteful as almost neither of these pins needs the high speed output capability of a GPIO pin. Heaters and fan PWM/PDM could be easily done by via shift registers. Some logic chips could be used to share stepper pins for multiple drivers as only one motor needs to run at any given point (ignoring mixing hotends for the moment). DACs with SPI interface (goes nicely with shift registers) would provide analog inputs. The pin count would drop to SPI (SCLK, CS, MOSI, MISO) and stepper interface (ENABLE/DIR/STEP) for a max of 8 hotends/extruder. 

The CPU requirements (cycles and RAM) per hotend are pretty low, mainly a PID loop, safety and possibly filament run-out checks, the CPU cycles for step generation are only needed during extrusion and therefor don't increase with each hotend.

The purpose of this post should be mainly to figure out if you (the devs) are actually interested in such a contribution (code, schematic, pcb design, documentation etc) for V1, as I'm unsure about the status of V1 wrt new features. 

*) It was featured in a hackady blog post on Sunday which sparked much more interest than I expected, please forgive if I sound overly enthusiastic.

Having 5+ hotends for a printer will continue to be a niche requirement.