Digging around in the Firmware and documentation, got me to https://duet3d.dozuki.com/Wiki/M578 and from there to using the P parameter on the G1 command. It allows you to set a pattern of bits, mapped to GPIO pins defined in the M670 command.

This I think is exactly what I am looking for, and it seems the SUPPORT_IOBITS flag is enabled for the Duet2 WIFI (i.e. DuetNG), so it should work out of the box. I'll will try it as soon as I have the opportunity.

@deckingman Thanks for the reply. This is indeed something that could possibly work. I would have to do an experiment in how fast the tool change command is processed, as when I used M42 to set an GPIO output in between two G1 commands, I observed a delay.

I'll post an update when I get the chance to try this.

(Still, having an extruder set per GPIO would be the most elegant solution, still wonder if that is possible)

@deckingman Ok, i'll upgrade the firmware asap and see if that allows for some new possibilities.

I think I didn't explain myself correctly. The problem I am having is that all nozzles need to be active all the time. While the print head makes a scanning move, along the path I need to repeatatly open/close each nozzle (not necessarily at the same time), without any delays/slow down of the printhead.

I guess using different tools would not be the cleanest, as then you'd have to change tool midway, which isn't a buffered command.

If I could use three extruders, one for each nozzle, I imagine the gcode could look something like:

...
G1 X50 E1:0:1    ; move to X50 with nozzle 1 and 3 open, 2 is closed
G1 X100 E0:1:1   ; continue moving to X100, but close nozzle 1 and open 2. Leave nozzle 3 open.
...

I hope I made myself more clear.

Is there maybe a way to create a new extruder based on a GPIO instead of a drive?

I'm building a machine to print cement using an array of nozzles, currently three of them.
My goal is to move in a scanning-type fashion, moving over the print bed in straight lines, with the line of nozzles perpendicular to the motion. Then I switch the nozzles on/off where water needs to be deposited, kind of like a dot matrix printer.

The nozzles/valves accept a 3.3V and/or 5V signal to open. I'm using a Duet2 Wifi board, currently with firmware 2.03.

My question now is how to best configure the Duet. Currently I have it working for one nozzle: , I connected the nozzle to the FAN1 output, disabled the fan, set up a tool that uses extruder 0 and then using M571 made the FAN1 (=P21) to turn on with the extruder. Then with G0/G1 with a Ex.xx command, the fan will turn on.

However, as far as I could find, the Duet2 Wifi only allows two extruders to be defined (M563), so I cannot perform this trick for 3 nozzles.

I have read about laser mode, but I can also not find how to control more thane one 'laser' in that mode.
I have tried as well to use M42 to just switch the nozzles on/off in between move commands, but this results in a very slow switching. It needs to be bound the G0/G1 commands for smooth/fast switching.

Any ideas about the best way to approach this?