PWM Fan2 toggles on start up (Firmware 3.2beta3)



  • Hi - just raising an issue with PWM fan2 toggling on and off after the controller is reset. Before this behaviour was just on fan1 and not fan0 or fan2. I am using the pins to control relays which will have an extractor and air filter connected too them for a CNC set up. If this behaviour is going to remain in the new version because the controller is primarily for 3D printing use, could I get some advice or be pointed in the right direction for changing the pins behaviour in the bootloader. Cheers, Savvas



  • Ok this was a complete noob moment but I swapped over to the thermistor 1 and 2 terminals on the duet board. For some reason I imagined that output and inputs were somehow different and missed that you can change between those with M950. Massive face palm.

    That does beg the question how are inputs and output different? Are some terminals opto-isolated and other not?



  • @EducatingSavvas said in PWM Fan2 toggles on start up (Firmware 3.2beta3):

    That does beg the question how are inputs and output different?

    Fan1 output has a hardwired pull up, the other Fan outputs doesn't (on the Duet2) if thats what you refer to?



  • @bearer Just had a complete noob moment... as it clicked to me that I can just use the thermistor terminals instead. It took me a while to realise you can change an input into an output.



  • @EducatingSavvas said in PWM Fan2 toggles on start up (Firmware 3.2beta3):

    It took me a while to realise you can change an input into an output.

    can you? haven't tried but you should be aware inputs and outputs have very different protection circuitry making them less than ideal in the opposite role in some circumstances.

    refer to https://github.com/Duet3D/ for both Duet 2 and Duet 3 schematics for more details if you're into that sort of thing.



  • @bearer As far as I can tell the probe and endstops use 2n2 capacitor and thermistor 2u2 capacitors which I guess act like noise suppression (smoothing?). I am also using analogue convertors to go from the terminals from thermistor 1 and 2 to the relay modules which are also opto-isolated. Hopefully that is enough protection.



  • Was thinking more in the lines of outputs having a few hundred ohms, if any series resistance, while the inputs have 10k. That will greatly effect your ability to sink or source current from an input pin, and limit the swing of the output.

    Some outputs are also buffered making them unable to be used as inputs, but you'll find all the details in the schematics and I'm not 100% visualizing what you're trying to do.



  • @bearer I'll have to get the voltmeter out to see what it's doing when I next have time - but when I left it, they seemed to be behaving as expected. The schematic side of things is quite new to me.

    I'm now using two thermistor pins as outputs to switch two relay modules which will atomically start a dust extraction unit and an air filter. I wrote some conditional code to check the machine state at the start, pause, resuming and stopping / cancelling stages of a CNC job with the hope to automate some of the processes involved in CNC routering.



  • If the relay module works with an input current that is okay for the pin on the mcu (2mA?) then happy shiny.


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