VSSA and GND?



  • Hi as mentioned in my other thread Im starting to wire things up a bit...

    But im a little confused....

    Now I had some assumptions which turned out to be wrong... For instance i had thought the thermsister where reading back off a 3V or 5V rail... Now this is where my second confusion comes in... On looking at the header pin diagram https://d17kynu4zpq5hy.cloudfront.net/igi/duet3d/YN2oahyGpcPNN1Pw.huge

    I can see that the IO actually reads to ground? well VSSA... Using a continuity tester, the VSSA seems to be connected to ground directly.

    Is there any reason why I couldnt have the VSSA side of my therm, just connected to a ground port?

    This is all for the print carriage, I have basically run out of pins on the plugs I'm using and Im trying to work out what to do...

    As I previously mentioned I had thought Ill just be using 3 or 5v and a single return to the thermsister header.

    I have 2 plugs one 14 pin one 4 pin...

    1. 5V

    2. 5V GND (assuming each rail has its own isolated ground)

    3. 3V (for endstops)

    4. BL SIG

    5. BL Z

    6. Therm 1 (the thermister gpio side)

    7. Therm 2 (the thermister gpio side)

    8. MOTOR1

    9. MOTOR2

    10. MOTOR3

    11. MOTOR4

    12. End Stop 1 (min Powered off 3v above)

    13. End Stop 2 (max Powered off 3v above)

    14. Part Cooler Fan IO return (powered from 24v)

    15. 24V

    16. 24V GND (assuming isolated ground from other 5v 3v ground)

    17. Heat Cart 1(returns Powered off 24V above)

    18. Heat Cart 2 (returns Powered off 24V above)

    Is 24v Gnd isolated in anyway from the ground returns of the 5 and 3v? if not I could just use a single ground line, either the pin i marked as 24v GND or 5vGND...

    And is VSSA different from common ground, and can I just wire my therms to one of the two grounds i have? (if the two grounds are isolated)

    Id rather not sacrifice my Endstop Max as its a safety feature more for when im tinkering not wanting to crash the carriage.

    If anyone can clear this up for me... I could get on with finishing this bit of wireing...



  • Right moments after I posted this thread, i did a search and just found another thread asking about the VSSA and ground... So it appears that VSSA is fuse protected and also placed on the ground plane near the MCU to minimise noise....

    Damn.

    In that case, can i share the 5V ground and 24V Ground? Are they just the same ground plane? That would free up one pin for VSSA.


  • administrators

    @mangy_dog 5V and 24V ground on the Duet are the same. There is 1 GND plane and the VSSA (analog ground) that should be kept separate.



  • Ahhhh great 😄

    that solves that one... Ill make the pin i marked as 24V GND the common ground pin, and 5VGND to VSSA


  • administrators

    I strongly recommend you use separate 24V ground and 3.3V/5V ground. the reason is that if you use a common ground, and the ground wire breaks or loses contact, then 24V will be fed to the common ground through the fan, and from there into the components connected to 5V or 3.3V.

    Additionally, you may find it useful to control the hot end heatsink fan thermostatically; in which case you won't have a 24V ground at all, just another fan return pin.

    What endstops are you using? Do they really need 3.3V?



  • Just using the normal end stops... I thought the endstops used 3V IO though? Thats why I put 3V on there, or is it perfectly fine to use 5V for the stops?


  • administrators

    @mangy_dog said in VSSA and GND?:

    Just using the normal end stops... I thought the endstops used 3V IO though? Thats why I put 3V on there, or is it perfectly fine to use 5V for the stops?

    Many "normal" endstops are just microswitches and don't need the power pin. But if your endstops do need power and your Duet is revision 1.04, then it's safe to use 5V power from the expansion connector to the endstops, instead of the 3.3V power from the endstop connector centre pin.



  • Yeah i mean the micro switch type...
    You mention that they dont need power? What are they looking for? Is the IO pulled up? and looking for a pulled down signal to ground?


  • administrators

    The simplest type of endstops is a bare microswitch, with (usually) the NC contacts connected to the two outer terminals of the endstop connector (one of which is signal ground).

    Often you see microswitches mounted on PCBs along with a couple of resistors and an LED. These don't really add anything useful. You will probably need to print a plastic part to mount the microswitch or PCB on the printer frame anyway.



  • Oh the micro switches are all on there mounted parts for the end stops....

    It was just the signalling i wasnt sure of...

    So ok, I dont need to set them to read off a voltage... just need to connect them to a ground... that saves another pin...


 

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