Recently i had to control a functionality of an ECU using a separate system (another ECU).
ECU "A" provides an input pin which if measured in open circuit shows 12V, so i guess it is a pull-up input. ECU "B" provides a sourcing output 0-24V that is used to control the functionality of ECU "A". Basically when there is 24V, the ECU "A" does something while when there is 0V, the ECU "A" does another thing.
What i'm experiencing is that when i ask for 24V and i measure it on the input pin it is actually what i am expecting, while when i ask 0V the input voltage shows 9V (hence it doesnt work).
It is as if the ECU "B" can't bring to GND the input of ECU "A".
The temporary solution i've adopted is to use a relay with directly ground and VCC coming from the power system rather than the control system (ECU).
ECU "B" output can be configured as Push/Pull, Sourcing or Sinking
Can you explain me in detail what's happening?