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?

  • \$\begingroup\$ Why can't you get the necessary information from the manufacturer of "ECU A"? Is it that you control "ECU B" but have no information about "ECU A" or is my guess wrong? It will help if you explain more of the context. Opening "ECU A" and looking at its PCB may give more clues about its input circuit. It seems that "ECU B" may be powered from 24V and "ECU A" may be powered from 12V, but you don't state that clearly. Can you add that type of information? Also, what sensor / device / thing should normally be connected to the input you are using on "ECU A"? Are you sure nothing else is driving it? \$\endgroup\$ – SamGibson Aug 30 '18 at 13:47
  • \$\begingroup\$ ECU "A" can't be touched nor opened. Their specification just states that when you bring +24V on the pin it does something when you bring 0V it does the other. If you don't want to do anything then you dont bring voltage at all. It is as if there is a sort of voltage divider between the output-cable and the input pull-up resistor but from what i know a pull-up resistor should be MOhm. Im trying to get info from them but they are slow. Regarding power supply it is as you say. There isn't a device connected normally since it has special function but always digital input \$\endgroup\$ – user217354 Aug 31 '18 at 14:13
  • \$\begingroup\$ I think i grasped the main idea, since i'm currently using a sourcing output i can actually provide +24V since im closing to +VCC. When i try to bring 0V since im sourcing i'm not able to because i'm not closing to GND, i should use sinking. Since i'm using only a single pin i should then use push/pull configuration to do both. I'll give it a try! \$\endgroup\$ – user217354 Aug 31 '18 at 14:18

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