Poll: Do you use Sinking or Sourcing IO?


Thread Starter

Ken E


I know the merits and pitfalls of Sinking Vs. Sourcing IO have come up time and time again (I just did a forum search), so please lets not argue which is better.

What I'd like to know is if there are industries out there preferring one method vs. the other and which regions these are from. So if you all could chime on the following I'd appreciate it:

- Which IO types you use (Sourcing or sinking for both inputs and outputs)

- What region of the world you are using these products.

- What is/are your main application(s).

For definition purposes:

- Sinking inputs expect 0V applied to their terminals to activate.

- Sinking outputs switch their terminals to 0V when they are activated.

- Sourcing inputs require +V on their terminals to activate.

- Sourcing outputs supply +V on their terminals when they are activated.

Thank you,
I don't run neutral as a signal level for a remote motor control station so the motor can start autonomously from the conduit getting crushed nor put my light switches in the neutral, and I don't use ground as an I/O level.

I'm not negative on negative logic per se, it would be fine if they grounded the positive so you were still switching on the ungrounded side.

Industries: just about everything, from small wet process and plastics equipment to packaging automation.

curt wuollet

Nearly all the foreign (non-US) equipment I've seen uses sourcing outputs and sinking inputs. I think there might be EU standards for this. Some robots use negative logic as well. This is with conventional current flow. I've heard the reason is that an open will be false.


Gabriele Corrieri


I'm working in copper industry, Europe, from 16 year we use exclusively sourcing output and sinking input, because they're more safety than the reverse logic. Time by time I've encountered some devices (mainly made in japan) that uses sinking output and sourcing input.

I'm not sure when you tell sinking input when they're absorb current, for definition sourcing output with sinking input, they must be complementary: sourcing with sinking, PNP output transistor with NPN input transistor (or optocoupler)


Ken Emmons Jr.

In my definition for sinking inputs the switch external to the input module would be tied to ground to "sink" current away from the pullup resistors/optos in the input module, thus turning the input "on". I guess you could call it active low, or 0V logic, or NPN (in the case of inputs NPN refers to the transistor required on an external sensor to sink current to ground).

It sounds like when you say sourcing inputs you mean that the common for the inputs is +V (i.e. +24V). This would make the input terminals active when you apply 0V to them, or sinking/NPN by my definition.