Help with Open Drain output (pull-up resistor?)

Thread Starter


Joined Jan 10, 2018

I am to use this pump with I/O control pins. It is controlled through a PLC. I am not able to get output from pin 1 (of connector 1) when I'm filling. I need that signal to process my filling sequence.
I've done some research about Open Drain configuration, and it seems like they're NMOS type used for GPIO.
I've also learned that they need to be used alongside a pull-up resistor (otherwise they're useless).
Knowing that I did not connect 24 VDC to PIN 3 (which I assume is for activating the internal pull-up resistor as depicted below), this is what I get:
case 1: 0 v when I'm not filling.
case 2: around 200 mV when I'm filling.

I have also learned that if we have two states if no pull-up resistor is connected to Vcc (24 VDC in my case): GND and FLOATING.
My question is: Should I feed 24 VDC to PIN 3 to make PIN 1 output HIGH when filling and LOW when not filling?


Thread Starter


Joined Jan 10, 2018
I tried it this morning but it did not work. I actually lost the ability to give a Fill command to the pump when Pin 3 is connected to power source 24


Joined Sep 17, 2013
I find those instructions confusing.
Are you sure about the logic operation? Pin 1 is stated to be 'Active when filling', but does that mean the internal transistor is active (i.e. turned on)? If so, then pin1 would be at logic level 0. Or does 'Active' mean logic level 1?


Joined Sep 17, 2013
Try an external resistor (say 10k) between pins 1 and 3. Any change in the meter readings?

At any time have you connected pin 1 to pin 3 without an external resistor?
Last edited:


Joined Dec 2, 2017
Open drain outputs are not "useless" unless you need to pullup the output, where this is not always the case. IE: driving a simple load.

This image is incorrect as it shows an open "collector" and the description says "open drain".



Joined Jun 28, 2022
If you are using the pin 1 output as a status input to the PLC, you need some form of pullup resistor to a voltage that your PLC can handle. You could use the 24V output on pin 2 to pullup your PLC input. A 10K would be 2.4 mA that is a small percentage of the output capability of pin 2, 250mA.
If concerned about 24V into PLC you could use a voltage divider say 10K/10K that to create a 12V pullup

When Pin 1 Off, PLC sees 12V and current draw on Pin 2 is only 1.2ma
When Pin 1 On, PLC sees 0V and current draw on Pin 2 is 2.4ma
Both are much less than pin 2's capability of 250mA.
Now this all assumes PLC input is high-impedance (i.e. negligible current input).
Calculation changes if PLC requires any input current other that uA..
Last edited: