Connecting open collector signal outputs to PLC current sourcing input modules

Discussion in 'Embedded Systems and Microcontrollers' started by meld2020, May 24, 2016.

  1. meld2020

    Thread Starter New Member

    Mar 14, 2016
    21
    0
    My device's documentation says to "please provide a 820 ohm minimum pull-up resistor" for this schematic. Isn't the point of a pull-up resistor to enable us to differentiate between floating and open states? If my application is not concerned with such a thing, do I really need to include this? My primary reservation in not doing so is that I worry it is offering me some sort of current protection, although in a switched state there would be no such protection. This is for a turbine flow meter. I am not seeing any information regarding protecting the magnetic pickup inside this turbine flow meter as far as current limitation goes. Resistor in series, maybe? Or should I look to my current sourcing input module to provide an internal resistance? Would most people agree this is a standard feature in most modern PLC modules?

    Thanks,
    Mel

    image1.jpg

    Mods Note:
    Before you upload the photos, please compress to a clear resolution as 800x600 or 1024x768, this photo already compressed to 800x492 and the file size from around 2.9MB to 61KB.
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    The output is a open collector so a load is required, if the destination source (customer Interfacing equipment) has a pull up on the input then it may not be required.
    Otherwise the output will be permanently floating and you will not have a decisive result.
    Max.
     
  3. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,388
    1,605
    Is that picture from the flow meter documentation?

    If so, it is saying you run the meter off 9 to 35 VDC, and one can assume they did all the protection properly as long as you feed it this voltage.

    The pull up resistor spec is implying not to sink more that 42 mA into the meter.

    I don't use PLC devices so please explain what the Sourcing input module is or does. Or just wait till someone who knows does it for you.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    The fact that is is pull up to +ve indicates it is NPN output.
    Many PLC's have the option of setting the digital Inputs sink or source, but you seem to using a dedicated analogue input of some kind if using a transducer, or is this a counter module of some kind.
    What PLC is it?
    Max.
     
  5. meld2020

    Thread Starter New Member

    Mar 14, 2016
    21
    0
    My sourcing digital input module is being used as a pulse counter to determine flow rate from the meter. It provides Vsup to a sinking device (meter) that, when switched, provides the path to common ground. The module / all of it's IO is sourcing input. It's a NI 9426.
     
  6. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    With using a standard input module for counting do you not miss any counts due to scan time?
    Are you using rising edge detection?
    Max.
     
  7. meld2020

    Thread Starter New Member

    Mar 14, 2016
    21
    0
    I am using dedicated FPGA hardware for sampling the input, so the scan time hasn't really been an issue. The frequency range of this device is 37Hz to 370Hz, so the ~2.7ms sampling rate on the fast end is handled fine, although with my application I won't even near half of that. I am doing a period based calculation, so I opted to use rising edge detection only so as not to throw off the periods since the waveform wouldn't be perfectly on/off.

    I am just getting really thrown off by this schematic and it's frustrating. It almost seems as though the schematic assumes a sinking input on the customer side.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    Not really, the module shows to be sinking (NPN), to a PLC sourcing, so it would be compatible to a sourcing input.
    You most likely do not need the pull up as the PLC will supply the NPN load.
    Max.
     
  9. meld2020

    Thread Starter New Member

    Mar 14, 2016
    21
    0
    The white wire is essentially providing 24V through the module's internal resistor since it's a current sourcing input module, so I'm not really understanding what the point of the pull-up resistor is in this schematic if the same thing is basically happening coming out of the Customer Interface Equipment bottom wire. Shoot me!
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,548
    2,373
    IF you don't have any pull up or source from the equipment it is being connected to (PLC) then a pull up resistor has to be provided.
    In your case a PLC is typically a opto isolator as a load which provides the pull up source.
    Max.
     
    meld2020 likes this.
Loading...