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



Mods Note:
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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.

 

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.

 

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.

 

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.

 

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.

 

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.

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.

 

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.

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.

 

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!

 

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.