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Sinking and sourcing field devices and PLC I/O cards

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Hello all, I am working on a project where my company is contracted to design, install and commission traffic management control system. We are going to install Overheight Detectors to detect vehicles above certain height. The overheight detection is monitored and controlled through a PLC. The signal output from the receiver of the overheight detector pair is the input to the PLC digital input card. I am supposed to choose the type of overheight detector (sensors) based on the output. The two options are: 1. NPN transistor output, and 2. PNP transistor output. Researching from the web, I was able to know that NPN is a "sinking" device and PNP is "sourcing". The logic of detection is that when the overheight detectors are in normal state the input to PLC is "HIGH" and when the overheight vehicle is detected the input to PLC goes"LOW". The PLC card is a sinking input card. The dilemma is the selection of the sensors from the above mentioned outputs. I am also a bit unclear on the concept of sinking and sourcing from the perspective of field devices as well as PLC I/O cards. If someone could please explain this??? Can I select a sensor with NPN transistor output and connect it to the PLC input card to obtain the desired input signal? If yes, how will this work? Could anyone please suggest what type of sensor do I need and why would that type of sensor be suitable in regards to sinking and sourcing as well as the control logic? Thanks.

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To get to the crux of the issue, the output type of the sensor makes very little difference. Select your sensor based on the application considerations (environment, range, signal type) first, then worry last about the output. It's just the electrical interface between the sensor and the PLC, and doesn't make a difference to the application. Generally, you can get any sensor in either output type, so I usually determine the PLC first. To answer more specifically, different PLC manufacturers use the terms "sink" and "source" differently. While one way may be "better", what really matters is how the PLC manufacturer defines it.

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http://www.theautomationstore.com/npn-pnp-devices-and-connections/

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since you already have PLC, you must know it's inputs. what are inputs of your plc expecting? PNP or NPN?

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The PLC input card we have is positive switching and the reference ground for all points is 0V power contact.

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That means you want a PNP sensor. If for some strange reason the perfect sensor for your application is only available as an NPN output, you can get either a relay or an NPN/PNP signal converter.

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Sorry, but I am still confused with the concept of sourcing and sinking. I believe that the PLC card being positively switched and have reference as 0Vdc means it is a "sinking" input card and hence you are suggesting to use a sensor with PNP (sourcing) transistor output.

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The members here are advising you correctly. Because you have 0VDC as your system reference point, it demands a positive switching input and output. Thus, you need PNP or "sourcing" sensors.

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Forget about the names matching up as some hardware manufactures interpret this different from others, there does not appear to be a clear standard. Here is how I keep it straight in my mind. An NPN input will switch the Negative or 0-Volt line of the DC circuit. A PNP input will switch the Positive or 24-VDC line. An NPN device will need to be installed to switch the Negative or 0-Volt side of the supply lines. A PNP device will need to be installed to switch the Positive side of a 24VDC circuit.

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The terminology is confusing because of how different manufacturers use it. Here are some brief definitions: The "common" is literally the common point in an electrical circuit. Think about a light bulb and a switch connected to a power source. The side of the power source that both the light bulb and the switch is connected to is the common. Because the switch is just a contact (i.e. not solid state), you can wire the circuit with either side of the power source being common. Here in the US it's most usual to see 0VDC as a circuit "common", but that's not the case in other parts of the world. There are a number of reasons why you'd choose one over the other, but that's unimportant to our discussion. Follow the standard in your country if possible. "Sourcing" refers to electrically pulling a voltage up from 0V to the level of the source voltage, which is usually 24V. This would make 0V be the logical LOW (aka OFF or "0") and 24V be the logical HIGH (aka ON or "1"). Note that the common in a sourcing circuit would be 0V. "Sinking" refers to electrically pulling a voltage down to 0V from the level of the source voltage, which is usually 24V. This would make 24V be the logical LOW (aka OFF or "0") and 0V be the logical HIGH (aka ON or "1"). Note that the common in a sinking circuit would be source voltage (e.g. 24V). Here's where things get dicey and the discussion turns into a fist-fight. You were warned. A PNP transistor when turned on provides source voltage at the output. The would make it a sourcing device. The opposite is true of an NPN transistor. However!! To turn on a PNP transistor, you need to pull it's input down to 0V. And again, the opposite is true for NPN. Let me say that a different way. It takes an NPN transistor to turn on PNP transistor and vice versa. This is, I believe, the key point, because it explains why PLC inputs that are called "sinking" by one company are electrically identical to those called "sourcing" by another company. Here's why. A sensor is manufactured with either a PNP or NPN output. This refers to the final transistor output in the circuit, and so the output of the sensor is either sourcing (PNP) or sinking (NPN). I don't think there's any argument about the terms at this point. But think about what I said above. A sensor with a PNP output must be matched with a PLC input that has an NPN transistor in it. So do we call the input "sinking" (referring to the transistor), or "sourcing" (referring to the electrical circuit of which it is a part? Different companies make different decisions on this point. I remember one project I was on where the engineer who specified sensors asked the engineer who specified the PLC what the inputs were. "Sinking", he said, so the sensor engineer ordered NPN sensors. Unfortunately, the PLC was an Allen-Bradley product, so when I went onsite for start-up, none of the sensors worked. In the Allen-Bradley world, a sinking input is an NPN input. So the NPN sensors and the NPN inputs didn't work together. We ended up switching input cards.
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The most common of IO types are NPN, PNP, dry contact, 4-20 mA or voltage outputs. Once you know this and that you can accurately “see” your product or “measure” your product with your sensor. You may find many different type of Remote I/O modules based on different protocols ( like Modbus, Ethernet, Profibus, CAN bus, etc) and different I/O type Sink/Source Wet/Dry etc at below link: http://www.icpdas-usa.com/remoteiomodules.html?r=mohsen And also see below link which is very useful to select what you want: http://www.icpdas-usa.com/m7000_analog_digital.php#digital?r=mohsen you may also use below table: (For sensors with NPN, PNP or dry contact outputs, use this table and select the module with the correct number of sensor inputs (or greater) and the matching output style:

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When talking about the IO cards, sourcing means they send out 24V and sinking means they are expecting 24V in. Sensors are generally not referred to as sinking or sourcing and instead referred to as PNP or NPN. PNP = Sinking inputs (you put in 24V to turn an input on) and Sourcing outputs (24V comes out of them when you turn them on).NPN = Sourcing inputs (24V comes out of the input and connecting that 24V to common turns the input on) and Sinking outputs (when turned on, the output connects to common).Generally, you always use PNP, Sinking inputs, sourcing outputs. The reason NPN stuff exists is because low powered microchips with inputs and outputs built right in can't provide any power themselves, so they are always sinking no matter if they are outputs or inputs. That explains sinking outputs, I forget the very legitimate reason sourcing inputs exist, but there is one. I am of the opinion that if you are not in the very rare case of actually needing NPN for a legitimate electrical reason, you should always use PNP. It makes a lot more intuitive sense (put voltage into inputs take voltage out of outputs) and there is a much wider selection of sensors available. The next thing to think of is Normally Open or Normally Closed (NO or NC). NC sensors always give you 24V until they see the thing they are supposed to sense. That way, if the sensor is broken or the wire is cut, it will behave as if it is sensing the thing (in your case, everything will be an over height truck, which is better than missing an over height truck). NO are the opposite, only giving 24V when sensing the thing/event. These are better for when a you are waiting for a thing to happen rather than making sure a thing isn't happening. For example, you're looking for a cylinder to extend would use NO, but making sure a cylinder is not extended would use NC.
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Great explanation CaptainWinky! Now I understand the origin of the terms. Doesn't change my experience that some brands (e.g. Mitsubishi) choose to refer to a "sinking" input as being an input that switches with a "sinking" output. Right or wrong, it just is. Possibly just an attempt to simplify things. Amazing how often "simplification" just makes a bigger snarl of things.

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There has been some great discussion regards NPN and PNP, sinking sourcing etc.... With Omron input cards they are bipolar.... so you choose the input type by what you wire to the common of the input card.. I always think of it simply in that if I have 24vdc coming in from the sensor for the input then the COM must be a 0VDC. Visa Versa... if 0VDC is coming in as the input then the COM must be 24VDC.... A PNP sensor switches the positive.. hint "P" A NPN sensor switches the negative... hint "N" If in doubt check the wiring diagram for the card to make sure you understand regardless of the IO card manufacture. However I have a bigger concern which was stated in the original post... Are you kidding me..... I hope that company has all the Public Liability and Professional Indemnity insurances up to date.
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