Sinking and Sourcing PLC Inputs Explained | What is the Difference?

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⌚Timestamps:
00:00 - Intro
00:31 - PLC digital vs analog input module
01:13 - Detecting input field device conditions
02:48 - Sinking and sourcing
04:05 - 1) Sinking DI
04:38 - 2) Sourcing DI
04:53 - Active input field devices
05:48 - PNP and NPN devices
06:15 - Siemens - 6ES7521-1BH50-0AA0
06:50 - Phoenix Contact - AXL SE DI16/1
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In this video, we’re going to talk about Sinking and Sourcing PLC digital input modules and how they are connected to field devices.
We will be using a Siemens digital input module 6ES7321-1BP00-0AA0. bit.ly/3NE266x
A digital input module accepts inputs from digital devices such as switches and proximity sensors. The input module will detect one of the two possible conditions of an input device: open or closed.
For example, a push-button switch is a digital device and can either be open or closed.
If the switch is open, there is 0V at the module input terminal. If the switch is closed, there is +24V at the module input terminal.
The input module will detect either a no-voltage or voltage condition at the input. These voltage conditions represent the open or closed conditions of the switch.
These voltage or no-voltage conditions are represented by 1’s and 0’s in the PLC memory.
Let’s talk about the current flow between the Power supply and the input module.
There is a current flow, but it is very small and usually under 10 milliamps.
Depending on the type of PLC input module and how it is connected with the input device, the current could flow in or out of the module. That’s where the terms Sinking and Sourcing come into the picture.
When we talk about sinking and sourcing, it’s all about defining the direction of conventional current flow between two devices.
- The current is flowing from Device #1 to Device #2. Therefore, we say Device #1 is Sourcing the current, and Device #2 is Sinking the current.
- What if the current is flowing from Device #2 to Device #1? In this case, we say Device #2 is Sourcing the current, and Device #1 is Sinking the current.
In every situation where you have a current flow between two devices, one of the devices will be Sourcing and the other will be Sinking.
1) Sinking PLC digital input modules
PLC vendors manufacture both Sinking and Sourcing Digital Input Modules.
Let’s have a look at two devices with current flowing between them.
One device is a PLC input module, and the second device is a push-button switch. Based on the connection polarity, the Sinking module will have the current flowing into it from the switch.
2) Sourcing PLC digital input modules
OK… let’s do that again, but this time our PLC input module will be a Sourcing module.
Based on the connection polarity, the sourcing module will have the current flowing out of it.
Current can flow in either direction through a push-button switch because it is a passive device and not polarity-dependent.
There are many active digital input devices used in industry today that are polarity-dependent. They must be wired correctly or they will not work at all. These active devices will be labeled as either Sinking or Sourcing.
1) If the input device is a sourcing type, the PLC input module must be a sinking type.
2) If the input device is a sinking type, the PLC input module must be a sourcing type.
The Siemens Digital Input module 6ES7521-1BH50-0AA0 is a Sourcing type. bit.ly/4ai6wtA
The Phoenix Contact Digital Input module AXL SE DI16/1 is a Sinking type. bit.ly/43fC6Ee
Adding confusion to already murky concepts, some vendors refer to Sinking and Sourcing input devices as NPN and PNP.
- An NPN device is a Sinking device
- A PNP device is a Sourcing device
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To learn more, you might want to review our other articles:
How to Wire Discrete DC Sensors to PLC - Part 2 realpars.com/d...
What is a Transistor | Working Principles realpars.com/t...
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#RealPars #PLC #automationengineer

[@kmndza]

I'm working as an automation engineer. This video is very nice, well explained.

[@realpars]

Glad to hear that, Kevin! Happy learning.

[@stevenlee5755]

Do you need PLC or another products?

[@natriumguy8537]

Literatures and PLC manufacturers differ from one another when it comes to the concepts of sinking and sourcing. The important thing is that you have to maintain the potential difference of 24V between digital input and common terminal. In many PLCs, we can connect either 0V or 24V to common terminal and opposite of that (24V or 0V) must be connected to digital input. It is better to refer installation guides of PLCs to decide wiring connections. Some PLCs even have jumpers to switch from sinking to sourcing type wirings and vice-versa.

[@stevenlee5755]

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[@jothain]

Thank you. I've been self learning very basic logic stuff and I've been trying to find out very clear information about how PLC card types and PNP and NPN sensors actually work in practice. Now I'm finally pretty much 100 confident how they're supposed to work. Very good video, thank you 👍👍

[@realpars]

Glad it was helpful, Jothain! If you have any questions along the way, please feel free to contact me at any time!

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[@stevenlee5755]

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Best automation teacher on KZbin thank you 😊 RealPars 😎

[@realpars]

Thank you very much!

[@dutch6649]

Thank God. I've been struggling with this concept for years.

[@Ryarios]

Every circuit has a power source and a load. This this case, the input card is the load on the circuit. It’s the device that regulates the amount of current the flows through the circuit. Sourcing or sinking just determines whether you switch the + side of the power supply or the ground side of the circuit. The circuit works the same when current flows but it has ramifications that are important to note. For instance, let’s take the sinking input. You have +24v going out to the field and when the switch out there closes it comes back, through the input card (load) to ground. Now let’s say you accidentally ground out one of those field wires. It could me anything. A tech could drop on and hit the metal conduit. The conduit can be damaged, or the switch can fail and become grounded. If you do that to the +24v wire, then you immediately short the power supply. If you do it to the return wire, then you short the power supply when the switch closes. Either one can take down the whole control system unless you put in overload protection. Now, in a sourcing input card. The two field wires are placed in the circuit AFTER the load (input card). If you short the wire coming from the card to ground, the input goes active immediately. If you short the wire after the switch (going to ground) then the input goes active after the switch is closed. This makes sourcing inputs more able to deal with failure modes. Unfortunately, sourcing type inputs can be trickier to troubleshoot and the tech definitely needs to know it’s a sourcing input card. Sinking cards are by far the most common found in the US. I believe that’s because they more closely resemble how most electric circuits work and so, are easier to envision for troubleshooting and for designers.

[@realpars]

Thanks for sharing your knowledge and experiences with us! We always appreciate such in-depth comments.

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[@kevin00861]

Such an amazing explanation ! And yes; we faced such ground in GE Speedtronic Control System and it took us some time to figure out the problem ... Thanks for your detailed comment I already screenshot it as a reference for me and for my fellows

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[@realpars]

Glad you like them! Thanks for your support, Alejandro!

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[@CreoleGuh504]

Thank you so much. I came to class a little late and missed the lecture on this. This helps so much! Not hard to understand at all.

[@realpars]

Glad it was helpful! Always feel free to let us know if anything is unclear. Happy learning!

[@mannyvillagrana1267]

Thank you for this!!!! really helped me understand this better than my professor's instruction. Subscribed

[@realpars]

Awesome, thank you!

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Thanks a million times for your informative and detailed videos i became literally a great follower to all your automation videos ans also literally video by video

[@realpars]

Great to hear that! Thanks for sharing and happy learning!

[@anonichechannel]

Finally, I got it. Thanks for your video.

[@realpars]

Glad it helped! You're more than welcome, and always feel free to leave us any questions if anything is unclear. Happy learning

[@HeI2naNLeo]

Excellent video, very didactic and applicative, could you upload a video of the same thing with analog inputs, it would be very helpful since I am trying to connect a sinking-type analog instrument and I want to know if I have to modify the connection of the module to a sourcing type

[@realpars]

Great suggestion! I will happily pass this on to our course developers, hopefully they are able to create a video course on this in the near future. Thanks again for sharing and happy learning!

[@joyflowers6693]

Excellent. Very clear explanation. Thank you.

[@realpars]

Glad it was helpful!

[@genghizalseitov3807]

Very good explanation better then academic ones

[@realpars]

Thank you, Genghiz!

[@stevenlee5755]

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[@rajshekharghosh2]

isn't that current arrow reversed? Sinking current goes out, and sourcing current flows in. please help me if I misunderstood the whole video.

[@realpars]

Hi @Ra Gho. Thanks for your question. We use conventional current flow terms when describing current flow. Conventional current flows from positive to negative. If you are referring to electron current flow, then you are correct.

[@HoHoMoMoo]

Mechatronics engineering student here. Your videos are very helpful

[@realpars]

Amazing! We're happy to receive such positive feedback! Feel free to leave us any questions you might have. Happy learning!

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[@deniskalugin7984]

Great video! I have never seen such a nice explanation! Thank you! P.S. Liked and subscribed

[@realpars]

Awesom, thank you!

[@hfe1833]

After several tutorial I found sinking or sourcing this one I understand

[@dushyantdave5637]

Very nicely explained

[@realpars]

Thank you!

[@nayrbsworld3048]

so if i have plc D.I. sourcing and i measure the voltage in each side of the switch(off state). what would be the reading? 24VDC? given 24 vdc is the plc

[@realpars]

Hi @nayrbs world. Thanks for your question. If I understand you correctly, you are referring to an open passive SPST switch connected to an input of a sourcing digital input card. You will measure 24 volts on the switch terminal connected to the digital input terminal of the module. If the switch is open, there will be 0v on the other side of the switch.

[@nayrbsworld3048]

i mean if i will use a switch with a (Normally open & normally closed switch SPDT) i wire a normally open to a d.i. sourcing plc. when i put the (+) leads of my multimeter to N.O. then (-) to common of that switch at idle state. what would be the reading in my multimeter. the same as what will be the reading when in NC state. thanks real Pars

[@faizalraazy91]

Beautiful presentation

[@realpars]

Thank you, Faizal!

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[@Senthilsdrawingdesk]

Your videos are awesome. Videos are created in such a way that automation concepts are very easily understandable..keep up your good work👍

[@realpars]

Thanks a million for your positive feedback! We always appreciate such kind comments.

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[@parshuramkolpuse9819]

Thank you Sir,This is very helpful video.

[@realpars]

You're very welcome!

[@stevenlee5755]

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[@chickenz4604]

Hi, I am planning to power up my Digital Inputs and Analog Inputs Extension module with 24V DC from PLC 24V DC Supply (500mA). Is it okay to do this? For the PH meter I use AC source, Flowmeter I use other DC Supply, and the pt100 temp sensor I use two wiring transmitter

[@realpars]

Hi Chickenz. It's common to connect input devices to the 24 v power supply. But, you need to be cautious so as not to overload the power supply. Not knowing what you have currently connected to the power supply, it is not possible to determine the current draw. You might have to do a bit of homework to find current draws for your connected devices. As for the PLC inputs themselves, they draw very little current. I'm, sorry but I don't know what you mean by "Analog Extension module". Can you elaborate please?

[@chickenz4604]

@@realpars Currently I have PLC unit with 16 DI and 16 DO and Analog extension module with 4 AI. This analog module also requires 24V DC Supply. For the PLC it uses AC source and have 24V DC Supply Out. Thx

[@albertjohnson4668]

DC current flows from negative to positive poles.

[@danieljuneau229]

So if a plc common input or output has positive, then it’s sinking? A negative common would be sourcing? The field device being the opposite.

[@realpars]

Hi Daniel Juneau. Yes, good summary in a nutshell. Thanks.

[@surendrakverma555]

Thanks Sir 👍

[@realpars]

You're very welcome!

[@t4c4]

when sensor is in closed state. what will be the voltages in sourcing and sinking plc inputs?

[@realpars]

Hello @t4c4. Thanks for your question. Let's assume our Power Supply voltage is 24VDC and the sensor is in the closed, or low resistance state. There will be 24 vdc at the Sinking plc input terminal and 0V at the Sourcing plc input terminal. Interestingly, with the sensor in the open, or high resistance state, the voltages will be opposite.

[@t4c4]

@@realpars Thanks

[@TheGrateCanadianDream]

Sir what are the condition under which we use npn or pnp sensors

[@realpars]

Hi. Thanks for your question. It really depends on what you want to switch with your sensor. Do you want to switch ground/return or supply voltage? It also depends on the input module itself. If your module is a sinking type, you need PNP sensors. If your module is a sourcing type, you need NPN sensors.

[@KevinNguyen-tw5ml]

Thank you very much!

[@realpars]

You're welcome! Happy learning

[@jothain]

Stupid question. At my work it seems to be pretty much standard for all different manufacturers to use PNP sensors in their devices. But why does both still exist in industry? Is it simply backwards compatibility for older equipment or does NPN offer great benefits in some configurations? Oh and more, does ie. Siemens CPU's support to plug in both type of input cards simultaneously? I'd assume they would, though wiring would be great pia and very misleading to people who wouldn't notice that odd configuration.

[@realpars]

Hi jothain. Both still do exist in the industry for a number of reasons. Your question is excellent and has appeared a few times for this video. If you scroll down and read through the comments, you'll see many responses from us and also other viewers. Siemens PLCs have the ability to have both sinking and sourcing modules at the same time. This is true for most PLC vendors as far as I know. And you are correct when you say it could be misleading and possibly create some errors dues to incorrect wiring faults.

[@stevenlee5755]

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[@AmadiGoodluckNdubuisi]

thank you

[@realpars]

You're very welcome!

[@jordaz87]

Great video 💯

[@realpars]

Glad you enjoyed it! Thank you for sharing

[@hamelh.m4190]

Thank you .. very informative

[@realpars]

You're welcome

[@stevenlee5755]

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[@alsas1004]

Hello, i really like your videos, i'd like to know (IF possible) what program do you use to make those great animations?, Keep up the good work :D

[@realpars]

Hi there, Thanks for your kind comment! I am actually not sure about this as this is done by our graphic and animation department.

[@stevenlee5755]

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[@johancampos]

Good explanation

[@realpars]

Thanks a lot, Johan!

[@stevenlee5755]

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[@jasperlumantao5000]

Good day sir ..can you explain ..how vs motor working principle. .

[@realpars]

Hi Jasper! Thanks for your comment and your suggestion. I will pass this on to our course developers! Thanks for sharing and happy learning!

[@jasperlumantao5000]

Thank you very much sir .. I'll wait for your video.. Good Bless

[@stevenlee5755]

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[@kurtmatchitt8563]

Can you have sinking and sourcing Analog inputs?

[@realpars]

Hi @Kurt Matchitt. Thanks for your question. Quick answer - No, the terms sinking and sourcing are only used when referring to digital inputs and outputs. We can expand on that answer if you would like more detail. Just let us know by replying.

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[@realpars]

Thank you for your comment! Have you tried purchasing it from your local TIA Portal seller?

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[@ikpalian4413]

Wow!

[@joes.8351]

Sinkn(pn) like sinking. PuNP up power source.

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[@palashabid]

Very nice 👍👍👍

[@realpars]

Thanks a lot, Abid!

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[@roseelectronics4582]

Nice work!

[@realpars]

Thank you!

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[@lauroolmos3362]

Great explanation!!

[@realpars]

Glad it was helpful!

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[@khoanguyen4780]

what about misubishi PLC?

[@khoanguyen4780]

for input signal, sensor is device 1

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[@mustafaimad3109]

Helo how i can take PLC cources from u?

[@realpars]

Hi Mustafa, Great to hear your motivation in regard to learning PLC Programming. 

 Our course library is specifically designed in a way where it provides all the tools needed for beginners to evolve into an advanced level through our hands-on courses. Feel free to browse through our course library to see all the different topics we are covering. bit.ly/30AVJaR Just to let you know, that we also have a free course on PLC Hardware bit.ly/2XnnUrF
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 Hope this helps! If I can be of any further assistance, please let me know - I’m more than happy to help!

[@mustafaimad3109]

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[@stevenlee5755]

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[@mustafaimad3109]

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[@jdnoble6945]

So with that in mind it's easy to remember a NPN card is recieving the current and a PNP card is the source of the current. If your like me a bit dyslexic you need these little rhymes to stick things in your head

[@danielkoryciak2357]

Thank You! But When we use sinking and sourcing. Somebody can give example for a life 😌?

[@danielkoryciak2357]

In most cases, we use sinking, so when we use sourcing?

[@sjholmesbrown]

@@danielkoryciak2357 Sometimes a control system is setup with sourcing PLC inputs as a design choice, but the main reason I have seen it used is when some of your digital field devices only support NPN outputs. Some rotary encoders, reed switches or other devices are not always available in a PNP output in the brand and size you want. In this situation you either need a converting relay to change the device output polarity to suit a sinking input, or to use a sourcing input setup.

[@stevenlee5755]

Do you need PLC or another products?

[@stevenlee5755]

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[@bobsmith3291]

I don’t get what the point is in having two different types? I feel like plcs are overly complicated for absolutely no reason what so ever it could all be done so much easier than the way it is

[@realpars]

Hi @Bob Smith. That's a great question. Thanks for reaching out. Bottom line, it comes down to what voltage level is required. A sourcing input or output circuit provides the necessary voltage for the circuit. A sinking input or output circuit provides the necessary ground for the circuit. I'm sorry to hear that you think the graphics and illustrations are overly complicated. We do our best to simplify concepts for our viewers. Any modification suggestions are appreciated.

[@bobsmith3291]

@@realpars no I didn’t mean your video was overly complicated I meant plcs in general . They can be done in a much easier way than they do them but they like making it harder for people for some reason