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 with this part number.
If you want to order this module online
you can head on over to the RealPars marketplace
or simply click on this link here.
The two types of PLC input modules are Digital and Analog.
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.
We will examine how the input module detects the input device condition
later in this video.
The focus of our video is on digital input modules.
OK… let’s go.
As we said earlier,
a digital input module
will detect one of the two possible conditions of an input device: open or closed.
So, the question is
“How does the input module know if the input device is open or closed?”
All we need to do is construct a closed circuit containing the input device,
the PLC input module,
and a 24V DC power supply.
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.
Every input device is connected the same way.
If you have a 64 input module,
it's possible to have 64 separate circuits…
We'll show you more examples later.
Alright, now that we know how the digital input module
determines the input device condition,
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.
Notice in our example that the current is flowing “in” to the PLC module.
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.
Let’s spend some time clearing up any misconceptions
you might have about these terms.
When we talk about sinking and sourcing,
it's all about defining the direction of conventional current flow
between two devices.
Think of it this way…
Current is flowing between Device #1 and Device #2.
Ask yourself “Which direction is the current flowing?”
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.
OK… 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.
PLC vendors manufacture both Sinking and Sourcing Digital Input Modules.
Let’s explore what they are and how they differ.
Remember what we discussed earlier…
it's all about the direction of conventional current flow.
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.
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.
You can learn more about active sensors in our other videos.
We’ll direct you there at the end of this video.
If you recall, earlier
we said that when there are two devices with current flowing between them,
one device will be sourcing
and one will be sinking.
If the input device is a sourcing type,
the PLC input module must be a sinking type.
If the input device is a sinking type,
the PLC input module must be a sourcing type.
Adding confusion to already murky concepts,
some vendors refer to Sinking and Sourcing input devices as NPN and PNP.
You can learn more about NPN and PNP in our other videos.
We’ll direct you there at the end of this video.
Here’s something to remember…
An NPN device is a Sinking device.
A PNP device is a Sourcing device.
The Siemens Digital Input module 6ES7521-1BH50-0AA0 is a Sourcing type.
If you want to order this PLC module to use in your project,
you can find the link in the video description.
Let’s connect a push button switch and a Sinking Proximity Sensor
to two of the inputs.
The current is flowing out of the PLC module
because it is a Sourcing type.
Let’s have a look at a Sinking module.
The Phoenix Contact Digital Input module AXL SE DI16/1 is a Sinking type.
You can find the link to order this PLC module
in the video description as well.
Let’s connect a push button switch
and a Sourcing Proximity Sensor to two of the inputs.
The current is flowing into the PLC module
because it is a Sinking type.
If you want to learn more,
you might want to review two of our other videos called
“How to Wire Discrete DC Sensors to PLC - Part 2”
and “What is a Transistor | Working Principles”.
You can find the links to these videos in the description.
OK… let’s review what we’ve covered in this video:
A digital input module
will detect one of the two possible conditions of an input device: open or closed.
Digital Input modules detect either a no voltage
or voltage condition at the input.
A Sinking input module will have the current flowing into it.
A Sourcing input module will have the current flowing out of it.
PLC vendors offer both Sinking and sourcing Digital input modules.
An NPN device is a Sinking device
A PNP device is a Sourcing device
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and take your career to the next level?
Head on over to realpars.com