Sinking and Sourcing PLC Inputs | What is the Difference?

In this article, 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.

PLC digital vs analog input module

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 article.

The focus of our article is on digital input modules. OK… let’s go.

Detecting input field device conditions

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 into 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.

Sinking and sourcing

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 digital input modules

1) Sinking PLC digital input modules

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.

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.

Active input field devices

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.

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.

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.

PNP and NPN devices

Adding confusion to already murky concepts, some vendors refer to Sinking and Sourcing input devices as NPN and PNP.

Here’s something to remember…

– An NPN device is a Sinking device
– A PNP device is a Sourcing device

SIMATIC S7-1500 – 6ES7521-1BH50-0AA0

The Siemens Digital Input module 6ES7521-1BH50-0AA0 is a Sourcing type.

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.

Phoenix Contact – AXL SE DI16/1 – 1088127

Let’s have a look at a Sinking module.

The Phoenix Contact Digital Input module AXL SE DI16/1 is a Sinking type.

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.

Summary

OK… let’s review what we’ve covered in this article:

– 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

We recommend checking the following related articles, if you haven’t already, to have a better understanding of PLC digital inputs and their connection to field devices:

How to Wire Discrete DC Sensors to PLC – Part 2

What is a Transistor | Working Principles

If you have any questions about the Sinking and sourcing PLC inputs or about PLC modules in general, add them in the comments below and we will get back to you in less than 24 hours.

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The RealPars Team

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