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. http://bit.ly/3l4xFbb

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. http://bit.ly/3boGMQt

The Phoenix Contact Digital Input module AXL SE DI16/1 is a Sinking type. http://bit.ly/3cdlzrW

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 https://realpars.com/discrete-sensors-part-2

What is a Transistor | Working Principles https://realpars.com/transistor

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https://realpars.com/sinking-output/

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