PLC Basics: Discrete Inputs and Outputs

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Understanding Discrete Inputs and Outputs in PLCs

While the CPU serves as the “brain” of a PLC with its processing power and memory, the real-world action lies in the Inputs/Outputs (I/O). I/O connections bring the PLC into interaction with the outside world, connecting sensors, switches, and actuators, which make I/O a key area for anyone installing or servicing PLC hardware.

I/O in PLCs is typically divided into two categories: discrete (digital) and analog. Here, we’ll focus on discrete I/O—those that handle simple on/off signals.

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What is Discrete I/O?

Discrete I/O (also called digital I/O) manages on/off signals. This means it recognizes binary states—either on or off, represented as 1 or 0 in the PLC’s memory (1 for TRUE and 0 for FALSE). In practical terms:

• Discrete Inputs: A switch in the “off” position would be read by the PLC memory as 0. When switched to the “on” position, it’s read as 1.
• Discrete Outputs: When the PLC sends a 1 from memory, the connected device (like an indicator light or motor) will be in the on state. If the output is set to 0, the device remains off.

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Exploring Discrete Inputs

Devices such as push buttons, toggle switches, and relay contacts are typical discrete inputs for PLCs. These switching devices form electric circuits with the PLC. Here’s how it works:

Open Switch: If the switch opens, current flow stops and the input as “off.” Based on our statement above, we know the PLC memory reads it as 0.

Closed Switch: When a switch closes, current flows and the input circuit is “on.” Based on our statement above, we know the PLC memory registers this as a 1.

Input Specifications

Input modules come in different types with different attributes. Typical operating voltages can be AC or DC and range from 5V to 240V. Depending on density, it is common for modules to have 8, 16 or 32 inputs. Then there is sinking and sourcing to consider which is related to the direction of current flow. The point is, it’s important to pay attention to the input specifications when wiring. You need to select a PLC or input module that is suited to the supply voltage and the connected devices.

For example, here is a diagram of an AC input module. Lets consider that list of attributes that was just mentioned.

• Number of inputs – in this case there are 8.
• Operating voltage – 240 VAC
• Sinking or Sourcing – That is not applicable when the module is designed to work with alternating current (AC). We will talk more about sinking and sourcing in a moment.

Considering the operating voltage of this module, it’s a good time to discuss electrical safety. Never assume, just because you are working on PLC control circuits, that you are working with safer, lower, voltages. In his case the control voltage is 240VAC. You could get a nasty shock. In fact, it could be a lethal one.

Sinking and Sourcing

Sinking and sourcing describe the direction of current flow in DC inputs. This is critical to know for proper wiring and compatibility, especially with semiconductor devices.

The explanation of sinking and sourcing is based on conventional current flow. Where positive potential is considered the “source” and negative potential is considered the “sink”.

Consider the diagram labelled Field Wiring For DC Sourcing Input Module below. The input module common terminal is connected to the positive. The switching devices are connected to the negative. In this configuration the input module is the electrical source. The red arrows indicate the conventional direction of current flow.

Next, consider the diagram labelled Field Wiring For DC Sinking Input Module. The input module common terminal is connected to the negative. The switching devices are connected to the positive. In this configuration the switching devices are the electrical source. That make the input module the sink. The red arrows indicate the conventional direction of current flow.

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Discrete Outputs and Their Role

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Discrete outputs control devices like indicator lights, solenoids, or relay coils—often called load devices. These load devices, when connected properly to PLC outputs, form electric circuits. It is the PLC outputs that will act as switching devices for each load device.

Output On: if the PLC memory for the assigned output is set to 1 the output will be in the ON state. When a PLC output is ON, current will flow and the load device will be energized.

Output Off: if the PLC memory for the assigned output is set to 0 the output will be in the OFF state. When a PLC output is OFF, current is interrupted and the load device will be de-energized.

Output Specifications

Similar to inputs, output modules are available in different. There are variations with different voltage ranges number of outputs. Outputs also be sinking or sourcing. It’s important to pay attention to the output specifications when wiring. You need to select a PLC or output module that is suited to the supply voltage and the connected devices.

Next, let’s look at the difference between sinking and sourcing outputs. Remember, conventional current flow is from positive to negative. Positive potential is considered the “source” and negative potential is considered the “sink”.

Consider the diagram labelled Field Wiring For DC Sourcing Output Module below. The output module common terminal is connected to the positive. The load devices are connected to the negative. In this configuration the output module is the electrical source. The red arrows indicate the conventional direction of current flow.

Next, consider the diagram labelled Field Wiring For DC Sinking Output Module. The output module common terminal is connected to the negative. The load devices are connected to the positive. In this configuration the load devices are the electrical source. That make the output module the sink. The red arrows indicate the conventional direction of current flow.

For those working with PLCs, understanding the concept of sinking and sourcing is essential to ensure proper field wiring, which in turn ensures device reliability.

If you are really interested in PLC fundamentals then you might want to take a look at this course on Udemy called the Pre PLC Programming Course. In my opinion, this course teaches what you absolutely need to know before you click on the PLC software and attempt to understand the programming. It is not geared toward any particular brand of PLC our programming software. The fundamentals are relevant to all types of PLCs.

Up Next – PLC Basics: Analog I/O in PLCs, Bridging the Digital and Real Worlds

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