Common PLC input signal

In industrial control systems, input components are essential for signal acquisition and system interaction. Commonly used input devices include switches such as toggle switches, push-button switches, limit switches, proximity switches, and encoders, as illustrated below:

These switches come in various wiring configurations—two-wire (e.g., push-button or toggle switch), three-wire (e.g., proximity switch), and four-wire (e.g., encoder). The type of input they provide can be either NPN or PNP. NPN switches typically connect to a low voltage (0V) when active, while PNP switches connect to a high voltage (usually 24V). Understanding the output behavior of these switches is crucial for proper integration with PLCs. When connecting these switches to a PLC, it's important to consider the input type. There are two main types: sink (leakage) and source inputs. In a sink input configuration, the current flows into the PLC input terminal from the common S/S terminal. In contrast, a source input allows current to flow out from the input terminal to the common S/S terminal. This distinction affects how the switches are wired and how signals are interpreted by the PLC.

For two-wire switches, both sink and source configurations may be possible, depending on the PLC model and setup. However, most PLC units are designed to support only one type of input at a time. For example, a three-wire NPN sensor should be connected to a sink-type input, while a PNP sensor requires a source-type input.

Proper wiring ensures reliable signal transmission and prevents damage to the PLC or sensors. It’s always recommended to double-check the switch type and PLC input configuration before making any connections. Whether you're working with proximity switches, encoders, or simple push buttons, matching the correct input type is key to achieving accurate and stable operation in your industrial control system.

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