I. Introduction
The digital input interface of a PLC is not complicated, but it plays a crucial role in ensuring the reliability of signal transmission. To enhance the anti-interference capability of the PLC, the input interface typically uses an optocoupler to isolate the external signal from the internal processing circuit. This means that the external input signal only needs to activate the LED inside the optocoupler, which then sends the signal to the receiving side via the optocoupler. This isolation ensures that the PLC can accurately receive and process the input signals without being affected by electrical noise or interference.
Currently, most PLCs use either a single-ended common point (S/S) or a double-ended input configuration. In the case of single-ended inputs, each manufacturer may have different conventions for connecting the common terminal. For example, Japanese PLCs often use a positive common point, while European models tend to use a negative one. Some PLCs now offer flexible options where the S/S terminal can be connected to either the positive or negative supply, depending on the application. Understanding these differences is essential when selecting sensors and designing the system to ensure proper operation and long-term stability.
II. Input Circuit Configurations
1. Types of Input Circuits
Digital input terminals in PLCs are generally categorized into DC and AC types based on the power supply used. Additionally, they can be classified as single-ended or double-ended based on the input configuration. A single-ended common point with a positive supply is referred to as SINK (sink current), whereas a single-ended common point with a negative supply is called SRCE (source current).
2. Key Terminology
- SINK (Sink Current): This refers to a configuration where current flows into the input terminal. It is typically used with NPN-type sensors and connects the input terminal to the negative side of the power supply.
- SOURCE (Source Current): In this configuration, current flows out of the input terminal. It is suitable for PNP-type sensors and connects the input terminal to the positive side of the power supply.
These terms can sometimes be confusing, especially since different manufacturers may use varying expressions. For instance:
- According to TI’s definition, sink current is the current drawn, while source current is the current supplied.
- Some systems refer to the polarity of the common point, distinguishing between a common positive and a common negative connection.
- SINK is often associated with NPN sensors, while SOURCE corresponds to PNP sensors.
- SINK may also be described as a "negative logic" connection, and SOURCE as "positive logic."
- Finally, some sources describe SINK as an active low-level signal and SOURCE as an active high-level signal.
Sensors such as proximity switches and photoelectric sensors often have three-wire or four-wire outputs, with NPN or PNP configurations. For non-detection states, NPN sensors output a high level, while PNP sensors output a low level. When a detection signal is present, the opposite occurs. However, this can vary depending on whether the sensor is normally open or normally closed. This can lead to confusion during setup if the user doesn't fully understand the sensor's output behavior and how it interacts with the PLC’s input type.
In some cases, users might connect a PNP sensor to a SINK input or an NPN sensor to a SOURCE input, and still see the PLC respond correctly. This is possible if the sensor has built-in pull-up or pull-down resistors, but it’s not always reliable. It depends on the specific PLC and sensor model, and it’s important to consult technical documentation to avoid compatibility issues.
III. Input Circuit Variations
1. DC Input Circuits
As shown in Figure 1, a DC input circuit requires the external signal to be a passive dry contact or a DC active switch. When the external component is activated, current flows through resistor R1, the optocoupler’s LED, and diode VD1 to the COM terminal. This activates the optocoupler, allowing the PLC to detect the input signal. The DC power can be provided internally by the PLC or externally. Resistor R2 helps prevent false triggering caused by static leakage currents from two-wire proximity switches.
2. AC Input Circuits
In an AC input circuit, the external signal must also be a passive dry contact or an AC active switch. The main difference from DC input is the inclusion of a step-down transformer, a bridge rectifier, and a filter capacitor before the optocoupler. This converts the AC signal into a DC signal that the PLC can process. AC input circuits are commonly used in environments with harsh conditions, where wiring changes are minimal. They are also useful for replacing traditional line switches with proximity switches.
IV. Single-Ended Common Point Configuration
In a single-ended common point configuration, all input circuits share a common terminal within the PLC. This reduces the number of required input terminals and simplifies wiring. For example, a PLC with N input points will require N+1 terminals (including the common point). Depending on the polarity of the common terminal, the configuration can be either SINK or SRCE.
If the common terminal is connected to the positive supply (24V+), it’s a SINK configuration, and the external common wire should be connected to the negative supply (24V-). This setup is ideal for NPN sensors. Conversely, if the common terminal is connected to the negative supply (24V-), it’s a SRCE configuration, and the external common wire should be connected to the positive supply (24V+), suitable for PNP sensors.
Some PLCs provide an S/S terminal instead of a fixed COM terminal, offering greater flexibility. This allows the user to choose whether to connect the common point to the positive or negative side of the power supply, making it easier to adapt to different regional standards and sensor types.
V. Conclusion
Understanding the various configurations of PLC input interfaces and the corresponding output signals from sensors is essential for successful system design and implementation. By carefully matching the input type with the sensor output, users can ensure accurate signal detection, reduce errors, and improve the overall performance and stability of the control system. Proper wiring and configuration lay the foundation for effective programming and long-term system reliability.
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