Overview

The digital input interface of a PLC is not complex. As we know, to enhance anti-interference capabilities, PLCs use optical isolators to isolate the input signals from the internal processing circuits. Therefore, the signal at the input end merely drives the internal LED of the optical isolator, which is then received by the phototransistor of the optical isolator, ensuring reliable transmission of external input signals.

Currently, PLC digital input ports are generally divided into single-ended common and double-ended inputs. Various manufacturers have single-ended common (Com) interfaces that differ between positive common and negative common. Japanese PLCs typically use positive common, while European PLCs prefer negative common. Japanese PLCs supplied to the European market also follow the European practice of using negative common. To allow flexible usage, single-ended common (S/S) options have been developed, which can connect to either negative or positive as needed. Due to these differences, users need to distinguish and understand the connections when selecting external sensors to correctly use them with the PLC, laying the foundation for subsequent programming work and system stability.

1. Input Circuit Types

1. Classification of Input Types

The digital input terminals of a PLC are classified by power supply into DC and AC. By input interface classification, they are divided into single-ended common input and double-ended input. Single-ended common connected to the positive power supply is called SINK (sink current), and single-ended common connected to the negative power supply is called SRCE (source current).

2. Explanation of Terms

SINK is a leakage type, while SOURCE is a source type.

SINK leakage type allows the current to flow out from the input terminal, thus it can be connected to the negative power supply, indicating that the internal optical isolator is single-ended common with the positive power supply, suitable for NPN type sensors.

SOURCE source type allows current to flow into the input terminal, thus it can be connected to the positive power supply, indicating that the internal optical isolator is single-ended common with the negative power supply, suitable for PNP type sensors. In China, these two methods are referred to in various expressions:

2.1 According to TI’s definition, sink current is pulling current, and source current is sinking current.

2.2 By the polarity of the single-ended common interface, it can be classified into positive common and negative common. This expression is easier to distinguish.

2.3 SINK refers to NPN connection, while SOURCE refers to PNP connection (according to the output form of the sensor).

2.4 SINK refers to negative logic connection, while SOURCE refers to positive logic connection (according to the output form of the sensor).

2.5 SINK refers to the sensor’s low level being effective, while SOURCE refers to the sensor’s high level being effective (according to the output state of the sensor).

This expression is the most encountered by the author and is also the most likely to cause confusion. Proximity switches and photoelectric switches with three or four-wire outputs are divided into NPN and PNP outputs. For NPN proximity switches and photoelectric switches without detection signals, the output is high level (considering internal pull-up resistors). When there is a detection signal, the internal NPN transistor conducts, and the switch output is low level.

For PNP proximity switches and photoelectric switches without detection signals, the output is low level (considering internal pull-down resistors). When there is a detection signal, the internal PNP transistor conducts, and the switch output is high level.

The above situations only apply when the sensor is in a normally open state. Currently, sensors produced by various manufacturers can be classified as normally open or normally closed; normally closed NPN outputs are low level, and normally closed PNP outputs are high level. Therefore, users often encounter discrepancies in selection and cooperation with suppliers.

Another situation is that users may encounter SINK connections with PNP type sensors and SOURCE connections with NPN type sensors, which can still drive PLC interfaces, while the status of PLC input signals can be modified by the PLC program. This is due to the presence of pull-up and pull-down resistors in the sensor output. For open-collector sensors, such connections are ineffective; moreover, the values of pull-up and pull-down resistors are significantly related to the leakage current parameters of the PLC interface. Not all sensors and PLCs can be used interchangeably.

3. Power Configuration Types

3.1 DC Input Circuit

As shown in Figure 1, the DC input circuit requires that the external input signal element be a passive dry contact or a DC active non-contact switch. When the external input element connects to the positive power supply, current flows through R1, the LED inside the optical isolator, VD1 (interface indicator) to the COM terminal, forming a loop. The internal receiving tube of the optical isolator receives the signal from the external element, transmitting it to the internal processing; this interface method powered by DC is called a DC input circuit. The DC power can be provided internally by the PLC or externally sourced for the external input signal element. R2 in the circuit serves to bypass the current of the optical isolator’s internal LED, ensuring that the LED is not activated by the static leakage current of two-wire proximity switches.

3.2 AC Input Circuit

As shown in Figure 2, the AC input circuit requires that the external input signal element be a passive dry contact or an AC active non-contact switch. The distinction from the DC interface is that a step-down circuit and a bridge rectification circuit are added before the optical isolator. After the external element connects to the AC power, current flows through R1, C2, and undergoes bridge rectification, transforming into reduced DC. The principle of the subsequent circuit is consistent with that of DC. AC PLCs are mainly suitable for relatively harsh environments with minimal wiring changes, such as using AC two-wire switches to directly replace original travel switches.

4. By Port Type

4.1 Single-Ended Common (Comcon) Digital Input Method

To save input terminals, the structure of single-ended common input connects one end of all input circuits (optical isolators) inside the PLC to an internal common terminal marked as COM. The other end of each input circuit connects to its corresponding input terminals X0, X1, X2, …, allowing N single-ended inputs to be made with N+1 terminals. Therefore, we refer to this structure as “single-ended common” input. Users must also connect one end of all external digital input components together, referred to as the external common wire; the other end connects to the PLC input terminals X0, X1, X2, ….

If COM is the power supply 24V+ (positive), the external common wire must connect to 24V- (negative), this connection method is called SINK (sink current) input; it is also referred to as PLC interface common positive.

If COM is the power supply 24V- (negative), the external common wire must connect to 24V+ (positive), this connection method is called SRCE (source current) input; it is also referred to as PLC interface common negative.

SINK (sink current) input method can connect to NPN type sensors, with the X port connected to the negative. SRCE (source current) input method can connect to PNP type sensors, with the X port connected to the positive. To accommodate usage habits in various regions, some manufacturers’ PLCs use S/S terminals, which can connect to either the positive or negative 24V power supply, allowing the PLC to use SINK (sink current) input method for NPN type sensors and SRCE (source current) input method for PNP type sensors. This is more flexible than PLCs using COM terminals. The development of S/S terminals is to suit the mixed use of Japanese and European PLCs in industrial control scenarios, serving a universal purpose. S/S terminals are also known as SINK/SRCE switchable types. (External input components can include button switches, travel switches, reed switches, Hall switches, proximity switches, photoelectric switches, light curtain sensors, relay contacts, and contactor contacts.)

SINK (sink current) Input Method

● Single-ended common SINK input wiring (internal common terminal COM→24V+, external common wire→24V-). As shown in Figure 3:

SRCE (source current) Input Method

● Single-ended common SRCE input wiring (internal common terminal COM→24V-, external common wire→24V+). As shown in Figure 4:

SINK/SRCE Switchable Input Method

The difference between the S/S terminal and the COM terminal is that COM is fixedly connected to the internal power supply positive or negative, while the S/S terminal is not fixedly connected and is connected to either the internal or external power supply positive or negative as needed.

● Single-ended common SINK input wiring (internal common terminal S/S→24V+, external common wire→24V-).

● Single-ended common SRCE input wiring (internal common terminal S/S→24V-, external common wire→24V+).

4.1.4 When Active Input Elements (Hall switches, proximity switches, photoelectric switches, light curtain sensors, etc.) Are Numerous and Consume More Power, External Power Supply Needs to Be Configured. Depending on the Requirement, a 24VDC Switching Power Supply of Certain Power Can Be Configured. The External Power Supply Should Not Be Connected in Parallel with the Internal Power Supply. Based on the Characteristics of COM and External Common Wiring, When Using SINK (sink current) Input Method, the External Power Supply Connects to the Internal Power Supply Positive; When Using SRCE (source current) Input Method, the External Power Supply Connects to the Internal Power Supply Negative.

4.1.5 A Simple Way to Determine SINK (sink current) Input Method is to Short Circuit the Xn Terminal to the Negative. If the Interface Indicator Lights Up, It Indicates SINK Input Method. The Common Positive Optical Isolator Can Connect to NPN Type Sensors. For SRCE (source current) Input Method, If You Short Circuit the Xn Terminal to the Positive and the Interface Indicator Lights Up, It Indicates SRCE Input Method. The Common Negative Optical Isolator Can Connect to PNP Type Sensors.

4.1.6 For Two-Wire Switch Inputs, If They Are Passive Contacts, SINK and SRCE Are Connected According to the Input Element Wiring in the Above Diagram. For Two-Wire Proximity Switches, the Polarity of the Proximity Switch Needs to Be Determined for Correct Connection.

4.2 High-Speed Dual-Ended Input Circuit

This is mainly used for hardware high-speed counters (HHSC) input, with an interface voltage of 5VDC. To ensure high speed and high noise resistance, a dual-line drive method (Line-Drive) is usually employed. If the operating frequency is not high and the noise is low, a 5VDC single-ended SINK or SRCE connection can also be used, with a series current-limiting resistor converted to a 24VDC single-ended SINK or SRCE connection.

4.2.1 Dual Input Dual-Line Drive Method (Line-Drive)

5VDC single-ended SINK or SRCE connection method.

24VDC single-ended SINK or SRCE connection method.

Note: For Sensors Powered by 24VDC, a Current-Limiting Resistor Needs to Be Connected in Series in the Input Circuit. R1 Should Be 10Ω, R2 Should Be 2KΩ. If No Current-Limiting Resistor Is Used, the Interface Circuit Will Be Burned Out. The Value of the Current-Limiting Resistor Should Be 2.7KΩ.

5. External Input Elements

1. Passive Dry Contacts (Button Switches, Travel Switches, Reed Magnetic Switches, Relay Contacts, etc.)

Passive dry contacts are relatively simple and easy to wire. There are no issues with power supply polarity or voltage drop factors. The input elements shown in Figures 3-6 are of this type. Here, we will not repeat the introduction.

2. Active Two-Wire Sensors (Proximity Switches, Active Reed Magnetic Switches)

Active two-wire proximity switches are divided into DC and AC. The characteristic of this sensor is that it has two wires. After the output terminal of the sensor is activated, a holding voltage is required to maintain circuit operation, usually between 3.5-5V, with a static leakage current of less than 1mA, which is a critical metric; if it is too high, the optical isolator at the PLC input will be activated without detection signals. Our company’s LJK series two-wire proximity switches control static leakage current between 0.35-0.5mA to adapt to various types of PLCs.

DC two-wire proximity switches are divided into diode polarity protection and bridge rectification polarity protection. The former requires attention to polarity when connecting to the PLC, while the latter does not. Active reed magnetic switches are mainly used for position detection on cylinders and also do not require attention to polarity due to the internal bidirectional diode circuit; AC two-wire proximity switches do not require attention to polarity either.

Single-ended common SINK input wiring (internal common terminal COM→24V+, external common wire→24V-). As shown in Figure 11:

Single-ended common SRCE input wiring (internal common terminal COM→24V-, external common wire→24V+). As shown in Figure 12:

S/S terminal wiring can refer to Figures 5-6 and Figures 11-12.

3. Active Three-Wire Sensors (Inductive Proximity Switches, Capacitive Proximity Switches, Hall Proximity Switches, Photoelectric Switches, etc.)

DC active three-wire proximity switches and photoelectric switches use transistors for output, thus sensors are divided into NPN and PNP outputs. Some products are four-wire, with dual NPN or dual PNP outputs, just in opposite states, and there are also combinations of NPN and PNP for four-wire outputs.

NPN type sensors output current flowing towards the negative when there is a detection signal VT conducting, with the output potential of OUT close to the negative, usually referred to as high level flipping to low level.

PNP type sensors output current flowing towards the output terminal OUT when there is a detection signal VT conducting, with the output potential of OUT close to the positive, usually referred to as low level flipping to high level. The resistor at the emitter of the transistor in the circuit is a short-circuit protection sampling resistor of 2-3Ω that does not affect output current. The resistors at the collector of the transistor are pull-up and pull-down resistors that provide output potential, facilitating the circuit of level interfaces. Another type of output with the transistor’s collector as open-collector does not connect to pull-up and pull-down resistors.

Simply put, when the transistor VT conducts, it is equivalent to a contact conducting, as shown in Figure 13:

Single-ended common SINK input wiring (internal common terminal COM→24V+, external common wire→24V-). As shown in Figure 14:

Single-ended common SRCE input wiring (internal common terminal COM→24V-, external common wire→24V+). As shown in Figure 15:

S/S terminal wiring can refer to Figures 5-6, Figures 11-12, and Figures 14-15.

6. Conclusion

The diversity of PLC input interface circuit forms and the output signal forms of external components (sensors) make it necessary to understand the PLC input circuit forms and sensor output signal forms before wiring the PLC input module to ensure correct wiring. This will facilitate practical application and lay the foundation for subsequent programming work and system stability.

Source: Technical Training Compilation

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