1. Overview
The digital input interface of a PLC is not complex. To enhance anti-interference capability, PLCs use optocouplers to isolate the input signals from the internal processing circuits. Therefore, the signal at the input end only needs to drive the internal LED of the optocoupler to turn on, and the phototransistor of the optocoupler receives it, allowing reliable transmission of external input signals.
Currently, PLC digital input ports are generally divided into single-ended common point and double-ended inputs. Due to these differences, users need to understand the wiring methods when selecting external sensors to ensure correct usage, which lays the foundation for subsequent programming and system stability.
2. Types of Input Circuits
1. Classification of Input Types
The digital input terminals of the PLC are classified by power supply into DC and AC. According to the input interface classification, there are single-ended common point inputs and double-ended inputs. A single-ended common point connected to the positive terminal of the power supply is called SINK (sink current), while a single-ended common point connected to the negative terminal of the power supply is called SOURCE (source current).
2. Overview of Terms
The SINK type allows current to flow out from the input terminal, so the input terminal should be connected to the negative terminal of the power supply, indicating that the internal optocoupler is a single-ended common point connected to the positive terminal of the power supply, suitable for NPN sensors.
The SOURCE type allows current to flow into the input terminal, so the input terminal should be connected to the positive terminal of the power supply, indicating that the internal optocoupler is a single-ended common point connected to the negative terminal of the power supply, suitable for PNP sensors.
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, when there is no detection signal, the output is high level (considering the internal pull-up resistor). 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, when there is no detection signal, the output is low level (considering the internal pull-down resistor). When there is a detection signal, the internal PNP transistor conducts, and the switch output is high level.
The above situations are only applicable when the sensors are in a normally open state.
3. By Power Supply Configuration Type
(1) DC Input Circuit
As shown in Figure 1, the DC input circuit requires that the external input signal component be a passive dry contact or a DC active non-contact switch. When the external input component is connected to the positive terminal of the power supply, current flows through R1, the internal LED of the optocoupler, VD1 (interface indicator), to the COM terminal, forming a loop. The internal receiving tube of the optocoupler receives the signal from the external component, transmitting it to the internal processing; this interface powered by DC is called a DC input circuit.
DC power can be provided by the PLC internally or by an external DC power supply for the external input signal components. R2 in the circuit serves to bypass the current of the optocoupler’s internal LED, ensuring that the LED is not turned on by the static leakage current of the two-wire proximity switch.
(2) AC Input Circuit
As shown in Figure 2, the AC input circuit requires that the external input signal component 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 rectifier circuit are added before the optocoupler. After the external component is connected to the AC power, current flows through R1, C2, and the bridge rectifier, converting it into a step-down DC. The subsequent circuit operates on the same principle as the DC circuit.
AC PLCs are mainly suitable for relatively harsh environments, where wiring techniques do not change much; for example, proximity switches can directly replace the original limit switches with AC two-wire connections.
4. By Port Type
(1) Single-ended Common Point (Comcon) Digital Input Method
To save input terminals, the structure of single-ended common point input connects one end of all input circuits (optocouplers) inside the PLC to a common internal terminal marked COM, while the other end of each input circuit connects to its corresponding input terminals X0, X1, X2, …
Common points with N single-ended inputs can create N digital inputs (N+1 terminals), hence this structure is referred to as “single-ended common point” input. When wiring external digital input components, users should also follow the same practice, connecting one end of all input components together, referred to as external common wiring; the other end connects to the PLC input terminals X0, X1, X2, …
The SINK input method can connect to NPN sensors, meaning the X terminal is connected to the negative terminal.
The SOURCE input method can connect to PNP sensors, meaning the X terminal is connected to the positive terminal.
(External input components can include button switches, limit switches, reed switches, Hall switches, proximity switches, photoelectric switches, light curtain sensors, relay contacts, contactor contacts, and other switch elements.)
(2) SINK (sink current) Input Method ● Single-ended Common Point SINK Input Wiring (Internal Common Terminal COM→24V+, External Common Wiring→24V-). As shown in Figure 3:
(3) SOURCE (source current) Input Method
● Single-ended Common Point SOURCE Input Wiring (Internal Common Terminal COM→24V-, External Common Wiring→24V+). As shown in Figure 4:
(4) SINK/SOURCE Switchable Input Method
The S/S terminal differs from the COM terminal in that COM is fixedly connected to the internal power supply’s positive or negative terminal, while the S/S terminal is not fixedly connected and can be connected to either the positive or negative terminal of the internal or external power supply as needed.
● Single-ended Common Point SINK Input Wiring (Internal Common Terminal S/S→24V+, External Common Wiring→24V-).
● Single-ended Common Point SOURCE Input Wiring (Internal Common Terminal S/S→24V-, External Common Wiring→24V+).
(5) When there are many active input components (Hall switches, proximity switches, photoelectric switches, light curtain sensors, etc.) with high power consumption, and the built-in power supply of the PLC cannot meet the demand, an external power supply needs to be configured. Depending on the requirements, a 24VDC power supply of a certain power rating can be used. The external power supply should not be connected in parallel with the internal power supply. According to the characteristics of COM and external common wiring, in the SINK (sink current) input method, the external power supply is connected to the positive terminal of the internal power supply; in the SOURCE (source current) input method, the external power supply is connected to the negative terminal of the internal power supply.
(6) A simple way to determine the SINK (sink current) input method is to short the Xn terminal to the negative terminal. If the interface indicator light is on, it indicates that it is the SINK input method. The common positive optocoupler can connect to NPN sensors. For the SOURCE (source current) input method, short the Xn terminal to the positive terminal. If the interface indicator light is on, it indicates that it is the SOURCE input method. The common negative optocoupler can connect to PNP sensors.
(7) For two-wire switch inputs, if they are passive contacts, the SINK and SOURCE connections follow the input component wiring shown above. For two-wire proximity switches, it is necessary to determine the polarity of the proximity switch for correct connection. Some of our company’s two-wire LJK series proximity switches can also be connected without polarity; please refer to the accompanying product manual for specifics.
(8) 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 immunity, a dual-line drive method (Line-Drive) is usually adopted. If the working frequency is not high and the noise is low, a 5VDC single-ended SINK or SOURCE connection can be used, with a current-limiting resistor in series to convert it into a 24VDC single-ended SINK or SOURCE connection.
(9) Dual Input Dual-line Drive Method (Line-Drive).
(10) 5VDC Single-ended SINK or SOURCE Connection.
(11) 24VDC Single-ended SINK or SOURCE Connection.
Note: For sensors powered by 24VDC, a current-limiting resistor must be connected in series on the input circuit. R1 should be 10Ω, and R2 should be 2KΩ. If a current-limiting resistor is not connected, it will burn out the interface circuit. The value of the current-limiting resistor should be 2.7KΩ.
3. External Input Components
1. Passive Dry Contacts (Button Switches, Limit Switches, Reed 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, as shown in the input components in Figures 3-6. This will not be repeated here.
2. Active Two-wire Sensors (Proximity Switches, Active Reed Switches)
Active two-wire proximity switches are divided into DC and AC. The characteristic of this sensor is that it has two wires, and when the output terminal is conductive, a holding voltage is required to maintain the circuit’s operation, usually with a voltage drop of 3.5-5V, and the static leakage current must be less than 1mA. This specification is very important; if it is too high, the optocoupler at the PLC input terminal will conduct even when there is no detection signal. Our company’s LJK series two-wire proximity switches control the static leakage current between 0.35-0.5mA, making them suitable for various types of PLCs.
DC two-wire proximity switches are divided into diode polarity protection and bridge rectifier polarity protection. The former requires attention to polarity when connecting to the PLC, while the latter does not. Active reed switches are mainly used for position detection on cylinders and do not require attention to polarity due to the internal bidirectional diode circuit; AC two-wire proximity switches also do not require attention to polarity. As shown in Figure 10:
(1) Single-ended Common Point SINK Input Wiring (Internal Common Terminal COM→24V+, External Common Wiring→24V-). As shown in Figure 11
(2) Single-ended Common Point SOURCE Input Wiring (Internal Common Terminal COM→24V-, External Common Wiring→24V+). As shown in Figure 12:
(3) S/S Terminal Wiring Reference Figures 5-6 and Figures 11-12.
3. Active Three-wire Sensors (Inductive Proximity Switches, Capacitive Proximity Switches, Hall Proximity Switches, Photoelectric Switches, etc.) Active three-wire proximity switches and photoelectric switches use transistors for output, thus the sensors are divided into NPN and PNP outputs. Some products are four-wire with dual NPN or dual PNP outputs, which are just opposite states, and there are also NPN and PNP combined four-wire outputs.
For NPN type, when the sensor has a detection signal VT conducting, the output terminal OUT current flows to the negative terminal, and the output terminal OUT potential approaches the negative terminal, usually referred to as high level flipping to low level.
For PNP type, when the sensor has a detection signal VT conducting, the positive current flows to the output terminal OUT, and the output terminal OUT potential approaches the positive terminal, usually referred to as low level flipping to high level.
The resistor on the emitter of the transistor in the circuit is a short-circuit protection sampling resistor of 2-3Ω, which does not affect the output current. The resistor on the collector of the transistor is a pull-up and pull-down resistor, providing output potential, facilitating the level interface circuit. Another type of output from the transistor’s collector is open-collector output without pull-up and pull-down resistors.
In simple terms, when the transistor VT conducts, it is equivalent to a contact conducting, as shown in Figure 13:
(1) Single-ended Common Point SINK Input Wiring (Internal Common Terminal COM→24V+, External Common Wiring→24V-). As shown in Figure 14:
(2) Single-ended Common Point SOURCE Input Wiring (Internal Common Terminal COM→24V-, External Common Wiring→24V+). As shown in Figure 15:
(3) S/S Terminal Wiring Reference Figures 5-6, Figures 11-12, and Figures 14-15.
Due to the diversity of PLC input interface circuit forms and the output signal forms of external components (sensors), it is essential to understand the PLC input circuit forms and the sensor output signal forms before wiring the PLC input module to ensure correct wiring. This knowledge will facilitate practical applications and lay the foundation for subsequent programming and system stability.
