1
Overview
The digital input interface of a PLC is not complex. To enhance its anti-interference capability, PLCs employ optocouplers to isolate the input signal from the internal processing circuit. Therefore, the signal at the input terminal merely drives the internal LED of the optocoupler to conduct, which is then received by the photocell of the optocoupler, ensuring reliable transmission of the external input signal.
Currently, PLC digital input ports are generally classified into single-ended common-point and dual-ended inputs. Due to these differences, users need to have a certain understanding of the connection methods when selecting external sensors to ensure correct use and lay the foundation for future programming work and system stability.
2
Forms of Input Circuits
1. Classification of Input Types
PLC digital input terminals are classified into DC and AC based on power supply, and into single-ended common-point and dual-ended inputs based on input interface. When connected to the positive terminal of the power supply, the single-ended common-point is SINK (sink current), and when connected to the negative terminal of the power supply, it is SRCE (source current).
2. Overview of Terms
SINK leakage type involves current flowing out from the input terminal, indicating that the input terminal is connected to the negative terminal of the power supply. This means that the optocoupler inside the interface is set to single-ended common-point with the positive terminal of the power supply, allowing for the connection of NPN sensors.
SOURCE type involves current flowing into the input terminal, indicating that the input terminal is connected to the positive terminal of the power supply. This means that the optocoupler inside the interface is set to single-ended common-point with the negative terminal of the power supply, allowing for the connection of PNP sensors.
Proximity switches and photoelectric switches have three or four wire outputs, which are classified into NPN and PNP outputs. When there is no detection signal, the output of the NPN proximity switch and photoelectric switch is high level (for internal pull-up resistors), and when there is a detection signal, the internal NPN transistor conducts, and the switch output becomes low level.
For PNP proximity switches and photoelectric switches, the output is low level when there is no detection signal (for internal pull-down resistors), and when there is a detection signal, the internal PNP transistor conducts, and the switch output becomes high level.
The above situations are only for sensors in a normally open state.
3. According to the power supply configuration type
(1) DC input circuit
As shown in Figure 1, the DC input circuit requires the external input signal components to be passive dry contacts or DC-powered non-contact switch contacts. When the external input component is conducted to the positive terminal of the power supply, current flows through R1, the internal LED of the optocoupler, VD1 (interface indicator), and then to the COM terminal to form a loop. The internal receiving tube of the optocoupler receives the signal of the external component conduction and transmits it to the internal processing unit. This interface method, where DC power is used to provide power, is called a DC input circuit;
DC power can be provided internally by the PLC or externally by connecting an external DC power supply to the external input signal components. The role of R2 in the circuit is to bypass the current of the internal LED of the optocoupler, ensuring that the LED of the optocoupler is not conducted by the static leakage current of a two-wire proximity switch.
(2) AC input circuit
As shown in Figure 2, the AC input circuit requires the external input signal components to be passive dry contacts or AC-powered non-contact switch contacts. The difference from the DC interface lies in the addition of a step-down circuit and a bridge rectifier circuit before the optocoupler. After the external component is connected to the AC power, current flows through R1 and C2, undergoes bridge rectification, and becomes a stepped-down DC power. The principle of the subsequent circuit is the same as that of the DC circuit.
AC PLCs are mainly suitable for applications with relatively harsh environments and little change in wiring technology; for example, proximity switches can directly replace original travel switches with AC two-wire switches.
4. According to the port type
(1) Single-ended common-point (Comcon) digital input mode
To save input terminals, the structure of single-ended common-point input connects all input circuits (optocouplers) at one end together to an internal common terminal labeled COM, and the other end of each input circuit is connected to its corresponding input terminal X0, X1, X2, ...
With com common-point and N single-ended inputs, N digital inputs can be made (N+1 terminals), so we call this structure single-ended common-point input.
The SINK input mode can be connected to an NPN sensor, where the X port is connected to the negative terminal.
SRCE input mode, compatible with PNP sensors. Specifically, the X port is connected to the machine's common terminal. (External input components can be button switches, travel switches, reed switches, Hall switches, proximity switches, photoelectric switches, light curtain sensors, relay contacts, contactor contacts, and other switch-type components.)
(2) SINK (sink Current) input mode ● Single-ended common-point SINK input wiring (internal common terminal COM→24V+, external common line→24V-). As shown in Figure 3:
(3) SRCE (source Current) input mode
● Single-ended common-point SRCE input wiring (internal common terminal COM→24V-, external common line→24V+). As shown in Figure 4:
(4) Switchable SINK/SRCE input mode
The difference between the S/S terminal and the COM terminal is that COM is fixedly connected to the positive or negative terminal of the internal power supply, while the S/S terminal is non-fixedly connected and is only connected to 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 line→24V-).
● Single-ended common-point SRCE input wiring (internal common terminal S/S→24V-, external common line→24V+).
(5) When there are a large number of active input components (such as Hall switches, proximity switches, photoelectric switches, light curtain sensors, etc.) that consume a large amount of power and the built-in power supply of the PLC cannot meet the demand, an external power supply is required. Depending on the needs, a 24VDC switch-mode power supply with a certain power rating can be configured. In principle, the external power supply should not be connected in parallel with the built-in power supply. According to the characteristics of COM and the external common line, when in SINK (sink Current) input mode, the external power supply is connected to the positive terminal of the built-in power supply; when in SRCE (source Current) input mode, the external power supply is connected to the negative terminal of the built-in power supply.
(6) To easily determine the SINK (sink current) input mode, simply short-circuit the Xn terminal to the negative pole. If the interface indicator light is on, it indicates the SINK input mode. For optocouplers with a common positive pole, NPN sensors can be connected. For the SRCE (source current) input mode, short-circuit the Xn terminal to the positive pole. If the interface indicator light is on, it indicates the SRCE input mode. For optocouplers with a common negative pole, PNP sensors can be connected.
(7) For 2-wire switch inputs, if they are passive contacts, SINK and SRCE should be connected according to the input component connection method shown in the above diagram. For 2-wire proximity switches, it is necessary to determine the polarity of the proximity switch and connect it correctly.
(8) Ultra-high-speed dual-input circuit
It is primarily used for the input of the hardware high-speed counter (HHSC). The interface voltage is 5VDC. To ensure high speed and high noise immunity, a dual-line drive method (Line-Drive) is typically employed. If the operating frequency is not high and the noise level is low, a single-ended SINK or SRCE connection with 5VDC can be used, or a current-limiting resistor can be connected in series to convert it to a single-ended SINK or SRCE connection with 24VDC.
(9) Dual-input dual-line drive method (Line-Drive).
(10) Single-ended SINK or SRCE connection with 5VDC.
(11) Single-ended SINK or SRCE connection with 24VDC.
Note: For sensors powered by 24VDC, a current-limiting resistor is required in series on the input circuit. R1 is 10Ω, and R2 is 2KΩ. Without the current-limiting resistor, the interface circuit will be burned out. The value of the current-limiting resistor is 2.7KΩ.
3
External input components
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 factors such as power polarity or voltage drop to consider. The input components shown in Figure 3-6 are of this type. We will not repeat the introduction here.
2. Active two-wire sensors (proximity switches, active reed magnetic switches)
Active two-wire proximity switches are divided into DC and AC types. The characteristic of this sensor is that it has two wires. After the sensor output terminal is turned on, a holding voltage is required to maintain the circuit operation. Usually, the voltage drop is between 3.5-5V, and the static leakage current should be less than 1mA. This indicator is very important; if it is too large, the optocoupler on the PLC input terminal will be turned on when the proximity switch does not detect a signal.
DC two-wire proximity switches are divided into diode polarity protection and bridge rectifier polarity protection. When connecting to a PLC, attention should be paid to polarity for the former, while polarity does not need to be considered for the latter.
(1) Single-ended common-point SINK input wiring (internal common-point terminal COM→24V+, external common line→24V-) as shown in Figure 11
(2) Single-ended common-point SRCE input wiring (internal common-point terminal COM→24V-, external common line→24V+). As shown in Figure 12:
(3) Refer to Figures 5-6 and 11-12 for S/S terminal wiring.
3. Active three-wire sensors (inductive proximity switches, capacitive proximity switches, Hall proximity switches, photoelectric switches, etc.) use transistor output for DC active three-wire proximity switches and photoelectric switch output tubes, hence the sensors are divided into NPN and PNP outputs. Some products are four-wire systems, with dual NPN or dual PNP, only in opposite states, or a combination of NPN and PNP in a four-wire output.
When the sensor has a detection signal VT turned on, the current at the output terminal OUT flows to the negative pole, and the potential of the output terminal OUT approaches the negative pole, usually reversing the high level to a low level.
When the sensor has a detection signal VT turned on, the current at the positive pole flows to the output terminal OUT, and the potential of the output terminal OUT approaches the positive pole, usually reversing the low level to a 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 the output potential to facilitate the circuit of the level interface. Another type of output has the transistor collector open-circuited and does not connect to a pull-up or pull-down resistor.
Simply put, when the transistor VT is turned on, it is equivalent to a contact being turned on, as shown in Figure 13:
(1) Single-ended common-point SINK input wiring (internal common-point terminal COM→24V+, external common line→24V-). As shown in Figure 14:
(2) Single-ended common-point SRCE input wiring (internal common-point terminal COM→24V-, external common line→24V+).
(3) Refer to Figures 5-6, 11-12, and 14-15 for the wiring method of S/S terminals.
Due to the diversity in PLC input interface circuit forms and external component (sensor) output signal forms, it is essential to understand the PLC input circuit form and sensor output signal form before wiring the PLC input module. This ensures accurate wiring of the PLC input module, enabling proficiency in practical applications and laying a foundation for future programming work and system stability.
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