"Zhongtai PLC Automation Teaching" brings you the wiring method of sensors and PLC. Twenty wiring diagrams, aren't they super rich? Come and see it together! Special thanks go to "Zhongtai PLC Automation Teaching".

1. Overview 01

The digital input interface of PLC is not complicated. In order to improve the anti-interference ability, the input interface of PLC uses photoelectric couplers to isolate the transmission of input signals and internal processing circuits. Therefore, the signal at the input end only drives the internal LED of the optocoupler to turn on, and is received by the phototube of the optocoupler, so that the external input signal can be transmitted reliably.

At present, PLC digital input ports are generally divided into single-ended common-point input and double-ended input. Due to the difference, users need to distinguish and understand the connection method when selecting external sensors to correctly use the sensor and PLC for the later period. Lays the foundation for programming work and system stability.

2. Input circuit form 02

1. Classification of input types

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

2. Overview of words

SINK drain type means that the current flows out from the input terminal, then the input terminal can be connected to the negative pole of the power supply, indicating that the photoelectric coupler inside the interface is single-ended and the common point is the power supply Positive electrode, can be connected to NPN sensor.

SOURCE source type means that the current flows from the input end, then the input end can be connected to the positive pole of the power supply, which means that the photocoupler inside the interface is single-ended and the common point is the negative pole of the power supply, and can be connected to a PNP sensor.

The three- and four-wire outputs of proximity switches and photoelectric switches are divided into NPN and PNP outputs. When there is no detection signal, the output of NPN proximity switches and photoelectric switches is high level (for internal pull-up resistors) , when there is a detection signal, the internal NPN tube is turned on, and the switch output is low level.

When there is no detection signal, the output of the PNP proximity switch and photoelectric switch is low level (for internal pull-down resistors). When there is a detection signal, the internal PNP tube is turned on, and the switch output is high level. flat.

The above situation is only for when the sensor is normally open.

3. According to 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 contact or DC active non-contact switch contact. When the external input component is connected to the positive pole of the power supply, the current passes through R1, the internal LED of the photocoupler, VD1 (interface indication) to the COM terminal to form a loop, and the internal receiving tube of the photocoupler It accepts signals from external components and transmits them to internal processing; this interface method that provides power from DC is called a DC input circuit;

DC power can be provided internally by the PLC or can be provided by an external DC power supply for external input. Signal components. The function of R2 in the circuit is to bypass the current of the LED inside the photocoupler to ensure that the photocoupler 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 the external input signal components to be passive dry contacts or AC active non-contact switch contacts, which are connected to the DC The interface is distinguished by adding a first-level buck circuit and a bridge rectifier circuit before the photocoupler. After the external component is connected to the alternating current, the current passes through R1 and C2 and is rectified by the bridge to become a stepped-down direct current. The principle of the subsequent circuit is consistent with that of direct current.

AC PLC is mainly suitable for situations where the environment is relatively harsh and there is little change in wiring technology; for example, the proximity switch uses two AC wires to directly replace the original travel switch.

4. According to port type

(1) Single-ended common point (Comcon) digital input method

In order to save input terminals, the structure of single-ended common point input is Inside the PLC, connect one end of all input circuits (photocouplers) together to the internal common terminal marked COM, and the other end of each input circuit is connected to its corresponding input terminals X0, X1, X2,... .

com common point and N single-ended inputs can make N digital inputs (N+1 terminals), so we call this structure "single-ended common point" input. Users also need to do the same when wiring external digital input components. One end of all input components needs to be connected together, which is called the external common line of the input components; the other end of the input components is connected to the input terminal X0 of the PLC. , X1,

SRCE input mode, can be connected to PNP type sensor. That is, the X port is connected to the whole machine. (External input components can be push button switches, travel switches, reed switches, Hall switches, proximity switches, photoelectric switches, light curtain sensors, relay contacts, contactor electric shock and other switching 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 sink) input mode

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

(4) SINK/SRCE switchable input mode

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

● Single-ended common-point SINK input wiring (internal common-point 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 (Hall switches, proximity switches, photoelectric switches, light curtain sensors, etc.) and the power consumption is relatively large, and the PLC built-in power supply cannot meet the requirements, an external power supply needs to be configured. It can be equipped with 24VDC and a certain power switching power supply according to needs. In principle, the external power supply cannot be connected in parallel with the built-in power supply. According to the characteristics of COM and external collinearity, in the SINK (sink Current) input mode, the external power supply is connected to the positive pole of the built-in power supply; in the SRCE (source Current sink) input mode , the external power supply is connected to the negative pole of the built-in power supply.

(6) To simply judge the SINK (sink Current) input mode, you only need to short-circuit the Xn terminal and the negative pole. If the interface indicator light is on, it means the SINK input mode. The photocoupler with common positive pole can be connected to NPN type sensor. SRCE (source current sink) input mode, short-circuit the Xn terminal and the positive pole. If the interface indicator light is on, it means the SRCE input mode. A photocoupler with a common negative pole can be connected to a PNP sensor.

(7) For 2-wire switch input, if it is a passive contact, SINK and SRCE should be connected according to the input component connection method in the figure above. For 2-wire proximity switch, it is necessary to determine the proximity switch polarity, connect correctly.

(8) Ultra-high-speed double-ended input circuit

Mainly used for input of hardware high-speed counter (HHSC). The interface voltage is 5VDC. In application, it is used to ensure high speed and high noise. Resistance usually adopts dual-line drive method (Line-Drive). If the operating frequency is not high and the noise is low, you can also use a 5VDC single-ended SINK or SRCE connection, and connect a current-limiting resistor in series to convert it to a 24VDC single-ended SINK or SRCE connection.

(9) Dual-input dual-line drive mode (Line-Drive).

(10) Single-ended SINK or SRCE connection of 5VDC.

(11) Single-ended SINK or SRCE connection of 24VDC.

Note: For sensors powered by 24VDC, a current-limiting resistor needs to be connected in series on the input loop. R1 is 10Ω and R2 is 2KΩ. If the current-limiting resistor is not connected in series, the interface circuit will be burned. The current-limiting resistor value is 2.7KΩ.

3. External input components 03

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 supply polarity, voltage drop, etc. The input components in Figure 3-6 above are exactly this type. The introduction will not be repeated here.

2. Active two-wire sensor (proximity switch, active reed magnetic switch)

Active two-wire proximity switch is divided into DC and AC. The characteristics of this sensor are two After the output end of the transmitter is turned on, in order to ensure the normal operation of the circuit, a holding voltage is needed to maintain the circuit operation. Usually the voltage drop is 3.5-5V, and the static leakage current should be less than 1mA. This indicator is very important; if it is too large, When the proximity switch does not detect a signal, the photocoupler at the input end of the PLC is turned on.

DC two-wire proximity switch is divided into diode polarity protection and bridge rectifier polarity protection. The former needs to pay attention to the polarity when connecting to the PLC, while the latter does not need to pay attention to the polarity. Active reed magnetic switches are mainly used for position detection on cylinders. Since signal indication is required, there is a bidirectional diode circuit inside, so there is no need to pay attention to polarity; AC two-wire proximity switches do not need to pay attention to polarity. As shown in Figure 10:

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

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

(3) For the S/S terminal connection method, refer to Figure 5-Figure 6 and Figure 11-Figure 12

3. Active three-wire sensor (inductive proximity switch, capacitive proximity switch , Hall proximity switch, photoelectric switch, etc.) DC active three-wire proximity switch and photoelectric switch output tube use triode output, so the sensor is divided into NPN and PNP output. Some products are four-wire system, with double NPN or double PNP. It's just that the status is just the opposite. There is also a four-wire output that combines NPN and PNP.

NPN type When the sensor detects the signal VT and is turned on, the current of the output terminal OUT flows to the negative electrode, the potential of the output terminal OUT is close to the negative electrode, and the usually high level flips to a low level.

PNP type When the sensor detects the signal VT and is turned on, the positive current flows to the output terminal OUT, and the potential of the output terminal OUT is close to the positive terminal, and the usually low level flips to a high level.

The resistance on the emitter of the transistor in the circuit is a short-circuit protection sampling resistor of 2-3Ω and does not affect the output current. The resistance of the collector of the transistor is a pull-up and pull-down resistor, which provides the output potential and facilitates the level interface circuit. The open collector output of the other type of output transistor is not connected to the pull-up and pull-down resistors.

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 terminal COM→24V-, external common line→24V+). As shown in Figure 15:

(3) For S/S terminal connection method, refer to Figure 5-Figure 6, Figure 11-Figure 12 and Figure 14-Figure 15

PLC input interface circuit form and external components (Sensor) output signal forms are diverse. Therefore, before wiring the PLC input module, it is necessary to understand the PLC input circuit form and the sensor output signal form to ensure that the PLC input module wiring is correct and can be used with ease in practical applications. Later programming work and system stability.