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Sensor and PLC wiring

Last Updated on 2019-09-11 Hits:3645
The digital input interface of the PLC is not complicated. In order to improve the anti-interference ability of the PLC, the input interface uses an optocoupler to isolate the transmission of the input signal and the internal processing circuit. Therefore, the signal at the input only drives the internal LED of the optocoupler to be turned on, and is received by the photocoupler of the photocoupler, so that the external input signal can be reliably transmitted.


 


At present, the PLC digital input port is generally divided into single-ended common point and double-ended input. Because of the difference, the user needs certain distinction and understanding when selecting the external sensor to use the sensor and PLC correctly for the later programming work and system. Stabilize the foundation.
Input circuit form


1, the classification of the input type


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


 


2. Overview of the words


The SINK leakage type is the current flowing from the input terminal, then the input terminal is connected to the negative pole of the power supply, indicating that the photocoupler inside the interface is a single-ended common point for the positive pole of the power supply, and can be connected with the NPN type sensor.


The SOURCE source type is the current flowing from the input terminal, then the input terminal is connected to the positive pole of the power supply, indicating that the photocoupler inside the interface is a single-ended common point as the negative pole of the power supply, and can be connected to the PNP type sensor.


The proximity switch and the photoelectric switch have three- and four-wire output points of NPN and PNP output. For the non-detection signal, the NPN proximity switch and the photoelectric switch output are at a high level (for the internal pull-up resistor), when there is a detection signal, the internal The NPN transistor is turned on and the switch output is low.


For the non-detection signal, the PNP proximity switch and the photoelectric switch output are low (for the internal pull-down resistor), when there is a detection signal, the internal PNP transistor is turned on, and the switch output is high.


 


The above situation is only for the sensor, which is in a normally open state.


3, according to the power configuration type


(1) DC input circuit


The DC input circuit requires the external input signal component to be a passive dry contact or a DC active contactless switch contact. When the external input component is connected to the positive pole of the power supply, the current passes through R1, the internal coupler of the optocoupler, VD1 (interface indication ) forming a loop to the COM end, the internal receiving tube of the optocoupler receives the signal of the external component conduction, and transmits it to the internal processing; the interface mode for supplying power by the DC power is called a DC input circuit;


The direct current can be supplied from the PLC or an external DC power supply to the external input signal. The role of R2 in the circuit is to bypass the current of the LED inside the optocoupler, ensuring that the optocoupler 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 component to be a passive dry contact or an AC active contactless switch contact. The distinction between the DC input interface and the DC interface is preceded by a step-down circuit and bridge rectification in the optocoupler. Circuit. After the external components are connected to the AC power, the current is rectified through the bridge through R1 and C2, and becomes the DC current after the step-down. The principle of the subsequent circuit is consistent with the DC.


The AC PLC is mainly suitable for occasions where the relative environment is bad, and the wiring technology changes little. If the proximity switch is used, the AC line is directly replaced by the original line switch.


 


4, by port type


(1) Single-ended Common Point (Comcon) digital input method


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


Com can make N digital inputs (N+1 terminals) with N single-ended inputs, so we call this structure a "single-ended common point" input. The user also needs the same method when wiring the external digital input component. It is necessary to connect one end of all input components together, called the external collinearity of the input component; the other end of the input component is connected to the input terminal X0 of the PLC. , X1, X2, ....


The SINK input mode can be connected to the NPN type sensor, that is, the X port is connected to the negative pole.


SRCE input mode, can be connected to PNP type sensor. That is, the X port is connected to the entire pole.


(The external input component can be a switch component such as a push button switch, a travel switch, a reed switch, a Hall switch, a proximity switch, a photoelectric switch, a light curtain sensor, a relay contact, and a contactor electric shock.)


(2) SINK (sink Current) input method ● Single-ended common point SINK input wiring (internal common terminal COM→24V+, external collinear →24V-).


 


(3) SRCE (source current) input method


 


● Single-ended common point SRCE input wiring (internal common terminal COM→24V-, external collinear→24V+).


 


(4) SINK/SRCE switchable input method


 


The S/S terminal is different from the COM terminal. COM is fixedly connected to the positive or negative terminal of the internal power supply. The S/S terminal is not fixedly connected. It is connected to the positive or negative terminal 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 collinear → 24V-).


 


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


 


(5) When the number of active input components (Hall switch, proximity switch, photoelectric switch, light curtain sensor, etc.) is relatively large, the power consumption is relatively large, and the PLC built-in power supply cannot be satisfied, the external power supply needs to be configured. According to the demand, it can be equipped with 24VDC, a certain power switching power supply. 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, when the SINK (sink Current) input mode is used, the external power supply is connected to the positive terminal of the built-in power supply; SRCE (source current) input mode The external power supply is connected to the negative pole of the built-in power supply.


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


(7) For the 2-wire digital input, if it is a passive contact, SINK and SRCE are connected according to the input component of the above figure. For the 2-wire proximity switch, it is necessary to judge the polarity of the proximity switch and correctly access it. . Some of the 2-wire LJK series proximity switches of our company can also be connected to the interface without polarity. For details, please refer to the attached product manual.


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


It is mainly used for the input of hardware high-speed counter (HHSC). The interface voltage is 5VDC. In order to ensure high speed and high noise resistance, the line-drive method is usually adopted. If the operating frequency is not high and the noise is low, 5VDC single-ended SINK or SRCE connection can be used, and a current limiting resistor in series can be converted into a 24VDC single-ended SINK or SRCE connection.


(9), double-input two-line drive (Line-Drive).


 


(10), 5VDC single-ended SINK or SRCE connection.


 


(11), 24VDC single-ended SINK or SRCE connection.


 


The 24VDC-powered sensor requires a series current limiting resistor on the input circuit. R1 is 10Ω and R2 is 2KΩ. If the current limiting resistor is not connected in series, the interface circuit will be burned and the current limiting resistor will be 2.7KΩ.


External input component


1, passive dry contact (button switch, travel switch, reed magnetic switch, relay contacts, etc.)


Passive dry contacts are relatively simple and easy to wire. There are no factors such as the polarity of the power supply, voltage drop, etc., which is the input component in Figure 3-6 above. This is not repeated here.


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


The active two-wire proximity switch is divided into DC and AC. The characteristic of this sensor is two wires. After the output 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 at a voltage drop of 3.5-5V. The static leakage current is less than 1 mA. This indicator is very important. If it is too large, the photocoupler at the input of the PLC is turned on when the proximity switch does not detect the signal. Our company's LJK series two-wire proximity switch static leakage current control is between 0.35-0.5mA to adapt to various types of PLC.


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


 


(1) Single-ended common point SINK input wiring (internal common terminal COM→24V+, external collinear→24V-).


 


(2) Single-ended common point SRCE input wiring (internal common terminal COM→24V-, external collinear→24V+).


 


3, active three-wire sensor (inductance proximity switch, capacitor proximity switch, Hall proximity switch, photoelectric switch, etc.) DC active three-line proximity switch and photoelectric switch output tube use triode output, so the sensor is divided into NPN and PNP output, there are The product is a four-wire system with dual NPN or dual PNP, but the state is just the opposite, and there is a four-wire output combining NPN and PNP.


NPN type When the sensor has the detection signal VT turned on, the current of the output terminal OUT flows to the negative pole, and the output terminal OUT potential is close to the negative pole. Generally speaking, the high level is turned to a low level.


PNP type When the sensor has the detection signal VT turned on, the current of the positive pole flows to the output terminal OUT, and the potential of the output terminal OUT is close to the positive pole. Generally speaking, the low level is turned to a high level.


The resistance on the emitter of the triode in the circuit is short-circuit protected. The sampling resistor 2-3Ω does not affect the output current. The collector of the triode has a pull-up and pull-down resistor that provides an output potential to facilitate the level interface circuit. The other output of the triode open collector output does not have pull-up and pull-down resistors.


Simply put, when the transistor VT is turned on, it is quite conductive with a contact.


 


(1) Single-ended common point SINK input wiring (internal common terminal COM→24V+, external collinear→24V-).


 


(2) Single-ended common point SRCE input wiring (internal common terminal COM→24V-, external collinear→24V+).


 


The PLC input interface circuit form and the external component (sensor) output signal form diversity, so it is necessary to understand the PLC input circuit form and the sensor output signal form before the PLC input module wiring, in order to ensure the PLC input module wiring is correct, in practical application. In order to be able to work well, the post-programming work and system stability lay the foundation.

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