Research on Wiring Problems Between Commonly Used PLC Digital Input Modules and Field Instruments

This paper mainly analyzes several common interface circuit forms of plc digital input module (DI module) and sensor instrument, according to the different structures of DI module internal input circuit and sensor output interface electrical part, gives the interconnection The wiring method at the time ensures the correctness of the instrument type selection and on-site construction wiring in the future engineering design.
1 Introduction
In engineering design and on-site construction, various instrument wiring problems are often encountered. The correct circuit structure can be selected to ensure the correct connection between the selected instrument and the manufacturer’s PLC. Therefore, it is necessary to have a clear and systematic understanding of the input and output circuits inside the PLC input and output modules and the output interfaces of commonly used instruments.
In order to prevent the abnormal operation of the PLC caused by the interference generated by the external line, or even cause damage to the internal components of the PLC, the input interface circuit of the PLC digital input module (DI module for short) often uses a photoelectric coupling element to isolate the input signal from the connections between internal circuits. The signal at the input terminal drives the internal light-emitting diode of the photocoupler, the light-emitting diode emits light, and the phototransistor is turned on, so that the external input signal can be reliably transmitted to the processor.
The interface circuit of the common terminal of the PLC DI module of each manufacturer is divided into the positive common point and the negative common point of the photocoupler, and the output circuits of various instruments and meters include dry contact, active output, high level and low level Therefore, when we select an external instrument sensor, we need to distinguish and understand the output circuit of the selected PLC DI module and the structure of the output circuit of the sensor instrument in order to select the correct type to ensure Correct wiring with the DI module of the PLC manufacturer.
2 PLC DI modules are classified by input type
The input interface circuit of PLC DI module can be divided into DC and AC input circuits according to the type of external power supply; according to the flow direction of the input common terminal current It can be divided into source input and drain input circuits; according to the connection mode of the common terminal of the optocoupler light-emitting diode, it can be divided into common anode and common cathode input circuits.
3 Classified according to the type of external power supply
3.1 DC input circuit

Figure 1 is the DC input circuit (only one input circuit is drawn ), the input voltage of the DC input circuit is generally DC24V. The DC input circuit requires the external input signal to be a passive dry contact (the DC power supply is provided inside the PLC cabinet) or a DC active non-contact switch contact. When the external input contact signal is closed, the input terminal is connected to the positive pole of the DC power supply. The current passes through the resistor R1, the internal LED of the optocoupler, VD1 (interface indication) to the COM terminal to form a loop; the phototransistor is saturated and turned on, and the conduction signal is sent to the processor, so that the CPU thinks that there is a signal input on this channel. When the external input element is disconnected from the positive pole of the DC power supply, the light-emitting diode in the optocoupler goes out, the phototransistor is cut off, and the CPU thinks that there is no signal input in this channel. The DC power supply can be provided inside the PLC cabinet, or it can be an external DC power supply.
3.2 AC input circuit

Figure 2 is the AC input circuit, the input voltage of the AC input terminal is normal 120V or 230V. The circuit requires the contact of the external input signal to be a passive dry contact signal or an AC active non-contact switch contact signal. The difference between it and the DC input circuit is that a first-stage step-down circuit and a rectifier bridge circuit are added before the photocoupler. The AC current is limited by the resistor R and filtered by the capacitor C (to filter out the DC part of the power supply), and then through bridge rectification to become a DC current after step-down. The subsequent circuit principle is consistent with the DC input circuit. It can be seen from the figure that due to the addition of three links of current limiting, isolation and rectification in the AC input circuit, the delay time of the input signal is longer than that of the DC input circuit, which is its shortcoming. However, because its input terminal is a high voltage, the reliability of its input signal is higher than that of a DC input circuit. The AC input circuit is generally used in harsh environments such as oil mist and dust, where the responsiveness is not high, while the DC input is used in the environment where the environment is better, the electromagnetic interference is not serious, and the response time is required.
4 According to the incoming publicClassification of the flow direction of the common terminal current
Internally, the PLC DI module connects one end of all input circuits (photocoupler) to the common terminal (COM or M), and connects the other end of each input circuit to its corresponding input end, also called this structure as single-ended common point input. This practice can reduce the number of input terminals.
The current flows out from the input terminal of the DI module and flows into the negative pole of the power supply, which is the sink current or sink input; the current flows from the positive pole of the power supply to the common terminal, and the positive pole of the power supply is connected to the common terminal, that is, the common anode; the sensor is Active low.
The current flows from the positive pole of the power supply to the input terminal of the DI module, which is the sink current (Source Current) or sink input; the current flows from the common terminal to the negative pole of the power supply, and the negative pole of the power supply is connected to the common terminal, that is, the common cathode; the sensor is at a high level efficient.
4.1 Sink-type current source input circuit
The sink-type current source input circuit is shown in Figure 3. At this time, the current flows out from the input terminal, flows into the negative pole of the DC power supply, and flows into the PLC common from the positive pole of the DC power supply end (COM end or M end).

4.2 Source type sink current input circuit
Source type The input circuit is shown in Figure 1. At this time, the current flow is just opposite to that of the sink circuit. Current flows from the positive pole of the DC power supply to the input terminal of the DI module, and flows into the negative pole of the DC power supply from the common terminal.
4.3 Switching input circuit
In order to adapt to the usage habits of various regions, the internal public terminals of PLC DI modules of some manufacturers use S/S terminals, which can be connected to the 24V+ (positive pole) or 24V- of the power supply. (Negative) connection, combined with the change of external instrument wiring, the input circuit of the DI module can be either a sink input circuit or a source input circuit. It is more flexible than the DI module using the common terminal (COM terminal or M terminal). The development of the S/S terminal is to be suitable for industrial control occasions where Japanese and European PLCs are mixed, and it plays a general role. The S/S terminal is also called the SINK/SRCE switchable type. Its circuit form is shown in Figure 4. When used as a source input, the common terminal is connected to the negative pole of the power supply; when used as a sink input, the common terminal is connected to the power supplypositive pole. In this way, wiring can be done according to the needs of the site, which brings great flexibility to the wiring work. All the inputs of the basic unit can be set as sink input or source input by selection, but they cannot be mixed.

Sink type source current input circuit, source type sink current input circuit and Switching input circuits are all DC input circuits.
5 The internal circuit of the switch signal of the sensor and its connection with the PLC input circuit
During the engineering design process, it is often encountered various instruments whose output is a switch signal , such as: pressure switch, flow switch, level switch, temperature switch, valve status feedback, motor running status, fault status, etc. The output circuits for these sensors come in a variety of styles. Therefore, it is necessary to fully understand them in order to ensure matching wiring with the selected PLC DI module.
5.1 Passive dry contact
Limit switches of valves, travel switches, motor remote/local button switches, pull switches, deviation switches, relay contacts of motor running status, etc., are passive Dry contact signal, there is no polarity factor of the power supply, it is relatively simple, easy to connect, and can be applied to the above various types of DI input modules.

5.2 Active two-wire sensor
Active Two-wire sensors (such as proximity switches, etc.) are divided into DC and AC, and DC two-wire switches are divided into diode polarity protection Figure 5 and bridge rectification polarity protection Figure 6. The former needs to pay attention to the polarity when connecting to the PLC, and the latter does not. Pay attention to polarity.
The wiring of the DC two-wire switching instrument and the sink-type pull current PLC DI module is shown in Figure 7:

The wiring of the DC two-wire switching instrument and the source type sink current PLC DI module is shown in Figure 8:

AC two-wire switching instrument and source sink current PLC The wiring is shown in Figure 9:

5.3 Active three-wire sensor
The output signal of the sensor , In addition to some dry contact signals such as travel switches and relay contacts, some sensors also provide NPN and PNP open collector output signals. Its essence is to use the saturation and cut-off of the triode to output two states, namely high level and low level signals, which belong to the switch type sensor. The output signals of PNP and NPN sensors are diametrically opposite. For the input circuits of PLC DI modules from different manufacturers, it is sometimes at a loss whether to choose NPN or PNP output sensors. The following mainly introduces the wiring of these two output types of sensors and the input circuit of the PLC DI module.
5.3.1 The form of NPN and PNP output circuit

Figure 10 is the NPN sensor output circuit: NPN open collector output The output terminal (OUT terminal) of the circuit is connected to the negative pole of the DC power supply through a triode tube. In the initial state, when the sensor does not act, the triode is in the cut-off state, and the output terminal (OUT terminal) is close to the positive pole of the DC power, which is at a high level; when the sensor operates, the triode is saturated and turned on, and the current at the OUT terminal flows to the negative pole of the DC power supply, and the output is – low Level signal, the potential of the OUT terminal (output terminal) is close to the negative pole, that is, the high level is usually turned into a low level.
Figure 11 is the PNP sensor output circuit: the output terminal (OUT terminal) of the PNP open collector output circuit passes through the triode and DCPower supply positive connection. When the sensor operates, the triode is saturated and turned on, the positive current of the DC power supply flows to the OUT terminal, the potential of the OUT terminal (output terminal) is close to the positive pole of the DC power supply, and a high-level signal is output, that is, the low level is usually turned into a high level.
5.3.2 Connection of NPN and PNP output circuit and PLC input module
1) NPN collector open circuit output
From the above analysis, it can be seen that the output is 0V when the NPN collector is turned on, and when the output OUT When the terminal is connected to the input of the PLC DI module, the current flows out from the input terminal of the PLC and flows in from the common terminal of the DI module. The input circuit of the PLC DI module is a sink input circuit, that is, the sensor with NPN output can only be connected to a sink or The DI module of the switching input circuit is shown in Figure 12:

2) PNP open collector output
PNP collector conduction When it is on, the output is +V high level. When the output OUT terminal is connected to the input of the PLC DI module, the current flows in from the input terminal of the DI module and flows out from the common terminal of the DI module. The input circuit of the PLC DI module is a source input circuit , that is: PNP-type output sensors can only be connected to DI modules with source-type or switch-type input circuits, as shown in Figure 13:

6 Conclusion
Exactly Due to the variety of sensor output signals and PLC DI module input circuit forms, we must fully understand the type of PLC DI module input circuit and the form of sensor output signals in the process of engineering design. After that, the correctness of the instrument sensor selection can ensure the correctness of the wiring between the sensor and the PLC input module without adding additional components, so that it can be used in practical applications with ease.