Input circuit connection Input circuit is the port of PLC to receive signals (generally 0-40MA DC current or 0-10V DC voltage signal for analog), input connection refers to the external input device (any passive contact and collector open NPN triode) through the input circuit closed, at the same time the input indicating light emitting diode. Commonly used external input devices include buttons, proximity switches, transfer switches, dip switches, various inductors, etc., which are the main command appliances that send out various control signals to the system.
(1) Connection between PLC input module and master electrical equipment
Panasonic PLC in the figure for DC point input, that is, so the input point share a common terminal COM, COM end with DC24V power supply, when writing the program to pay attention to the use of external equipment is normally closed or normally open contact.
The stop button SB0 in the electrical schematic diagram of the input terminal uses normally closed contacts strung in the control line for shutdown control. Start button SB1 with normally open contact. The two trapezoidal diagrams in the design accomplish the same control function, but the stop signal X0 uses different contact types, so the external stop button contact types connected to the endpoint are different.
I/O allocates SB0-X0, SB1-X1, and outputs K0-Y0. When the external use of long closed contact, do not operate the button, the output Y0 is normally connected, in the PLC control system, the external switch whether start or stop generally choose normally open type.
(2) The connection between the proximity switch and the PLC input module in the PLC control system design of the wiring work proportion is small, but it is the basis of programming design. To ensure that the wiring work is correct, the input and output circuits of PLC need to have a clear understanding.
1 PLC DC input circuit: active type (common anode) input circuit, leakage type (common cathode) input circuit. So the COM end of the leakage input circuit PLC is the positive terminal of the external DC power supply, such as Siemens S7-400PLC DC input module COM terminal must be connected to the positive terminal of the external power supply. So Siemens PLC input signal for low voltage signal, if the external signal for high voltage signal should be converted through the intermediate relay.
2.PLC AC input circuit voltage is generally AC120V or AC230V, after resistance current limiting and capacitance isolation after rectifier into DC three links, so the input signal delay time is longer than DC circuit, but the input end is high voltage, input signal reliability is high, generally used for harsh environment, the response is not high occasions.
(3) Connection between switching signal and PLC input module
For different PLC input circuit should correctly choose the sensor (NPN or PNP) input mode, NPN type sensor action, OUT end is 0V, (NPN type output end OUT should be connected with PLC input end leakage type) output low level signal. PNP sensor action, OUT end is +V, output high level signal.