Working principle of the PLC for example

Below with a simple example to further illustrate the PLC scanning process. Figure 1 - The external wiring diagram and ladder diagram 6 shows the PLC, start button SB1 stop button SB2 and thermal relay FR normally open contacts, respectively, by the Numbers for X0 ~ X2 input of PLC, ac contactor KM of coil connect the output of the PLC in Numbers for Y0. Figure 1 - 6 b is the four input/output variables corresponding to the I/O image register, figure l - 6 c is the PLC ladder diagram, it - with figure 1 3 shows the function of the relay circuit is the same. But should pay attention to, ladder diagram is a kind of software, graphical is PLC program. In the figure is the ladder diagram programming elements such as X0, X0 ~ - X2 as input relay, Y0 is the output relay. Ladder diagram programming components X0 and meet in the input terminal of the SBl X0 normally open contacts and correspond to the input image register X0, Y0 programming components and output image register Y0 and connect the output terminal Y0 corresponding output circuit inside the PLC controller. PLC ladder diagram in the form of instructions stored in the user program memory, figure l - 6, the ladder diagram correspond to the following 5 instruction, '; 'After the instruction comment. LD X0: pick up on the left side of the bus bar X0 normally open contacts OR Y0; Normally open contact in parallel with X0 Y0 normally open contacts of ANI X1; In series and parallel electric circuits of the normally closed contacts ANI X1 X2; Series of X2 the normally closed contact of the OUT Y0; Figure 1 - Y0 coil The ladder diagram 6 complete logic operation for Y0 = ( X0 + Y0) , X1, X2 input processing stage, the CPU will SBl, SB2 and FR reads normally open contacts of the state of the corresponding input image register, the external contact is deposited in the register is a binary number l, whereas in the 0. , the execution of the first l from X0 corresponding to the input image register to retrieve binary number and save it. Article 2 instruction, take out the corresponding output Y0 image binary number in the register, and corresponding binary phase 'or' (X0 Corresponding 'or' parallel operation of the circuit) 。 Execution of article 3 or 4, respectively take xl or X2 input image register of the corresponding binary number, because it is a normally closed contact, after get the result from the previous phase 'and' ( Circuit in series of corresponding 'and' operation) And put them into operation result register. The result, the execution of 5 will register the binary Numbers into the corresponding output Y0 image register. In the output processing phase, the CPU will be the output of binary digital image register for output module and latches, if the corresponding output Y0 image register storage is a binary number 1, external coil of KM to electricity, instead will power outages. If reading input image register are binary 0 X0 to X2, during the program execution phase, after the above logic operation process, the result is still the Y0 = 0, so the KM of the coil is de-energized. Press the starting button SBl, X0 to l, Y0 into 1 after logic operation state, the output processing phase, the corresponding output Y0 image register 1 sent to the output module of PLC Y0 within the corresponding physical relay on the normally open contact, contactor coil electricity of KM.
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