First, the preparation of the demonstration experiment
The first step is to determine the logic variable and write the logical expression: the contact of the plc input relay (X) is regarded as the input variable, and the coil of the output relay (Y) is regarded as the output variable, and each of the logic gates is written according to the logical relationship of the logic gate. The logical expression of the logic gate.
The second step is to design a PLC ladder diagram based on the logical expression (as shown in Figure 1) and write the program statement.
The method is: when the input variable is "original variable", it is represented by "normally open contact"; when the input variable is "anti-variable", it is represented by "normally closed contact". "AND logic" is represented by "series of contacts"; "or logic" is represented by "parallel of contacts".
Step 3: Use the computer or handheld programmer to send the above programmed program into the PLC user memory.
The fourth step: I / O distribution, and wiring debugging. For the sake of wiring simplicity, we have the same input wiring for the six "doors": switch K0 ─ → X0, switch K1 ─ → X1 ("─→" means connecting between the two terminals); and output wiring They are: COM1─→24V DC power supply positive, COM2─→24V DC power supply positive.
"AND gate" output wiring: Y0 ─→L0 (L0---L5 is the LED indicator)
NAND gate output wiring: Y1 ─→L1
"OR gate" output wiring: Y2 ─→L2
"NOR gate" output wiring: Y3 ─ → L3
XOR gate output wiring: Y4 ─→L4
"Same OR gate" output wiring: Y5 ─→L5
After the wiring is debugged, the preparation for the experiment is over.
Second, demonstrate the practice of the experiment
For example, when doing the "AND-to-door" logic relationship experiment, you can press the "AND gate" logic relationship wiring: K0 ─ → X0 K1 ─ → X1 Y0 ─ → L0, and set the switch to close to 1, the switch is off 0; When 1, the indicator is off. Let the students observe the logical relationship between the switch K0, K1 on/off combination and the indicator L0 on and off, and list the truth table to analyze the logical relationship. In the same way, when doing the "NAND NAND" logical relationship experiment, it is wired according to the logical relationship of "NAND NAND": K0 ─→X0 K1 ─→X1 Y1─→ L1, let the students observe the switch K0, K1 on/off combination and The logical relationship of the indicator L1 is off, the truth table is listed, and the logical relationship is analyzed. And so on... Connect the logic according to the corresponding "gate", use "switch" as the input logic variable (cause), use the "light" to display the output logic variable (result), and analyze the logic by column truth table. Features. In short, students can fill out the following truth table by observing the demonstration experiment (as shown in Table 1).
Table 1: Six "Logical Gates" Logical Relationship Demonstration Experiment Truth Table
enter
Output
K0=X0
K1=X1
L0=Y0
L1=Y1
L2=Y2
L3=Y3
L4=Y4
L5=Y5
0
0
0
1
1
0
1
1
After completing the form, analyze the logic functions of the output and input one by one, and the logical relationship between Y1 and Y0, Y3 and Y2, Y5 and Y4.
Third, summary
1. Using the PLC test bench for digital circuit experiments is a new attempt, and such an attempt is feasible. Because the "logical control function" is the most basic control function of the PLC, we designed the purpose of this experiment not only for the experiment itself, but more importantly, to clarify the fact that in the digital circuit, various logic gates are used. The combinational logic circuit is completely achievable with the "program" of the PLC.
2. The "logic gate" demonstrated here is realized by the "program" of the PLC. Different "programs" correspond to different logic functions, and changing the "program" can change its logic function. Therefore, the "program" is the soul of the PLC.
3. It is advisable to use the PLC test bench for digital circuit experiments. Because the PLC test bench "universality", "flexibility", "reliability" are better. Not only can it be used to do two-input "AND", "OR gate" and other experiments, but also can be used to do three-input "and door", "or door" and other experiments, or do other experiments, just change the program, Very flexible, convenient and fast.
4. By means of “demo experimentsâ€, it is vivid and interesting for students to acquire knowledge from “practice†than to simply explain “book knowledgeâ€. Students can be guided to observe the "experiment" repeatedly and carefully, starting from analyzing the causal relationship of "event", determining logical variables in turn, assigning values ​​to logical variables, listing truth tables, determining logical functions, writing logical expressions, drawing logical symbols or The logical structure diagram finally allows students to remember various "logical functions" and several "logic function representations".
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