1. Repeating the coil Figure 9-2.5 (a) shows the use of a repetitive output (repetitive coil) in the plc ladder, which does not exist in the relay control loop. However, in the PLC program, it is sometimes necessary to use it for the convenience of programming.
When the ladder diagram uses repeated outputs, the final output state of Q0.6 is based on the result of the last executed program processing (second output). However, for the block before the second output, the internal state of Q0.6 is the first output state. Therefore, when IO.I and I0.2 are both "1", I0.3 and I0.4 When one is "0", QO.O in Figure 9-2.5 (b) will output "1" and Q0.6 will output "0".
2. Edge Processing Figure 9-2.6 (a) shows the "edge" output program that is often used in PLC ladder diagrams. Similar loop design in the relay control loop is meaningless (output MO.O is always "0"), but PLC The program is executed strictly in accordance with the "top to bottom" sequence of the ladder diagram. Therefore, in the first PLC cycle with IO.1 being "1", the state where MO.O and MO.1 are both "1" can occur. That is, a pulse output with a width of one PLC cycle can be obtained in MO.O, see Figure 9-2.6 (b).
Edge processing can be directly implemented using PLC programming instructions. Such as the S7-200 instructions "-IPI-", "-|N|-" and so on.
3. The series coil is used in the S7-300/400 PLC. Since the special command of the "intermediate operation result output" can be used, the effect is equivalent to the form in which the coils can be connected in series.
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