Interlocking functions of PLC program of ladder diagram

Using two or more normally closed contacts to ensure the coil does not power on function "interlock". Pros and cons of three-phase asynchronous motor control circuit for a typical interlock circuit, as shown in Figure 5-4. KMl and KM2 of which is to control the forward run and reverse operation of AC contactors.

Figure 5-4 the pros and cons of three-phase asynchronous motor control circuit
As shown in Figure 5-5 for PLC control of three-phase asynchronous motor positive inversion of the external I/O wiring diagrams and the ladder diagram. Achieving positive and negative rotation control functions the trapezoidal figure is made up of two stops of the ladder diagram and the interlocking between the two contact points.

Figure 5-5 PLC I/O wiring diagrams of the control motor positive inversion and ladder diagram
Should be noted that although in the ladder diagram has been soft interlock contacts of the relay (X1 and X0, Y1 and Y0), but must also be used in the I/O output circuit wiring diagram KM1, KM2, the normally closed contacts for hardware interlocks. PLC soft relay interlocking only one scan cycle, while external hardware disconnect contactor contacts are often larger than a scan cycle, no time to respond and disconnect time is generally longer than the closing time of the contacts. For example, Y0 while disconnected, may KM1-contact has not been broken, and in the case of no external hardware interlocks, contact KM2 got, cause the main circuit, so you must use hardware double interlock. Double interlock, while avoiding a contactor KM1 and KM2 main contacts motor winding short circuit caused by welding.