In fact, the cpu only knows the on-off of the external input circuit, and does not know what contact the external input circuit is, or a series-parallel circuit composed of multiple contacts. The input module can be externally connected with normally open contacts or normally closed contacts. No matter what the external contact is, you can use the normally open contact or normally closed contact of the input point in the ladder diagram. [b]
If the i0.0 terminal in the plc external wiring diagram is externally connected with the normally open contact of the emergency stop button, and the emergency stop button is pressed, the input process image register corresponding to i0.0 is in the state of 1, and the normally closed contact of i0.0 in the ladder diagram is disconnected, then the normally closed contact of i0.0 should be used in the ladder diagram.
If in the plc external wiring diagram, the normally closed contact of i0.0 terminal is externally connected with the emergency stop button, but the emergency stop button is not pressed, its normally closed contact is closed, the input process image register corresponding to i0.0 is in the state of 1, and the normally open contact of i0.0 in the ladder diagram is closed. When the emergency stop button is pressed, its normally closed contact is disconnected, and the input process image register corresponding to i0.0 is in the state of 0, and the normally open contact of i0.0 in the ladder diagram is disconnected, so the normally open contact of i0.0 should be used in the ladder diagram.
To sum up, when the i0.0 terminal is externally connected with the normally closed contact of the emergency stop button, the i0.0 normally open contact should be used in the ladder diagram.
If all normally open contacts are used in the plc external wiring diagram, then the ladder diagram is exactly the same as the normally open and normally closed contacts in the corresponding relay circuit diagram. It is suggested that normally open contacts should be used to provide plc input signals.
Hardware normally closed contacts of emergency stop button and limit switch used for safety protection are more reliable than normally open contacts. If the normally open contact of the external emergency stop button is in poor contact or the line is disconnected, pressing the emergency stop button will not work in an emergency. If the plc is externally connected with the normally closed contact of the emergency stop button, when the above problems occur, the equipment will stop, which is beneficial to find and deal with the contact problems in time. Therefore, it is suggested to use the emergency stop button and the normally closed contact of the safety protection limit switch to provide input signals to plc.