The so-called switching quantity is the digital quantity, that is, the "bit" status value of the input terminal is set to "0" or "1". The identification of the switching signal by plc is done by its digital input module.
When the PLC controls the electromechanical equipment, the various operation buttons in the equipment and the switching quantities such as pressure, temperature, liquid level, and stroke switch are directly connected to the input module terminals of the PLC, and each input terminal is assigned a "" in the data area of ​​the PLC. Bit", each "bit" is an address in memory. The internal circuit of the PLC can scan for the presence or absence of the switching signal. Reading the status value of the PLC input bit can be used as the basis for judging the switch quantity fault signal. The process of diagnosing the switch quantity fault is essentially the process of comparing the normal state value of the PLC input bit with the actual value of the corresponding input bit. If the result of the comparison is consistent, it indicates that the electromechanical device is in normal working condition, and the inconsistency indicates that the device part corresponding to the input bit is in a fault state. This is the basic principle of PLC diagnosis based on the failure of the switching signal.
General PLCs have LED indicators to help check if the fault is caused by an external device. Whether in analog debugging or in practical applications, if a loop of the system cannot be operated as required, first check whether the PLC input switch point contact is reliable (generally by looking at the input LED indicator or directly measuring the input). If the input signal is not transmitted to the PLC, it should check the external circuit corresponding to the input; if the input signal has been collected, then see if the PLC has the corresponding output indication. If not, it is an internal program problem or an output LED problem. If the output signal is confident that it is sent, check the external output loop (check back from the PLC output).
In the output loop, due to short circuit or other reasons, the PLC output point is internally viscous, just change its wiring to another reserved empty wiring point, and modify the corresponding program to change the original output label to the new address number. Just fine.
This diagnostic method provides accurate fault location and enables real-time online diagnostics. Through the ladder programming of PLC, fault diagnosis can also be integrated into process control to achieve the purpose of protecting electromechanical equipment.
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