With the continuous advancement of industrial automation, PLCs have become an indispensable part of industrial control and are widely used in industrial production. However, many engineers are unsure about their maintenance, troubleshooting, and operational techniques. This article summarizes some experiences and tips gained from using PLCs, which we hope will serve as a reference for our peers.
I. PLC Inputs and Outputs
A small PLC flexibly controls a complex system. What is visible are two rows of staggered input and output relay terminals, corresponding indicator lights, and the PLC serial number-much like an integrated circuit with dozens of pins. Without consulting the schematic diagram, anyone attempting to troubleshoot a faulty device would be at a loss, and the process of identifying the fault would be extremely slow. Given this situation, we have created a table based on the electrical schematic and posted it on the equipment's control console or control cabinet. This table lists the terminal numbers for each PLC input and output, along with their corresponding electrical symbols and Chinese names-similar to the functional descriptions of the pins on an integrated circuit.
With this I/O table, electricians who understand the operating process or are familiar with the device's ladder diagram can proceed with troubleshooting. However, for electricians who are unfamiliar with the operating process and cannot read ladder diagrams, it is necessary to create an additional table: the PLC I/O Logic Function Table. This table effectively illustrates the logical relationships between the input circuits (triggering elements and associated elements) and the output circuits (actuators) in most operating processes. Practice has shown that if you can skillfully utilize the I/O correspondence table and the I/O logic function table, you can troubleshoot electrical faults with ease, even without referring to the schematics.
II. Input Circuit Troubleshooting
To determine whether a specific input circuit-such as a pushbutton, limit switch, or wiring-is functioning properly, press the pushbutton (or other input contact) while the PLC is powered on (preferably in a non-operating state to prevent unintended equipment activation). This will short-circuit the corresponding PLC input terminal to the common terminal. If the PLC input indicator light corresponding to the pushbutton illuminates, it indicates that the pushbutton and its wiring are functioning normally. If the indicator does not light up, the pushbutton may be faulty, there may be poor contact in the wiring, or the circuit may be broken.
III. Output Circuit Troubleshooting
For PLC output points (here referring only to relay outputs), if the indicator light corresponding to the actuated object does not light up while the PLC is confirmed to be in operation, this indicates that the PLC's input-output logic function for that actuated object has not been satisfied. In other words, there is a fault in the input circuit; check the input circuit as described above. If the corresponding indicator light is on but the actuator (such as a solenoid valve or contactor) does not operate, first check the solenoid valve's control power supply and fuses. The simplest method is to use a voltage tester to measure the common terminal of the corresponding PLC output point. If the voltage tester does not light up, there may be a power supply fault, such as a blown fuse. If the voltage tester lights up, the power supply is good, and the corresponding solenoid valve, contactor, or wiring is faulty. If the system still does not function normally after troubleshooting the solenoid valve, contactor, and wiring, use a multimeter: connect one probe to the corresponding output common terminal and the other to the corresponding PLC output point. If the solenoid valve still does not operate, this indicates a fault in the output wiring.
If the solenoid valve operates at this point, the problem lies with the PLC output point. Since the voltage tester may sometimes give false readings, another method can be used for analysis: set the multimeter to the voltage range and measure the voltage between the PLC output point and the common terminal. If the voltage is zero or close to zero, the PLC output point is normal, and the fault lies in the peripheral circuit. If the voltage is high, it indicates that the contact resistance of this terminal is too high and it is damaged. Additionally, if the indicator light does not illuminate but the corresponding solenoid valve, contactor, etc., operates, this may indicate that the output terminal has been burned out due to an overload or short circuit. In this case, disconnect the external wiring from the output terminal and use a multimeter set to the resistance range to measure the resistance between the output terminal and the common terminal. If the resistance is low, it indicates that the contact is faulty; if the resistance is infinite, it indicates that the contact is intact, and the corresponding output indicator light is likely the issue.
IV. Program Logic Deduction
There are numerous types of PLCs commonly used in industry. For low-end PLCs, ladder diagram instructions are largely similar; for mid-to-high-end models, such as the S7-300, many programs are written in language tables. Practical ladder diagrams must include Chinese symbol annotations; otherwise, they are difficult to read. If you have a general understanding of the equipment's process or operating procedures before examining the ladder diagram, it will be easier to interpret. When performing electrical fault analysis, the reverse tracing method-also known as the backtracking method-is generally applied. This involves using the I/O correspondence table to locate the PLC's output relay corresponding to the fault point and then tracing back the logical relationships that trigger its operation. Experience shows that once a single issue is identified, the fault can generally be ruled out, as it is rare for two or more fault points to occur simultaneously on a single device.
Diagnosing PLC Malfunctions
Generally speaking, PLCs are extremely reliable devices with a very low failure rate; however, external factors can also cause them to malfunction.
A proximity switch with a 220V power supply had its two input signal contact wires sharing a 4-core cable with the switch's 220V power lines. When the switch failed and an electrician replaced it, they mistakenly swapped the power supply's neutral wire with the common input wire to the PLC. This caused three PLC input points to burn out when power was restored.
On another occasion, corrosion caused a break in the neutral line of the system power transformer, resulting in the 220V power supply to the PLC being elevated to 380V, which burned out the power module at the bottom of the PLC. During subsequent rectification, a 380/220V isolation control transformer was added.
On the Siemens S7-200 PLC, the common terminals for outputs are labeled 1L, 2L, etc., while the working terminals are denoted as AC L1 N. The +24V power supply is denoted as L+M. This can easily lead to confusion for beginners or those with limited experience. If L+M is mistakenly treated as the 220V power supply terminals, the PLC's 24V power supply will be damaged the moment power is applied.
The probability of hardware damage to the PLC, CPU, or similar components, or software runtime errors, is virtually zero. PLC input points are also unlikely to be damaged unless caused by high-voltage intrusion. The normally open contacts of PLC output relays have a very long service life, provided there is no short circuit in the peripheral load or design flaws causing the load current to exceed the rated range. Therefore, when troubleshooting electrical faults, the focus should be on the peripheral electrical components of the PLC. Do not automatically assume that the PLC hardware or program is at fault. This is crucial for quickly repairing faulty equipment and resuming production. Consequently, when troubleshooting electrical faults in a PLC control circuit, the focus should not be on the PLC itself, but rather on the peripheral electrical components within the control circuit.