Touch the CPU to check its temperature. A normal CPU operating temperature should not exceed 60°C. The ideal temperature for your hand is 37-38°C, which is the body's natural temperature.
Secondly, check whether the indicator lights of each module on each board are normal;
Third, smell for any unusual odors and check for any burnt electronic components or cables;
Fourth, listen for any unusual movements, check for loose screws and whether the relays are functioning properly, and listen to the feedback from the on-site personnel.
Fifth, locate the fault location based on drawings and process flow;
Six pairs of unidentified parts were replaced to confirm the fault.
When the PLC software is malfunctioning, check if the CPU's RUN status is normal. If not, clear the CPU and re-download the control program.
When the PLC hardware malfunctions, the following steps should be taken to check the operation:
1. Check if the PLC power supply has power: If it has power, measure whether the voltage is within ±5% of +24V. If it has power and is normal, proceed to the next step; if it has power but is abnormal, test the output and input terminals of the power module. If the output is abnormal but the input is normal, replace the module; if the input is abnormal, check the reverse current pattern of the input terminal, such as checking the AC voltage of the 24V AC/DC transformer at ±10% of 220V. If normal, replace the 24V DC transformer. If there is no power, trace the fault. Using the schematic diagram, site layout diagram, and wiring diagram, check whether the wiring of the output terminals of various electrical devices supplying power to the power module is correct. If incorrect, rewire. If correct, use a multimeter to check whether there is normal power supply at the input and output terminals of the air switch. If there is no normal power supply, determine whether the problem is external or internal. If it is external, analyze whether the voltage is insufficient, there is no voltage at all, the load is too heavy, or there is severe overcurrent, etc., until the fault is eliminated and the power supply is restored; if it is internal, replace the faulty component.
2. Understand CPU operating modes and priorities: High priority modes include STOP, HOLDUP, and STARTUP (WARMRESTART, COLDRESTART); low priority modes include RUN and RUN-P (PG/PC online read/write program). Check if the CPU is in RUN mode, STOP mode, or a combination of RUN and STOP modes (also called debug mode). If it is only in RUN mode, the CPU and boards are proceeding normally to step 3. If it is in debug mode, it may be due to a breakpoint in the user program during operation, or a breakpoint occurring in startup mode. In this case, re-debug the program and download the control program to the CPU again.
If it's in STOP mode, the possible causes are: A) No power: Analyze the cause of the power outage. Is it due to a problem with the power supply section, or an abnormal power outage (since a 1K3AH UPS ensures that abnormal power outages are rare)? Usually, it's because the power was cut off for maintenance. After maintenance is completed, manually restore power. Then use the PLC's online function to switch the CPU's operating mode from STOP to RUN; B) CPU failure: Replace it with a new, good CPU of the same type and version; C) A board failure: Replace the boards systematically. When replacing hardware, be sure to use the same type and version of the original equipment. Otherwise, the actual PLC equipment will be different from the hardware equipment database in the corresponding programming software, making it impossible for the user control program to execute normally.
3. Check the communication cables on each motherboard and expansion board, and inspect each module's LEDs to see if any faulty modules have a lit fault light. If so, the module is malfunctioning. The points on the digital output modules function similarly to a light switch in real life and are normally open. Therefore, when troubleshooting any point on this module online, simply check its continuity when there is no connection and the control program is not sending an output signal. If it's continuous, the point is faulty; if it's not continuous, it's normal. If it's not normal, reconnect the hardware cable to another point. Alternatively, you can replace it with a new module and measure the continuity of the replaced module's points. If it's continuous, the point is faulty; if it's not continuous, the point is good. The points on the digital input modules are like conducting coils and are normally closed. They can be tested online or offline. Use a multimeter to check; if a point is faulty, it's not continuous, so replace the point and reconnect the cable; a good point is continuous. After rewiring the hardware, the corresponding programming software must be used to replace the 0X or 1X address in the control software. Analog input modules are similar to digital input modules; each channel is equivalent to a wire, meaning it's a normally closed contact. Therefore, the method for testing a channel is to use a continuity tester. A good channel is one that is on, and a bad channel is one that is off. The testing method for analog output modules is the same as for digital output modules. If a bad channel is found, the hardware wiring needs to be replaced, and the corresponding 3X or 4X address in the control program needs to be replaced as well. For analog modules, the range selection needs to be checked, and the fuses need to be checked for blown connections. The software configuration should generally be 1-5V for voltage or 4-20mA for current, depending on the type of sensor and intelligent converter used. After replacing hardware points or channels, if possible, the PLC CPU should be stopped, and the program should be downloaded again. If this is not possible, the updated program should be downloaded directly without stopping the CPU. The good channels/good points of unnecessary input modules can be connected in series with the last used good channel/good point, or special settings can be made in the software.
4. The power supply modules on many output module boards cannot be powered off during normal production. Powering off at this time will cause the normally open relays in the relay cabinet to become normally open, which can easily lead to errors. Therefore, when testing such output modules, it is necessary to contact the on-site operators, manually operate the relevant equipment, and then disconnect the power supply line of the digital output module before performing the module test.
5. Testing of various switches: For devices such as relays, proximity switches, and circuit breakers, the testing is based on whether the switch is normally closed or normally open. A multimeter is used to check its continuity. If the result is the opposite of a good switch, the device is faulty and should be replaced. Normally open switches are used in most circuits for manual or automatic control of current flow; normally closed switches are mainly used in protection circuits. This allows you to understand the normal operating conditions of switches and protection devices, thus correctly identifying their condition.
6. The communication module test is performed by simply replacing the existing communication module with a good one to identify whether the module currently in use on the board is working properly.
7. Measurement method for conductors: Conductors are also measured using continuity testing methods. A conductor known to be continuous can be used to test a conductor whose condition is unknown. The method is to connect a good conductor to the unknown conductor and then measure the continuity.
8. Resistance detection: Detect voltage when energized, and detect the corresponding resistance when de-energized.
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