When the Rexroth PLC module cannot be turned on, most people prioritize checking the power supply and CPU module body, but tend to overlook key factors such as hidden power supply circuits, installation processes, environmental damage, and spare parts compatibility. These factors are often the "hidden culprits" of faults. The specific breakdown is as follows:

1、 Hidden faults in the power supply system

The CPU module of some Rexroth PLCs (such as RC series and VLC series) in the auxiliary power supply circuit needs to provide auxiliary power to the backplane bus and expansion interface in addition to the main power supply (DC24V or AC220V). If the wiring terminals of the auxiliary power supply are loose or the fuses (usually micro patch fuses) are blown, there will be a false impression of "the main power indicator light is on, but the module cannot start". Easy to ignore viewpoint: The auxiliary power fuse is integrated inside the module or hidden inside the terminal block, which is difficult to detect with the naked eye. A multimeter is needed to measure the voltage of the auxiliary power pin.

The power ripple exceeds the rated range of the main power supply voltage (such as DC24V ± 10%), but due to aging of the power module and bulging of the filtering capacitor, the output voltage ripple coefficient exceeds the 5% threshold allowed by the Rexroth PLC, which can cause the internal voltage stabilization circuit of the CPU module to fail to work properly, resulting in the module being unable to start up. Easy to overlook viewpoint: Only measuring the effective value of voltage is not enough, and an oscilloscope is needed to detect ripple; Poor compatibility between low-quality switch mode power supplies and Rexroth PLCs can also cause this issue.

If the PLC system adopts a distributed layout (such as separate cabinet power supply for CPU modules and remote I/O modules), the grounding resistance of each cabinet will be inconsistent, resulting in a ground potential difference. The instantaneous circulating current generated during startup will trigger the overcurrent protection of the CPU module, causing it to fail to start. Yi Hu Viewpoint: The multimeter measures the grounding resistance of a single cabinet normally, but ignores the potential difference between multiple cabinets; Insufficient cross-sectional area of grounding cables (less than 2.5mm ²) can also exacerbate this problem.

2、 Process defects in installation and connection

Poor contact between the backplane bus slot and the CPU module of the Rexroth PLC needs to be in close contact with the backplane bus. If there is severe vibration during transportation of the control cabinet, or if the module is not fully inserted into the slot during installation (the buckle is not fastened), it will cause bus communication interruption and the module cannot self check and start up. Easy to ignore viewpoint: Metal shrapnel inside the slot oxidizes or deforms (difficult to distinguish with the naked eye), which can cause excessive contact resistance; The fatigue caused by repeatedly plugging and unplugging modules is also an implicit cause.

Expansion module overload drag ": When the CPU module is connected to too many expansion modules (such as exceeding the maximum expansion quantity specified in the manual), or when there is an internal short circuit in a certain expansion module, it will cause the load to be too high when the CPU module is turned on, triggering a self-protection shutdown, manifested as the CPU module being unable to start normally. Easy to ignore viewpoint: Only investigate the CPU module, without disconnecting and testing the expansion modules one by one; The faults of some expansion modules are "hidden", and the multimeter cannot detect a short circuit in the power-off state.

The "hidden damage" of live plugging and unplugging: If a CPU module or expansion module has been unplugged and unplugged before the power is turned off, even if there is no obvious fault at that time, it may still penetrate the bus driver chip inside the module, and subsequently fail to start due to chip failure during startup. Yi Hu Viewpoint: This type of damage will not immediately appear and may erupt after multiple power on/off cycles, with no visible burn marks on the appearance, making it difficult to locate using conventional measurements.

3、 Hidden aging of environment and components

The moisture resistance level of the circuit board of the Rexroth PLC module for "moisture corrosion" and "dust short circuit" is IP20. If the sealing strip of the control cabinet ages and the humidity in the workshop exceeds the standard during the rainy season, the circuit board will absorb moisture, causing pin oxidation and insulation resistance decrease; Dust and oil stains adhering to the surface of the circuit board can form an implicit short-circuit circuit, which triggers protection when the current is abnormal during startup. Easy to ignore viewpoint: There is no obvious rust on the surface of the module, but the tiny capacitors and resistor pins inside have oxidized; Modules that have not been dedusted for a long time, and the "smoldering" aging caused by blocked heat dissipation holes, can also cause the module to suddenly fail to start up.

The "chain reaction" of internal battery failure in the Rexroth PLC's CPU module is equipped with a backup battery, which is used to store programs and system parameters. If the battery voltage is lower than 3.0V (rated 3.6V), not only will the program be lost, but it will also cause the module to fail the self-test when turned on. Easy to ignore viewpoint: Battery failure will not trigger any indicator light alarm, and most users only check the battery when the program is lost, ignoring its impact on startup.

The design life of the "component fatigue" Rexroth PLC module that has exceeded its service life is 8-10 years. After exceeding the service life, the internal electrolytic capacitors, crystal oscillators, and other components will naturally age, causing a decrease in capacitor capacity and crystal oscillator frequency drift, resulting in the inability to complete clock calibration and program loading when the module is turned on. Yi Hu Viewpoint: The appearance is undamaged and has not experienced overload or short circuit, but suddenly cannot be turned on due to "expiration of lifespan"; High temperature workshops will accelerate the aging of components (for every 10 ℃ increase, the lifespan of capacitors is reduced by half).

4、 Hidden faults in programs and systems

If there are infinite loop instructions and unreasonable interrupt nesting in the PLC program, the CPU module executing the program will fall into a dead loop during startup, unable to complete the self-test process. This is manifested as the module indicator light being constantly on but unresponsive (seemingly not powered on). Yi Hu Viewpoint: Users default to 'program is fine', but ignore that the program has not been fully tested after downloading; Incorrect operation of special registers in the program can also cause system parameter disorder.

If the firmware version is incompatible and the PLC firmware has been upgraded or the firmware version of the replaced spare parts is inconsistent with the original system, it will cause the module to fail to match the hardware parameters when turned on, trigger the firmware protection mechanism, and fail to start normally. Yi Hu Viewpoint: The firmware of some series of Rexroth PLCs (such as MPC series) needs to match the version of the expansion module. Upgrading only the CPU firmware may cause compatibility issues.