Troubleshooting the plc system


A ground loop condition occurs when two or more electrical paths exist on a ground line. For example, in the Figure, the transducers and the transmitter are grounded in the chassis (or device housing) and connected to an analog input card through a shielded cable. The shield connects to both terrains of the chassis, creating a way for the current to flow from one floor to another, since both have different potentials. The current flowing through the shield could be as high as several amps, which would induce significant magnetic fields in the transmission of the signal. This could create interference that could result in a possible erroneous reading of the analog signal. To avoid this problem, the screen must be grounded on only one side of the chassis, preferably on the PLC side.

In the example shown in the figure, the protector should only be grounded at the analog input interface.

To check for a ground loop, disconnect the ground wire at the ground termination and measure the resistance from the wire to the termination point where it is connected (see Figure below). The meter should read a large ohm value. If a low ohm value occurs across this gap, circuit continuity exists, meaning that the system has at least one ground loop. 1


If the field device connected to an input module does not seem to turn on, problem may exist somewhere between the L1 connection and the terminal connection to the module. The status indicators of an input module can provide information about the field device, the module and the wiring of the field device to the module that will help identify the problem. The first step to diagnose the problem is to place the PLC in standby mode, so that it does not activate the output. This allows the field device to be manually activated (for example, a limit switch can be manually closed). When the the field device is activated, the module’s power status indicator should turn on, indicating that there is continuity of power. If the indicator is on, then the wiring is not the cause of the problem

The next step is to evaluate the PLC reading of the input module. This can is achieved by using the PLC test mode, which reads the inputs and executes the program but does not activate the outputs. In this mode, the The PLC screen should show a 1 in the bit of the image table corresponding to the activated field device or the reference instruction of the contact must it will be highlighted when the device provides continuity (see Figure). Yes the PLC is reading the device correctly, then the problem is not found in the input module. If you do not read the device correctly, then the module could be defective The logical side of the module may not work properly, or its Optical insulator can be blown. On the other hand, one of the interfaces of the module the channels may be defective. In this case, the module must be replaced. 1

If the module does not read the field device’s signal, then further tests are required. Bad wiring, a faulty field device, a faulty module, or an improper voltage between the field device and the module could be causing the problem. First, close the field device and measure the voltage to the input module. The meter should display the voltage of the signal (e.g., 120 volts AC). If the proper voltage is present, the input module is faulty because it is not recognizing the signal. If the measured voltage is 10–15% below the proper signal voltage, then the problem lies in the source voltage to the field device. If no voltage is present, then either the wiring or the field device is the cause of the problem. Check the wiring connection to the module to ensure that the wire is secured at the terminal or terminal blocks.


The PLC output interfaces also contain status indicators that provide useful information to solve problems. Like the troubleshooting of PLC inputs, the The first step in solving exit problems is to isolate the problem either module, the field device or the wiring.

In the output module, make sure the power supply to switch the output It is at the correct level. In a 120 VAC system, this value must be within 10% of the nominal value (that is, between 108 and 132 volts AC). In addition, it examines the Output module to see if it has a blown fuse. If you have a blown fuse, Check the nominal value of the fuse. Also, check the current of the output device requirements to determine if the device is pulling too much current. If the output module receives the command to turn on from the processor however, the output state of the module does not turn on as a result, then the output the module is defective. If the indicator turns on but the field device does not energize, check the voltage at the output terminal to ensure that switching the device is working If there is no voltage present, then the module must be replaced. If there is voltage present, then the problem lies in the wiring or field device. At this point, make sure that the field wiring to the module terminal or the terminal block has a good connection and there are no broken wires.

After checking the module, verify that the field device is functioning correctly. Measure the voltage that reaches the field device while the output module is ON, making sure that the return line is properly connected to the device. Yes there are is the power but the device does not respond, then the field device is defective. Another method to verify the field device is to test it without using the exit module. Remove the output wiring and connect the field device directly to the source of power. If the field device does not respond, then it is defective. If the field device responds, then the problem lies in the wiring between the device and the output module. Check the wiring, looking for broken wires along the wire path.

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