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first of all check the plc device and power supply and the error leds
then check the inputs and outputs terminals
check the devices including such as contactors and relays
then check the program steps
The Basics
Decide if the problem is internal to the processor or in the I/O system.
The first thing to check is the integrity of the PLC's power and ground. Check the DC supplies for AC ripple.
Measure the voltage of any batteries in the system.
Electro-magnetic interference (EMI) or radio frequency interference (RFI). Try to correlate the erratic behaviour with an external EMI or RFI event like a large motor starting, arc welding in the area, lightning strikes, or even the use of handheld radio transmitters. Handheld radios commonly used by maintenance personnel emit powerful RF radiation and can seriously disrupt the operation of unprotected electronic equipment.
Verify that the program is correct, verify the program with the backup, and reload the program if problems are encountered.
Keep the program backups up-to-date and safely away from temperature extremes, high humidity, and EMI and RFI exposure to ensure they will always be usable.
Troubleshooting inputs and outputs
Find out why the internal status of the PLC (what the PLC thinks is happening) does not agree with the external situation (what is actually happening). The first thing that must be done is to determine the relationship between physical I/O modules and the I/O instructions in the PLC program.
Determine if the power for the input in question is present, as faults in field wiring and devices can blow a fuse, trip a breaker, or cause some other power disruption.
sensor/realswitches of led trace the communication wires
to know how to think and how to find the problem first , because50% of solving te problem to find it first , then to think how to solve this exact problem
well check the power supply
check the status leds
check the response for digital inputs on the modules
if all is ok go online see if something wrong with analogs maybe some major error
modules not responding maybe bad connection or defective
check outputs forcing and monitor program execution
