Program troubleshooting
There are severall causes off alteration to the user
 extreme environmental conditions,
 electromagnetic Interference (EMI),
 improper grounding,
 improper wiring connections, andUnauthorized tampering.
If you suspect the memory has been altered, check the program against a previously saved program on an EEPROM, UVPROM or flash EPROM modulle.

Hardware troubleshooting
Tips for troubleshooting control system
If installation and start-up procedures were followed closely, controller will give reliable service.
If a problem should occur, the first step in the troubleshooting procedure is to
identify the problem and its source.
Do this by observing your machine or process and by monitoring the diagnostic LED indicators on the CPU, Power Supply and I/O modules.

By observing the diagnostic indicators on the front of the processor unit and I/O modules, the majority of faults can be located and corrected. These indicators, along with error codes identified in the programming device user manual and programmer’s monitor, help trace the source of the fault to the user’s input/output devices, wiring, or the controller.

Troubleshooting Controller
In identifying the source of the controller’s operation problem use troubleshooting considerations table including status indication, trouble description, probable causes and recommended action.
To receive the maximum benefit, follow these steps:
 Identify Power Supply and CPU LED status indicators;
 Match processor LEDs with the status LEDs located in troubleshooting tables;
 Once the status LEDs are matched to the appropriate table, simply
move across the table identifying error description and probable causes;
 Follow the recommended action steps for each probable cause until
the cause is identified;
 If recommended actions do not identify the cause, contact manufacturer or distributor for assistance.

Troubleshooting Input modules
An input circuit responds to an input signal in the following manner:
An input filter removes false signals due to contact bounce or electrical interference; Optical isolation protects the backplane circuits by isolating logic circuits from input signals; Logic circuits process the signal; An input LED turns on or off indicating the status of the corresponding input device.
The processor receives the input status for use in processing the program logic.

Troubleshooting Output modules
An output circuit controls the output signal in the following manner:
The processor determines the output status; Logic circuits maintain the output status. An output LED indicates the status of the output signal, Optical isolation separates logic and backplane circuits from field signals; The output driver turns the corresponding output on or off.