P L C

Programming Support

Program troubleshooting


There are severall causes off alteration to the user program:
 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 programmerís monitor, help trace the source of the fault to the userís input/output devices, wiring, or the controller.


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.


Power distribution


The master control relay must be able to inhibit all machines motion by removing power to the machine I/O devices when the relay is de-energized. The DC power supplly shoulld be powered directly from the fused secondary of the transformer. Power to the DC input, and output, circuits is connected through a set of master control relay contacts. Interrupt the load side rather the AC line power. This avoids the additional delay of power supply turn-on and turn-off.



Power LED


The POWER LED on the power supplly indicates that DC power is being supplied to the chassis. This LED could be off when incoming power is present when the:
Fuse is blown;
Voltage drops below the normal operating range;
Power supply is defective.

Safety Considerations


Actively thinking about the safety of yourself and others, as well as the condition of your equipment, is of primary importance.
When troubleshooting, pay carefull attention to these general warnings:
 Have all personnel remain clear of the controller and equipment when power is applied.
 The problem may be intermittent and sudden unexpected machine motion could result in injury.
 Have someone ready to operate an emergency-stop switch in case it becomes necessary to shut off power to the controller equipment.
 Never reach into a machine to actuate a switch since unexpected machine motion can occur and cause injury.
 Remove all electrical power at the main power disconnect switches before checking electrical connections or inputs/outputs causing machine motion.
 Never alter safety circuits to defeat their functions. Serious injury or machine damage could result.


Calling for assistance


If you need to contact manufacturer or local distributor for assistance, it is helpful to obtain the following (prior to calling):
 Processor type, series letter
 Processor LED status
 Processor error codes
 Hardware types in system (I/O modules, chassis)
 Revision of programming device (HHT or APS).


System documentation


The documentation is the main guide used by the users and for troubleshooting and fault finding with PLCs.
The documentation for a PLC installation should include:
 A description of the plant.
 Specification of the control requirements.
 Details of the programmable logic controller.
 Electrical installation diagrams.
 Lists of all inputs and outputs connections.
 Application program with full commentary on what it is achieving.  Software back-ups.
 Operating manual, including details of all start up and shut down procedures and alarms.


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