Troubleshooting II

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.