Once we have modeled the system we wish to diagnose, we need to identify test points within the system.

 

These test points typically occur at the outputs of components or functional groups.

 

For example a functional test could be:

 

Do both the hoist brakes release when the hoist command is given?

 

Obviously this test consists of two visual checks one for each of the hoist brakes.

 

A components specific output could also be tested, for example:

 

Test for 380V at the hoist brake motor terminals when the hoist command is given.

 

Tests are sometimes placed at a different point in the model to where the actual test occurs.

 

For example:

 

Check if PLC output 22 LED is on. Could be at PLC output 22.

 

Physically we are not actually checking output 22 of the PLC for a voltage, but are relying on the LED to be a reliable indication of its status.