Automation System
Relay ladder logic is used in a PC because:

it is impossible to write a control program for a machine in any other logic
it is the best industrial control language ever developed.
All of these
most industrial control personnel are familiar with relay ladder logic diagrams and this is the easiest for them to use
None of these

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Automation System
There are several procedures you can implement to provide maximum security. The most important of these is:

Verify the reputation of the people installing your system
do not allow unbonded service people to work on your equipment
run regular simulated tests using TARP (testing of Applications Recovery Planes)
do not leave training manuals unsecured
None of these

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