4 Programmable Logic Controllers

Question 1

Explain the term cyclic execution.

Answer 1

There are four repeating steps:
– Internal Checks
– Reading Hardware Inputs
– Proram Execution
– Writing Hardware Outputs

Question 2

What are the two worst-case assumptions that are made when calculating the
maximum reaction time?

Answer 2

The two assumptions are that the input changes just after the inputs are read and
the maximum cycle time is reached.

Question 3

How are maximum cycle time and maximum reaction time linked?

Answer 3

Maximum reaction time ≈2· Maximum Cycle time

Question 4

What to do, if reaction time is too long?

Answer 4

▪ Optimize Program
▪ Choose more performant PLC
▪ Use interrupts

Question 5

How does the estimation of real-time behavior work?

Answer 5

▪ Determine longest program execution path.
▪ Measure or estimate cycle time for this execution path.
▪ Apply formula for maximal reaction time

Question 6

Idea behind the Function Block language (FB):

Answer 6

Idea:
▪ Resembling switching circuit design by gates
▪ blocks with input and output

Question 7

Idea behind the Ladder Diagram (LD)

Answer 7

Idea:
▪ Resembling switching circuit design by relays („schematics“)
▪ „contacts“ and „coils“

Question 8

What does the “set” command do?

Answer 8

set variable to 1

Question 9

What does the “reset” command do?

Answer 9

set variable to 0

Question 10

Name three different timer block functions:

Answer 10

▪ TON Timer On Delay
▪ TOF Timer Off Delay
▪ TP Timer Pulse