Starter kit for ecomatController EC2121
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4.2
Time response
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4.2.1
Cycle time
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The time a controller needs for one cycle is called cycle time.
Typically, this time is in the ms range.
The cycle time is also the time an input signal has to be present to be reliably detected by the
controller.
If a signal is shorter, it depends on whether it happens to come at that point of time at which the inputs
are read. Only then will it be recognised.
With the ecomatController, the cycle time is determined by the configuration of the tasks.
It is recommended to define as few tasks as possible.
That means, usually one task is sufficient. You may need a second task for a bus system.
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4.2.2
Watchdog
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A PLC monitors itself if the cycle is correctly processed. The case that a program cycle is not complete
is dangerous. Then no inputs are read. A change of the state on the input, e.g. "limit switch reached"
cannot be processed.
The status of the output cannot change, e.g. the drive keeps on running although the limit switch has
been reached.
The watchdog function exists to avoid such states. If the end of the cycle has not been reached within
a certain time which is started with each new cycle, the watchdog becomes active. The controller is
stopped and the outputs are disconnected.
What can cause such an event?
1. Hardware error
If a processor fails, for example, the controller must pass into a safe state.
2. Program error
If loops or jumps are used in programming, it is possible that processing takes too long or in the
worst case a closed loop leads to the situation that the end of the cycle cannot be reached.
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4.2.3
Response time
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This value is often used to characterise the time response of a PLC.
This is the time that elapses from a change of an input signal to the corresponding response of the
output.
If an input signal changes only shortly after the PII has been written and is therefore not copied to the
PII before the following cycle, the output signal changes after two cycles.
In the worst case, the response time = 2 * cycle time.
