8 Operation as state machine
In the previous chapters, the PLC functions were introduced as a sequence of various instruc-
tions. In addition, a state machine sequence (also referred to as finite state machine) can be
integrated by the specified instruction types. A state machine is often used for representing
sequences schematically and for easier implementation of solutions.
In order to realize a state machine sequence, the jump functions are of particular importance.
The jump functions are required for changing the state. Inputs 1 and 2 of the jump function are
used for checking the condition for the transition. Inputs 3 and 4 set the input buffer and write
the output buffer. In the state machine, inputs 3 and 4 are generally set to TRUE at the jump
functions for this reason in order to update the changing signals for changing the state.
8.1 Example of a controller
Example:
A lifting gear with two positions ("up" and "down") is to be controlled by the function table. The
target position is defined via a toggle switch. Each position is equipped with an initiator which
informs the frequency inverter that the target has been reached. As soon as the position is
reached, the frequency inverter is to stop and the respective LED "top" or "bottom" is to be
switched on. As soon as the drive starts again, the LED is turned off.
Both positions are provided with a door which can be opened manually by the user. As soon as
one of the two doors is open, the warning lamp "top" or "bottom" is pulsed on and off at an
interval of 100 ms. Note that the "door open" signals from the two doors are connected in
series.
Initiator top
Initiator bottom
Ho
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