Introduction.

Workshop PG5
Program structures
5.5 Exception Block (XOB)
What’s a XOB:
The exception blocks are treated automatically once a special event occurs. These
events are defined by the firmware, and they cannot be modified by the user. It’s
up to the user whether he want’s to program these events or not.
START-UP PCD
XOB 16
The moment an event occurs,
the PLC stops the task it is
working on, and it starts to
treat the XOB which corresponds
to the event.
When a XOB is
called, the current
program is
stopped.
Once the XOB is
finished, the
program continues:
COB 2
XOB 20
Once the PLC has finished the
XOB, it returns to the program
point where the event appeared
COB 5
If an event appears but there is no program in the corresponding XOB, then the
error LED on the PCD is lit and the PLC continues to run.
Example:
Turn on your PCD, take out the battery and the error LED will come up. If there
was a XOB 2 ( see table on the next page ) in your program, then the LED won’t
come up and the XOB 2 is executed instead.
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