Introduction.

Workshop PG5
Program structures
5.1 Cyclic Organization Block (COB 0 to 15)
What’s a COB:
COB's (Cyclic organization blocks) are foreseen for program parts which are
executed nonstop, without ever waiting stopping. (For example in order to wait
for internal or external events to happen). After Startup of the PCD, the program
starts executing the first COB in the program file and then executes one COB after
the other. If needed you may create up to 16 COB’s. All the COB's are called
automatically one after the other in a never ending cycle.
All signals which need to be treated on a regular basis (for example end-switches for
motor movements, external power-cut or emergency-stop signals, human protection
devices ect) have to be inside a COB.
There has to be at least one COB in the PCD!
It is very important to understand, that the notion of the blocks which
are always executed, is the most important aspect of all PLC's. The
lack of waiting loops is not a failure in the concept of PLC programming,
but a very important security feature. It's the only way you can
guarantee your client that important signals are checked on a regular
basis.
If you write your programs with the “FUNCTION BLOCK EDITOR”, a new file in
a COB is automatically opened for you.
You can then change the block type or comment as you wish.
In instruction list programs, the block is defined by instructions which surround the
program code.
© SAIA-Burgess Controls AG Page 5-1