The PLC not only requires electronic components to operate, it also needs a software program.
The PLC programmer is not limited to writing software in one format. There are many types
available, each lending itself more readily to one application over and above another. Typical is
the RLL type previously discussed. Other S/W programs include "C," State Language, and SFC
(Sequential Function Charts).
Regardless of which software is chosen, it will be executed by the PLC's CPU module. The
software can be written and executed with the processor in an online state (while the PLC is
actually running) or in the off-line state (whereby the S/W execution does not affect current
operation of the I/O base).
In the RLL software program, we find several types of programming elements and functions to
control processes both internal to the PLC (memory and register) as well as external (field)
devices