Program execution

The central unit microprocessor ensures the cyclic execution of the system as shown in

Figure 1-4.

The internal processing:

– PLC monitoring and control

– and processing requests from the terminal operator,

is executed in parallel with the previously described cycle.

The main program is processed sequentially. It may call up to a maximum of 12 sub-programs.

Each sub-program may be called numerous times in the main program.

Three types of interruptions may be executed parallel to the main program:

– A cyclic interruption

– A warning interruption triggered by an event on the I 62.03 input

– A warning interruption triggered by an event on the I 62.02 input

The interruptions have priority over the main program execution. If all three interruptions are

triggered simultaneously then the interruption triggered by I 62.03 has priority over the I 62.02

input interruption which in turn has priority over the cyclic interruption. Once an interruption has

been launched it cannot be interrupted by another (Figure 1-5).

The execution duration of a cycle (bus cycle + program cycle) is controlled by the central unit.

Any excess of the cycle time defined by the user in AC31GRAF is signaled by the ERR Led, at

the front of the central unit, as of the first program cycle.