Thursday, June 17, 2010

Interface between Operating System and Interrupt Service Routine of PLC 135 WB

PLC 135 WB has interrupted processing capabilities. In this mode, the cyclic program in interrupted and an interrupt service routine executed. Once the interrupt service routine has terminated, the processor returns to the point of interruption and resumes execution of the cyclic program.

The interrupt service routine is initiated in two different ways:
• I/Os causing interrupt (OB 2).
• Signal state change of selected input bit (edge-controlled) (OB 3).

OB 2 and OB 3 constitute the interface between the operating system and the interrupt service routines.

Organization block OB 2
OB 2, the block with the highest priority, is called by interrupts of the process I/Os causing the interrupt. It can only be processed in special mode. The alarms are retained in flag bytes FY 8 to FY 10 and must be acknowledged by the user.

Organization block OB3
OB 3 is always invoked when the signal state of a bit in up to four interrupt input bytes changes. The user may select the input bytes and default it via machine data (PLC MD 124 to 127).

The system program checks the interrupt bytes every 10 ms, and invokes an interrupt service routine when required. OB 3, however, is only processed once per 10 ms scan, even, if several signal state change occur.

OB 3 can be processed in normal or special mode and has a lower priority than OB 2, e.g.
OB 3 can not interrupt OB2.

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