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Isochronous interrupt OB (OB 6x)
You use the isochronous mode interrupt OB (e.g., OB 61) for the isochronous connection of the user program.
Call of isochronous mode interrupt OB
The isochronous interrupt OB is triggered by the system clock, or more precisely by an adjustable delay time after the system clock. The effect is illustrated by the following figure:
| T_DC | Data cycle |
Delay time
STEP 7 initially automatically calculates a suitable value for the delay time. The delay time compensates the transfer time of the inputs of the isochronous IO devices to the IO controller through the PROFINET IO-network. The execution of OB 6x is linked to the system clock and the specified delay time. The delay time can also be manually corrected, if necessary. For example, you can shorten the delay time in order to process program sections that do not access the isochronous I/O at the start of OB 6x. This gains you runtime for the isochronous user program.
Connection of isochronous I/O to the isochronous mode interrupt OB
You access the isochronous I/O via a process image partition, i.e., the addresses of isochronous modules must be in a process image partition.
Any access to isochronous I/O must be programmed by the SYNC_PI and SYNC_PO instructions within the isochronous mode interrupt OB.
You call SYNC-PI at the start of OB 6x, provided you have selected the automatic setting for the delay time. You call SYNC_PO at the end of OB 6x.
T_DC, application cycle, and send clock
The time T_DC designates the time for one data cycle, i.e., this time is the shortest possible time for reading in, processing, and outputting the data (IPO model). T_DC is always a multiple of the send clock.
If IRT is activated for an IO device, T_DC is then equal to the send clock. The update time corresponds to the send clock.
The application cycle is a multiple of the data cycle. It can be identical to the data cycle if the runtime of the isochronous mode interrupt OB is correspondingly short.
Program processing according to the IPO model with short time
The IPO model with application cycle factor = 1 is characterized by the fact that the I/O data are completely processed within one system clock T_DC. This model is used to achieve the shortest response times.