Basics Of Programming - Siemens SIMATIC PROFINET Function Manual

Functions
5.7 Isochronous mode
T is the compensation arising from the backplane bus and the digital-to-analog conversion
O
within the IO device. T
outputs are output. The time T
automatically assigning the bias time T
Isochronous mode and non isochronous mode distributed I/O
It is possible to combine isochronous mode distributed I/O with non isochronous mode
distributed I/O on one IO controller.
5.7.5

Basics of Programming

Programming in the isochronous mode interrupt OBs
The isochronous part of the program is always programmed in the isochronous mode
interrupt OBs OB 61 to OB 64. As the isochronous mode interrupts are processed with high
priority, only the time-sensitive elements of the program should be processed in OB 6x. The
isochronous mode interrupt is called with a configured delay time.
Accessing isochronous I/O by calling system functions
The isochronous I/O are updated exclusively by calling the "SYNC_PI" and "SYNC_PO"
instructions and therefore in the corresponding process image partition.
Note
To prevent inconsistent data from being returned to OB6x, you should avoid using the
"DPRD_DAT" and "DPWR_DAT" instructions (direct data access) in the isochronous mode
OB.
Direct access to the I/O areas returns current values of the process that do not, however,
necessarily match the values supplied by "SYNC_PI" and "SYNC_PO".
The "SYNC_PI" and "SYNC_PO" instructions can update the process image partition only
within the permitted execution window. The execution window for the possible call of the
"SYNC_PI" and "SYNC_PO" instructions stretches from the end of the cyclical data
exchange on the PROFINET IO until the point in time before the end of T_DC at which the
outputs can still be copied on time. The data exchange must be started within this time
window. If the "SYNC_PI" and "SYNC_PO" instructions are not processed within the
execution window, the system functions will indicate this with a corresponding error
message. An image in which the execution window and the corresponding times are
displayed can be found in the next section.
Program execution models
Depending on the order of the "SYNC_PI" and "SYNC_PO instructions in OB6x and the
application cycle factor used, the program execution can be divided into two basic models:
● IPO model (read Inputs - Processing - write Outputs), CACF = 1
● IPO model (read Inputs - Processing - write Outputs), CACF > 1
186
is the "flashlight" for the outputs; at this instant the synchronized
O
can be configured by STEP 7or by the user. We recommend
O
of STEP 7.
o
PROFINET with STEP 7 V14
Function Manual, 09/2016, A5E03444486-AG