Siemens SIMATIC ET 200AL - Basics of Programming

Functions
5.7 Isochronous mode
PROFINET with STEP 7 V15
Function Manual, 12/2017, A5E03444486-AH
201
The start of the I/O read cycle is advanced by the amount of a bias time T
I
in order to make
all input data available for transfer on the PROFINET IO line at the start of the next
PROFINET IO cycle. T
I
is the "flashlight" for the inputs; at this instant all synchronized inputs
are read in. T
I
is necessary in order to compensate for analog-to-digital conversion,
backplane bus times, and the like. The bias time T
I
can be configured by STEP 7 or by the
user. We recommend automatically assigning the bias time T
I
of STEP 7.
The PROFINET IO line transports the input data to the IO controller. The application can be
started synchronized to the cycle, i.e., the isochronous mode interrupt OB is called after a
parameterizable delay time. The user program in the isochronous mode interrupt OB decides
the process reaction and provides the output data in time for the start of the next data cycle.
The length of the data cycle is always configured by the user.
T
O
is the compensation arising from the backplane bus and the digital-to-analog conversion
within the IO device. T
O
is the "flashlight" for the outputs; at this instant the synchronized
outputs are output. The time T
O
can be configured by STEP 7or by the user. We recommend
automatically assigning the bias time T
o
of STEP 7.
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, howe
ver,
necessarily match the values supplied by "SYNC_PI" and "SYNC_PO".