Siemens SIMATIC ET 200AL, System Manual

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Description
2.1 Introduction to PROFINET
PROFINET with STEP 7
Function Manual, 05/2021, A5E03444486-AL
25
Update time
The update time is a time interval. IO controller and IO device/I-device exchange IO data
cyclically in the IO system within this time interval. The update time can be configured
separately for each IO device and determines the interval at which output data is sent from
the IO controller to the IO device (output module/submodule) as well as input data from the
IO device to the IO controller (input module/submodule).
STEP 7 calculates the update time automatically in the default setting for each IO device of
the PROFINET IO system, taking into account the volume of data to be exchanged as well as
the set send clock.
For additional information on the update time, refer to section Real-time communication
(Page 1).
Watchdog time
The watchdog time is the time interval that an IO controller or IO device permits, without
receiving IO data. If the IO device is not supplied by the IO controller with data within the
watchdog time, the IO device detects the missing frames and outputs substitute values. This
is reported in the IO controller as a station failure.
In STEP 7, the watchdog time is made up from an integral multiple of the update time and
can be set by the user.
Send clock
The period of time between two consecutive communication cycles. The send clock is the
shortest possible interval in data exchange and thus also the smallest value that can be set
for the update time.
Relationship between the update time and send clock
The calculated update times are reduction ratios (1, 2, 4, 8, ..., 512) of the send clock. The
minimum possible update time thus depends on the minimum send clock of the IO controller
that can be set and the efficiency of the IO controller and IO device. Depending on the send
clock, it can be that only some of the reduction ratios are available (STEP 7 guarantees this
through a pre-selection).
The following tables illustrate the dependency of the update time that can be set on the send
clock, using an example of the CPU 1516-3 PN/DP. The update times satisfy the requirements
of the PROFINET standard IEC 61158.
Table 2- 2 With real-time communication the following applies:
Send clock Update time Reduction ratios
250 μ s 250 μ s to 128 ms 1,2, ..., 512
500 μ s 500 μ s to 256 ms 1,2, ..., 512
1 ms 1 ms to 512 ms 1,2, ..., 512
2 ms 2 ms to 512 ms 1,2, ..., 256
4 ms 4 ms to 512 ms 1,2, ..., 128
Additional information
For information on real-time communication, refer to the section Real-Time Communication
(RT) (Page 1).