Additional Information; Programming Data Transfer Via Tcp Connections - Siemens S7-300 Configuration Manual

SEND/RECEIVE interface in the user program

4.4 Additional information

FC calls slower than the transfer time
If a program block is called again before the data has been completely sent or received, the
reaction on the interface of the program blocks is as follows:
● AG_SEND / AG_LSEND / AG_SSEND:
The job is positively acknowledged; the Ethernet CP is ready to accept a new send job (at
the earliest, however, with the next call).
● AG_RECV / AG_LRECV / AG_SRECV:
The job is acknowledged with "New data accepted" if the data has been accepted in the
user program. Following this, the FC can be called again.
Note
Note that if there are different processing speeds (sender faster than receiver), resource
bottlenecks can occur at the send and receive end.
If this is the case, the sender receives feedback from the program blocks (condition code
"No receive resources on the target station"). (Not with AG_SRECV)
4.4 Additional information
4.4.1

Programming data transfer via TCP connections

Purpose of TCP connections
TCP connections should be used primarily to link third-party systems if these do not support
the protocol extension RFC1006.
For the communication between devices of the SIMATIC family, ISO-on-TCP connections
should be used because these are more convenient. The following section points out certain
special features.
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Configuring and commissioning S7 CPs for Industrial Ethernet
Configuration Manual, 09/2013, C79000-G8976-C182-13