Technical data

Table Of Contents
Overview over the S7 Communication and the S7 Basic Communication
System Software for S7-300/400 System and Standard Functions - Volume 2/2
A5E00739858-01
20-9
SFCs for Internal Communication
Block Brief Description
SFC 72 I_GET Reading a variable of a communication partner without your having to
place a corresponding SFC on the communication partner. This
functionality is implemented in the communication partner of the
operating system.
SFC 73 I_PUT Writing a variable into a communication partner without your having to
place a corresponding SFC on the communication partner. This
functionality is implemented in the communication partner of the
operating system.
SFC 74 I_ABORT Aborting of an existing connection without data being transferred. The
corresponding connection resources are thus released again on both
ends.
Example Programs
Two example programs for the SFCs for S7 basic communication are supplied with
STEP 7. They are contained in the directories step7\examples\com_SFC1 and
step7\examples\com_SFC2.
Maximum User Data Length
The communication SFCs for non-configured S7 connections are integrated on all
CPUs of the S7-300 and S7-400.
It is guaranteed that 76 bytes of user data can be transferred with all SFCs
(parameter SD or RD). The exact number if found at each block.
Connection to the Communication Partner
With the communication SFCs for non-configured S7 connections, the connection
is established while the SFC is being executed. Depending on the value you assign
to the CONT input parameter, the connection either remains established or is
terminated on completion of the data exchange. This means that the
communication has the following characteristics:
The number of communication partners that can be reached one after the other
is higher than the number of communication partners that can be reached
simultaneously (the number depends on the specific CPU, see /70/, /101/).
If no connection can currently be established to a communication partner
because the connection resources (on the local CPU or on the communication
partner) are all being used, this is indicated in RET_VAL. You must then trigger
the job again later at a suitable point in time. There is, however, no guarantee
that later connection establishment will be successful. If necessary, check the
use of connection resources in your program and use a CPU with more
resources.