Siemens SIMATIC S7 System Manual page 505

You must not change the input parameters of the ISO_CONNECT while the instruction is
busy. The CPU requires this so that it knows that this is a continuation of the call that started
the connection process.
You assign the connection ID (ConnID) input to the connection and then use this ConnID to
reference this connection when sending, receiving, or disconnecting.
The Active input bit determines whether this is an active connection (Active set to TRUE) or
a passive connection (Active set to FALSE).
If this is an active connection (a client), then the S7-200 SMART CPU attempts to contact
and create a connection to the specified IP address and remote TSAP (RemTsap). The CPU
opens a local TSAP (LocTsap) to receive messages from the remote device.
When the Active input is FALSE, the S7-200 SMART CPU creates a passive (server)
connection. In this case, the CPU opens the requested local TSAP (LocTsap) and accepts
connection requests from a remote device. You should set the IP address to 0.0.0.0 if you
wish to accept a connection request from any remote IP address. If the IP address is non-
zero, the CPU accepts a connection request only from the specified IP address. The CPU
ignores the remote TSAP string (RemTsap) for passive connections, and the RemTsap can
be set to an empty string (for example, "").
You can call the ISO_CONNECT instruction anytime to determine the current status of a
connection. Set the Req input to FALSE and provide a valid connection ID (ConnID), and
ISO_CONNECT returns the following:
● Busy if the connect process is still progressing.
● Done if the connection is active and ready to send or receive.
● Error if the connection is not usable. Status contains one of the error codes to specify the
problem.
Note that active connections can require up to 30 seconds to determine if the remote device
will allow the connection or not. Passive connections show a Busy status until a remote
device attempts to connect to the CPU.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
9.4 Open user communication library
Libraries
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