Siemens SIMATIC S7 System Manual page 538

Libraries
9.4 Open user communication library
Network 4: Start the connection process. This is a server so set the Active input FALSE. Set
the IPaddr inputs to zero so that the server accepts a connection from any address. Set the
RemPort to zero because you do not need it for a server connection. Call the
TCP_CONNECT instruction with the Req input TRUE to start the connection process.
Network 5: If Done is TRUE, the CPU established the connection so go to the "Idle" state.
If Busy is TRUE, go to the "Connect Wait" state to wait for the connection to be established.
If Error is TRUE, there is probably an error with the input parameters, so check to see what
state to go to next.
In all cases, exit the state machine for this scan. The program continues to the next state on
the next scan.
Network 6: State "Connect Wait"...
Wait in this state until the active partner creates a connection to this CPU.
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System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART