Siemens SIMATIC S7 System Manual page 530

Libraries
9.4 Open user communication library
Network 15: Send the new message to the passive device. Set Req to TRUE to initiate the
new send operation.
Network 16: If Done is TRUE, the send is complete (probably will never happen this quickly),
so go to the "Receive" state on the next scan.
If Busy is TRUE (this is the normal situation), go to the "Transmit Wait" state to wait for the
transmit to finish.
If Error is TRUE, check the reason, and you may need to change state if there is a
connection issue.
Network 17: State "Transmit Wait"...
Wait in this state until the transmit is complete.
Network 18: Call the TCP_SEND instruction with Req = FALSE to determine if the send is
complete. Be sure to use the same send length and buffer pointer that you used to initialize
the send request.
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System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART