Siemens SIMATIC S7 System Manual page 541

Network 15: Send the data back to the partner.
Network 16: If Done is TRUE, the send is complete (probably 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 when the send
is complete. Be sure to use the same send length and buffer pointer that you used to
initialize the send request.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
9.4 Open user communication library
Libraries
541