Siemens SIMATIC S7 System Manual page 540

Libraries
9.4 Open user communication library
Network 11: If Done is TRUE, the CPU received new data so go to the "Receive Check"
state.
If Busy is TRUE, stay in the "Receive" state until you receive some data.
If Error is TRUE, check the error code to determine what to do next.
In all cases, exit the state machine for this scan. The program continues to the next state on
the next scan.
Network 12: State "Receive Check"...
Check and process the received data.
Network 13: In this program, you echo the data back to the partner. Copy all of the received
bytes to the send buffer.
Change the state to "Transmit", and exit until the next scan.
Network 14: State "Transmit"...
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System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART