Siemens SIMATIC S7 System Manual page 531

Network 19: If Done is TRUE, the send is complete, so go to the "Receive" state.
If Busy is TRUE, stay in the "Transmit Wait" state.
If Error is TRUE, check the reason for the error and change the state if there is a connection
issue.
Network 20: State "Receive"...
Clean up the receive data area and prepare to receive the response.
Network 21: Clear the receive buffer and the "RecvLength" so that there is no data left from
the last received message.
Capture the current interval time value (in "RecvStartTime") to support a receive timeout and
then go to the "Receive Wait" state.
Network 22: State "Receive Wait"...
Stay in this state until you either receive some data or time out.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
9.4 Open user communication library
Libraries
531