Siemens SIMATIC S7 System Manual page 532

Libraries
9.4 Open user communication library
Network 23: Call the TCP_RECV instruction to get any messages that the CPU has
received.
Network 24: If Done is TRUE, the instruction received new data, so go to the "Receive
Check" state.
If Busy is TRUE, stay in the "Receive" state until you reach the receive timeout value. If you
time out in this state, disconnect the device and then reconnect it.
If Error is TRUE, check the error code to determine what to do next.
Network 25: State "RecvCheck"...
Process the data returned from the passive device.
Network 26: This program only checks the returned data to ensure that you received the
same amount of data as was sent and that the first word matches the "Fill Pattern". Log
whether the response was good or bad, and then go to the "Idle" state to wait to send the
next message.
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System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART