Siemens SIMATIC S7 System Manual page 514

Libraries
9.4 Open user communication library
For example, let us suppose that there is a TCP client that sends four 20-byte messages to
the CPU in rapid succession, and your program does not call the TCP_RECV instruction
during this time. When the program does call the TCP_RECV instruction after all four
messages have been accepted by the CPU, the TCP_RECV instruction returns this as one
receive message of 80 bytes. Your program is responsible for calling the TCP_RECV
instruction often enough to receive each message as it is sent.
When you create a connection using the ISO-on-TCP protocol, the protocol itself delineates
the messages. The TCP_RECV instruction receives and holds all messages sent from the
remote device as separate messages in the CPU, no matter when or how often the program
calls the TCP_RECV instruction.
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System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART