Siemens SIMATIC S7 System Manual page 511

The instruction displays the Done or Error status for one call of TCP_SEND after the send
operation is complete. After that, the TCP_SEND responds with error code 24, which means
no operation pending, if called with the Req input set to FALSE. If the Req input is left set to
TRUE, the program initiates another send operation. The figure below shows the relationship
of the input and output parameters.
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S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Req is set TRUE so that the message send begins. Busy is set TRUE.
The message send is complete. Done is set, and Busy is cleared.
EN is TRUE, and Req is FALSE, but no message send is in progress. So, Error is set with
error code 24.
Req is set TRUE again, so another message send begins. Busy is set TRUE.
The message send is complete. Done is set, and Busy is cleared for one scan.
Req remains TRUE, so another message send begins.
The message send is complete.
9.4 Open user communication library
Libraries
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