Siemens SIMATIC S7 System Manual page 517

The instruction displays the Done or Error status for one call of UDP_SEND after the send
operation is complete. After that, the UDP_SEND responds with error code 24, which means
no operation pending, if called with the Req input set 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.
①
②
③
④
⑤
⑥
⑦
The maximum amount of data that you can send in one send operation is 1024 bytes. The
program copies the data from the send buffer in user memory to an internal buffer when the
UDP_SEND executes with the Req input set to TRUE. You can change the program send
buffer after the UDP_SEND executes and the instruction sets the Busy output.
The UDP_SEND instruction requires the IP address and port number on the remote device.
When the UDP_CONNECT created the connection, the local port was set. The IP address
(IPaddrx) and the remote port number (RemPort) are subject to the same rules and
restrictions as described earlier. (Refer to "Parameters common to the OUC library
instructions" (Page 499) for these rules.)
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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