Siemens SIMATIC S7 System Manual page 515

For example, let us suppose this time there is an ISO-on-TCP client that sends four 20-byte
messages to the CPU in rapid succession. Assume also that your program does not call the
TCP_RECV instruction during this time. The ISO-on-TCP protocol delivers the four
messages during four subsequent calls to the TCP_RECV instruction (one message per
call). This happens because ISO-on-TCP has start and end markers in the protocol to
delineate the messages and separate them in the receiving device.
Table 9- 18
Parameter
EN
ConnID
MaxLen
DataPtr
Done
Busy
Error
Status
Length
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Parameters of the TCP_RECV instruction
Declaration Data type
IN BOOL
IN WORD
IN WORD
IN WORD
OUT BOOL
OUT BOOL
OUT BOOL
OUT BYTE
OUT WORD
9.4 Open user communication library
Description
Enable input
The Connection ID (ConnID) is the number of the
connection for this receive operation (defined
during the connect process).
The MaxLen is the maximum number of bytes to
accept (for example, the size of the buffer at
DataPtr (1 to 1024)).
The DataPtr is the pointer to where the receive
data should be stored. This is an S7-200 SMART
pointer to I, Q, M, or V memory (for example,
&VB100).
The instruction sets the Done output when the
receive operation is complete with no errors.
When the instruction sets the Done output, the
Length output is valid.
The instruction sets the Busy output while the
receive operation is in progress.
The instruction sets the Error output when the
receive operation is complete with an error. Refer
to "Open user communication library instruction
error codes" (Page 524) for further information.
The Status output shows the error code if the
instruction sets the Error output. Status is zero
(no error) if the instruction sets the Busy or Done
outputs.
The Length is the actual number of bytes re-
ceived. Only when the instruction sets the Done
or Error outputs is the Length valid. If the instruc-
tion sets the Done output, then the instruction
received the entire message. If the instruction
sets the Error output, the message was greater
than the buffer size (MaxLen) and has been trun-
cated.
Libraries
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