Siemens SIMATIC S7 System Manual page 203

Receive data
①
Number of bytes received (byte field)
②
Start character
③
Message
④
End character
⑤
Characters of the message
If an interrupt routine is attached to the receive message complete event, the CPU generates
an interrupt (interrupt event 23 for port 0 and interrupt event 24 for port 1) after the last
character of the buffer is received.
You can receive messages without using interrupts by monitoring SMB86 (port 0) or
SMB186 (port 1). This byte is non-zero when the Receive instruction is inactive or has been
terminated. It is zero when a receive is in progress.
As shown in the following table, the Receive instruction allows you to select the message
start and message end conditions, using SMB86 through SMB94 for port 0 and SMB186
through SMB194 for port 1.
Note
The receive message function is automatically terminated in case of a framing, parity,
overrun, or break error.
You must define a start condition and an end condition (maximum character count) for the
receive message function to operate.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Program instructions
7.3 Communication
The Receive instruction lets you receive
a buffer of one or more characters, up to
a maximum of 255. The following figure
shows the format of the Receive buffer.
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