Siemens SIMATIC S7 System Manual page 205

Start and End conditions for the Receive instruction
The Receive instruction uses the bits of the receive message control byte (SMB87 or
SMB187) to define the message start and end conditions.
Note
If there is traffic present on the communications port from other devices when the Receive
instruction is executed, the receive message function could begin receiving a character in
the middle of that character, resulting in a possible parity or framing error and termination of
the receive message function. If parity is not enabled the received message could contain
incorrect characters. This situation can occur when the start condition is specified to be a
specific start character or any character, as described in item 2 and item 6 below.
The Receive instruction supports several message start conditions. Specifying a start
condition involving a break or an idle line detection avoids the problem of starting a message
in the middle of a character by forcing the receive message function to synchronize the start
of the message with the start of a character before placing characters into the message
buffer.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
Program instructions
7.3 Communication
205