Siemens SIMATIC S7 System Manual page 208

Program instructions
7.3 Communication
Any character:
6.
any and all characters and placing them in the message buffer. This is a special case of
the idle line detection. In this case the idle line time (SMW90 or SMW190) is set to zero.
This forces the Receive instruction to begin receiving characters immediately upon
execution.
Setup: il = 1, sc = 0, bk = 0, SMW90/SMW190 = 0, SMB88/SMB188 = don't care
Starting a message on any character allows the message timer to be used to time out the
receiving of a message. This is useful in cases where Freeport is used to implement the
master or host portion of a protocol and there is a need to time out if no response is
received from a slave device within a specified amount of time. The message timer starts
when the Receive instruction executes because the idle line time was set to zero. The
message timer times out and terminates the receive message function if no other end
condition is satisfied.
Setup: il = 1, sc = 0, bk = 0, SMW90/SMW190 = 0, SMB88/SMB188 = don't care, c/m = 1,
tmr = 1, SMW92 = message timeout in milliseconds
208
The Receive instruction can be configured to immediately start receiving
System Manual, V2.3, 07/2017, A5E03822230-AF
S7-200 SMART