Siemens S7-1200 System Manual page 677

Message start conditions
You can determine how the communication interface recognizes the start of a message. The
start characters and the characters comprising the message go into the receive buffer until a
configured end condition is met.
You can specify multiple start conditions. If you specify more than one start condition, all of
the start conditions must be met before the message is considered started. For example, if
you configure an idle line time and a specific start character, the CM or CB will first look for
the idle line time requirement to be met and then the CM will look for the specified start
character. If some other character is received (not the specified start character), the CM or
CB will restart the start of message search by again looking for an idle line time.
Parameter
Start on Any Character
Line Break
Idle Line
Special condition:
Recognize message start
with single character
Special condition:
Recognize message start
with a character sequence
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG
Definition
The Any Character condition specifies that any successfully received character indicates the
start of a message. This character is the first character within a message.
The Line Break condition specifies that a message receive operation starts after a break
character is received.
The Idle Line condition specifies that a message reception starts once the receive line has
been idle or quiet for the number of specified bit times. Once this condition occurs, the start
of a message begins.
①
Characters
②
Restarts the idle line timer
③
Idle line is detected and message receive is started
Specifies that a particular character indicates the start of a message. This character is then
the first character within a message. Any character that is received before this specific
character is discarded. The default character is STX.
Specifies that a particular character sequence from up to four configured sequences
indicates the start of a message. For each sequence, you can specify up to five characters.
For each character position, you specify either a specific hex character, or that the character
is ignored in sequence matching (wild-card character). The last specific character of a
character sequence terminates that start condition sequence.
Incoming sequences are evaluated against the configured start conditions until a start
condition has been satisfied. Once the start sequence has been satisfied, evaluation of end
conditions begins.
You can configure up to four specific character sequences. You use a multiple-sequence
start condition when different sequences of characters can indicate the start of a message. If
any one of the character sequences is matched, the message is started.
Communication processor and Modbus TCP
12.3 Point-to-point (PtP) communication
677