Siemens S7-1200 System Manual page 682
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Communication processor and Modbus TCP
12.3 Point-to-point (PtP) communication
Sample configuration - end message with a character sequence
Consider the following end message condition configuration:
In this case, the end condition is satisfied when two consecutive 0x7A characters are
received, followed by any two characters. The character preceding the 0x7A 0x7A pattern is
not part of the end character sequence. Two characters following the 0x7A 0x7A pattern are
required to terminate the end character sequence. The values received at character
positions 4 and 5 are irrelevant, but they must be received to satisfy the end condition.
Specification of message length within the message
When you select the special condition where the message length is included in the message,
you must provide three parameters that define information about the message length.
The actual message structure varies according to the protocol in use. The three parameters
are as follows:
● n: the character position (1-based) within the message that starts the length specifier
● Length size: The number of bytes (one, two, or four) of the length specifier
● Length m: the number of characters following the length specifier that are not included in
the length count
The ending characters do not need to be contiguous. The "Length m" value can be used to
specify the length of a checksum field whose size is not included in the length field.
These fields appear in the Receive message configuration of the device properties:
682
S7-1200 Programmable controller
System Manual, 03/2014, A5E02486680-AG
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