Data Transmission With The 3964(R) Procedure; Principle Of The Data Transmission With The 3964(R) Procedure; Control Characters Of The 3964(R) Procedure - Siemens Simatic S7-300 CP 341 User Manual

Basic Principles of Serial Data Transmission

2.4 Data Transmission with the 3964(R) Procedure

2.4 Data Transmission with the 3964(R) Procedure
2.4.1

Principle of the Data Transmission with the 3964(R) Procedure

The 3964(R) procedure controls the data transmission in a point-to-point connection
between the communication processor and a communication partner. As well as the physical
layer (layer 1), the 3964(R) procedure also incorporates the data-link layer (layer 2).
2.4.2

Control characters of the 3964(R) procedure

Control characters
During data transmission, the 3964(R) procedure adds control characters to the user data
(data-link layer). These control characters allow the communication partner to check whether
the data has arrived complete and without errors.
The 3964(R) procedure analyzes the following control characters:
● STX Start of text;
Start of character string to be transmitted
● DLE Data Link Escape;
Data transmission switchover
● ETX End of Text;
End of character string to be transmitted
● BCC Block check character (3964R only);
Block check character
● NAK Negative Acknowledge;
negative acknowledgment
Note
If DLE is transmitted as an information character, it is sent twice (DLE duplication) so that
it can be distinguished from the DLE control character on the send line for connection
setup and release. The receiver then reverses the DLE duplication.
Priority
With the 3964(R) procedure, one communication partner must be assigned a higher priority
and the other partner a lower priority. If both communication partners issue a send request at
the same time, the partner with the lower priority will defer its send request.
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CP 341 Point-to-Point Communication, Installation and Parameter Assignment
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Manual, 09/2008, A5E02191071-01