Data Transmission With The 3964(R) Procedure - Siemens SIMATIC CP 440 Manual

Basic Principles of Serial Data Transmission
Note
When you run the ASCII driver in RS422 multipoint or RS485 mode, you must
take steps in the user program to ensure that only one node sends data at any
one time. If two nodes send data simultaneously, the message frame is corrupted.
2.2.3

Data Transmission with the 3964(R) Procedure

The 3964(R) procedure controls data transmission via a point-to-point connection
between the CP 440 and a communication partner. In addition to the physical layer
(layer 1), the 3964(R) procedure also incorporates the data link layer (layer 2).
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 codes:
STX
DLE
ETX
BCC
NAK
Note
If DLE is transmitted as an information string, it is sent twice so that it can be
distinguished from the control code DLE during connection setup and cleardown
on the send line (DLE duplication). The receiver then reverses the DLE
duplication.
2-24
Start of Text; Start of character string for transfer
Data Link Escape; Data connection escape
End of Text; End of character string for transfer
Block Check Character (only with 3964(R)); Block check
character
Negative Acknowledge; Negative acknowledgment
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02