Siemens Simatic S7-300 CP 341 User Manual page 30

Basic Principles of Serial Data Transmission
2.1 Serial Transmission of a Character
Bidirectional data traffic - operating modes
The communication processor has two operating modes for bidirectional data traffic:
● Half-duplex mode (3964(R) procedure, ASCII driver, RK 512)
Data is exchanged between one or more communication partners in both directions
alternately. Half-duplex mode means that data is either being sent or received at any
given moment. The exception to this may be individual data flow control characters
(e.g., XON/XOFF), which can also be sent during a receive operation or received during a
send operation.
● Full-duplex mode (ASCII driver)
Data is exchanged between one or more communication partners in both directions
simultaneously; both sending and receiving can take place at the same time. Every
communication partner must be able to handle send and receive operations
simultaneously.
With an RS 485 (2-wire) setting, the X27 (RS 422/485) interface module can only be run in
half-duplex mode.
Asynchronous data transmission
With the CP 341, serial data transmission occurs asynchronously. The so-called time grid
synchronism (a fixed time grid for transmission of a fixed character string) is only maintained
during transmission of a character. Each character to be sent is preceded by a
synchronization pulse, or start bit. The length of the start-bit transmission determines the
clock pulse. The end of the character transfer is signaled by the stop bit.
Declarations
As well as the start and stop bits, further declarations must be made between the sending
and receiving partners before serial data transmission can take place. These include:
● Transmission speed (baud rate)
● Character and acknowledgment delay times
● Parity
● Number of data bits
● Number of stop bits
Sections "Configuration data (Page 79)" and "Transmission Procedure with a Point-to-Point
Connection (Page 33)" describe the role the declarations play in the various transmission
procedures, and how they are parameterized.
30
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CP 341 Point-to-Point Communication, Installation and Parameter Assignment
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Manual, 09/2008, A5E02191071-01