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Basic Principles of Serial Data Transmission
2.1 Serial Transmission of a Character
Bidirectional Data Traffic - Operating Modes
The CP 440 has two operating modes for bidirectional data traffic:
● Half-duplex operation (3964(R) procedure, ASCII driver)
Data is exchanged between the communication partners but only in one direction at a
time. In half-duplex operation, therefore, at any one time data is being either sent or
received. The exception to this may be individual control characters for data flow control
(e.g. XON/XOFF), which can also be sent during a receive operation or received during a
send operation.
● Full-duplex operation (ASCII driver)
Data is exchanged between two or more communication partners in both directions
simultaneously. In full-duplex operation, therefore, data can be sent and received at the
same time. Every communication partner must be able to operate a send and a receive
facility simultaneously.
Only half-duplex mode can be used with an RS 485 (2-wire) setting.
Asynchronous Data Transmission
With the communication processor, serial transmission occurs asynchronously. The so-
called time base synchronism (a fixed timing code used in the transmission of a fixed
character string) is only upheld during transmission of a character. Each character to be sent
is preceded by a synchronization impulse, or start bit. The length of the start-bit transmission
determines the clock pulse. The end of the character transmission 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 transmission can take place. These include:
● Transmission speed (baud rate)
● Character and acknowledgment delay times
● Parity
● Number of data bits and
● Number of stop bits
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Point-to-point connection CP 440 Installation and Parameter Assignment
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Manual, 09/2007, A5E00057742-03
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