Basic Principles of Serial Data Transmission
2.1 Serial Transmission of a Character
PtP coupling and configuration of CP 340
24 Manual, 04/2011, A5E00369892-03
Bidirectional Data Traffic - Operating Modes
The CP 340 has two operating modes for bidirectional data traffic:
● Half-duplex operation (3964(R) procedure, ASCII driver)
Data are 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 are exchanged between two or more communication partners in both directions
simultaneously. In full-duplex mode, 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.
You can choose between half-duplex operation (RS 485) and full-duplex operation (RS 422)
when using the CP 340-RS 422/485 module variant.
Asynchronous Data Transmission
With the communications processor, serial transmission occurs asynchronously. The so-
called timebase 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
● Number of stop bits
● Number of setup and transmission attempts permitted
Chapters "Transmission integrity (Page 30)" and "Transmission mode in Point-to-Point
Comm
unicati
on (Page 28)" describe the role the declaration
s play in the various
transmission procedures, and how they are parameterized.
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