Siemens SIMATIC CP 440 Manual page 21

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 while
data is being received or received while data is being sent.
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. Each communication partner must be able to
send and receive simultaneously.
Only half-duplex mode can be used with an RS 485 (2-wire) setting.
Asynchronous Data Transmission
With the CP 440, serial transmission takes place asynchronously. What is known
as timebase synchronization (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 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
two communication partners before serial transmission can take place. These
include:
Transmission rate (baud rate)
Character and acknowledgment delay times
Parity
Number of data bits
Number of stop bits
Section 2.2 describes the importance of the declarations in the various
transmission procedures, and how they are parameterized.
Point-to-point connection CP 440 Installation and Parameter Assignment
A5E00057742-02
Basic Principles of Serial Data Transmission
2-3