Siemens Simatic S7-400

Basic Principles of Serial Data Transmission
2
2.1 Serial Transmission of a Character
Introduction
The system provides various networking options for the exchange of data between two or more
communication partners. The simplest form of data interchange is via a point-to-point
connection between two communication partners.
Point-to-Point Communication
In point-to-point communication the communication processor forms the interface between a
programmable controller and a communication partner. Data are transmitted serially in the
point-to-point connection with the CP 440.
Serial Transmission
In serial transmission, the individual bits of each byte of information are transmitted one after
the other in a 󹪝xed order.
Drivers for Bidirectional Data Tra󹪟c
The CP 440 itself handles data transmission with communication partners via its serial interface.
The CP 440 is equipped with two di󹪜erent drivers for this purpose.
Bidirectional data tra󹪟c:
• ASCII driver
• 3964(R) procedure
The CP 440 executes data transfer via the serial interface depending on the selected driver.
Bidirectional Data Tra󹪟c - Operating Modes
The CP 440 has two operating modes for bidirectional data tra󹪟c:
• 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 󹪞ow 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.
Point-to-point connection CP 440 Installation and Parameter Assignment
Equipment Manual, 03/2022, A5E00057742-AE 17