Profibus Protocol; User-Defined Protocols (Freeport) - Siemens SIMATIC S7-200 System Manual

PROFIBUS Protocol

The PROFIBUS protocol is designed for high-speed communications with
distributed I/O devices (remote I/O). There are many PROFIBUS devices available
from a variety of manufacturers. These devices range from simple input or output
modules to motor controllers and programmable controllers.
PROFIBUS networks usually have one master and several slave I/O devices. The
master device is configured to know what types of I/O slaves are connected and at
what addresses. The master initializes the network and verifies that the slave
devices on the network match the configuration. The master writes output data to
the slaves and reads input data from them continuously. When a DP master
configures a slave device successfully, it then owns that slave device. If there is a
second master device on the network, it has very limited access to the slaves
owned by the first master.
For information about the EM 277 PROFIBUS-DP module and how to use it, see
the product specifications in Appendix A.

User-Defined Protocols (Freeport)

Freeport communication is a mode of operation through which the user program
can control the communication port of the S7-200 CPU. Using Freeport mode, you
can implement user-defined communication protocols to interface to many types of
intelligent devices.
The user program controls the operation of the communication port through the
use of the receive interrupts, transmit interrupts, the transmit instruction (XMT) and
the receive instruction (RCV). The communication protocol is controlled entirely by
the user program while in Freeport mode. Freeport mode is enabled by means of
SMB30 (port 0) and is only active when the CPU is in RUN mode. When the CPU
returns to STOP mode, Freeport communications are halted and the
communication port reverts to normal PPI protocol operation. See Section 9.15,
SIMATIC Communications Instructions in Chapter 9 for a description of the
Transmit and Receive instructions.
S7-200 Programmable Controller System Manual
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Setting Up Communications Hardware and Network Communications
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