Network Communication Protocols - Siemens SIMATIC S7-200 System Manual

Network Communication Protocols

The S7-200 CPUs support a variety of communication capabilities. Depending on
the S7-200 CPU that you use, your network can support one or more of the
following communication protocols:
Point-to-Point Interface (PPI)
Multipoint Interface (MPI)
PROFIBUS
These protocols are based upon the Open System Interconnection (OSI)
seven-layer model of communications architecture. The PPI and MPI protocols are
implemented on a token ring network which conforms to the Process Field Bus
(PROFIBUS) standard as defined in the European Standard EN 50170.
These protocols are asynchronous, character-based protocols with one start bit,
eight data bits, even parity, and one stop bit. Communication frames depend upon
special start and stop characters, source and destination station addresses, frame
length, and a checksum for data integrity. The three protocols can run on a
network simultaneously without interfering with each other as long as the baud rate
is the same for each of them.
The PROFIBUS network uses the RS-485 standard on twisted pair cables. This
allows up to 32 devices to be connected together on a network segment. Network
segments can be up to 1,200 m (3,936 ft.) in length, depending on the baud rate.
Network segments can be connected with repeaters to allow more devices on a
network and greater cable lengths. Networks can be up to 9,600 m (31,488 ft.) with
up to 9 repeaters (depending on the baud rate). (See Table 7-7.) For more
information about the EM 277 PROFIBUS-DP module, see the product
specifications in Appendix A.
The protocols define two types of network devices: masters and slaves. Master
devices can initiate a request to another device on the network. Slave devices can
only respond to requests from master devices. Slaves never initiate a request on
their own.
The protocols support 127 addresses (0 through 126) on a network. There can be
up to 32 master devices on a network. All devices on a network must have
different addresses in order to be able to communicate. SIMATIC programming
devices and PCs running STEP 7-Micro/WIN 32 have the default address of 0.
Operator panels such as the TD 200, OP3, and the OP7 default to address 1. The
programmable controllers have the default address of 2.
S7-200 Programmable Controller System Manual
A5E00066097-02
Setting Up Communications Hardware and Network Communications
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