Understanding The Basics Of S7-200 Network Communications - Siemens Simatic S7-200 System Manual

S7-200 Programmable Controller System Manual

Understanding the Basics of S7-200 Network Communications

Using Master and Slave Devices on a Network
The S7-200 supports a master-slave network and can function as either a master or a slave in a network,
while STEP 7–Micro/WIN is always a master.
Tip
If you use Windows NT and a PC/PPI cable, no other master can be present on the network.
Masters
A device that is a master on a network can initiate a request to another device on the network. A master
can also respond to requests from other masters on the network. Typical master devices include
STEP 7–Micro/WIN, human-machine interface devices such as a TD 200, and S7-300 or S7-400 PLCs.
The S7-200 functions as a master when it is requesting information from another S7-200 (peer-to-peer
communications).
Tip
A TP070 will not work on a network with another master device.
7
Slaves
A device that is configured as a slave can only respond to requests from a master device; a slave never
initiates a request. For most networks, the S7-200 functions as a slave. As a slave device, the S7-200
responds to requests from a network master device, such as an operator panel or STEP 7–Micro/WIN.
Setting the Baud Rate and Network Address
The speed that data is transmitted across the network is the baud rate, which is typically measured in
units of kilobaud (kbaud) or megabaud (Mbaud). The baud rate measures how much data can be
transmitted within a given amount of time. For example, a baud rate of 19.2 kbaud describes a
transmission rate of 19,200 bits per second.
Every device that communicates over a given
network must be configured to transmit data at
the same baud rate. Therefore, the fastest baud
rate for the network is determined by the
slowest device connected to the network.
Table 7-1 lists the baud rates supported by the
S7-200.
The network address is a unique number that
you assign to each device on the network. The
unique network address ensures that the data
is transferred to or retrieved from the correct
device. The S7-200 supports network
addresses from 0 to 126. For an S7-200 with
two ports, each port can have a network
address. Table 7-2 lists the default (factory)
settings for the S7-200 devices.
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Table 7-1 Baud Rates Supported by the S7-200
Network
Standard Network
Using an EM 277
Freeport Mode
Table 7-2 Default Addresses for S7-200 Devices
S7-200 Device
STEP 7–Micro/WIN
HMI (TD 200, TP, or OP)
S7-200 CPU
Baud Rate
9.6 kbaud to 187.5 kbaud
9.6 kbaud to 12 Mbaud
1200 baud to 115.2 kbaud
Default Address
0
1
2