Page 339 - Programmable Logic Controllers, Fifth Edition - Mobile version

Siemens Omron 3-to-1
1-to-2 2-to-3
Gateway Gateway

Bus network
Node Node Node
1 2 3
Gateway Gateway
Figure 14-31 Example of token passing.

Allen-Bradley Gould
Data highway Modbus Ethernet networks use a collision detection access
control scheme. With this access method, nodes listen
for activity on the network and transmit only if there are
Figure 14-30 Translating from one network-access
scheme to another. no other messages on the network. On Ethernet networks
there is the possibility that nodes will transmit data at the
same time. When this happens a collision is detected.
hardware, software, protocols, and network architecture Each node that had sent out a message will wait a random
that are needed for any two machines to communicate. amount of time and will resend its data if it does not detect
The actual connections between machines are physical, any network activity.
while the connections between layers in the model are The access method most often used in master/slave
logical. protocols is polling. The master/slave network is one
Gateways (Figure 14-30) make communication pos- in which a master controller controls all communications
sible between different architectures and protocols. They originating from other controllers. This configuration is
repackage and convert data going from one network illustrated in Figure 14-32 and consists of several slave
to another network so that the one can understand the controllers and one master controller. Its operation can be
other’s application data. Gateways can change the format summarized as follows:
of a message so that it will conform to the application
program at the receiving end of the transfer. If network- • The master controller sends data to the slave
access translation is their only function, the interfaces controllers.
are known as bridges. If the interface also adjusts data • When the master needs data from a slave, it will
formats or performs data transmission control, then it is poll (address) the slave and wait for a response.
called a gateway.
A bus topology network requires some method of con-
trolling a particular device’s access to the bus. An access Master controller
method is the manner in which a PLC accesses the net-
work to transmit information. Network access control
ensures that data are transmitted in an organized man-
ner preventing the occurrence of more than one message
on the network at a time. Although many access meth-
ods exist, the most common are token passing, collision
detection, and polling. Write Read
In a token passing network, a node can transmit data command command
on the network only when it has possession of a token. A
token is simply a small packet that is passed from node to
node as illustrated in Figure 14-31. When a node finishes
transmitting messages, it sends a special message to the
next node in the sequence, granting it the token. The token
passes sequentially from node to node, allowing each an
opportunity to transmit without interference. Tokens usu-
ally have a time limit to prevent a single node from tying Slave #1 Slave #2
up the token for a long period of time. Figure 14-32 Master/slave network.

320 Chapter 14 Process Control, Network Systems, and SCADA

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