Page 340 - Programmable Logic Controllers, Fifth Edition - Mobile version

Data highway (DH) network

Figure 14-33 Peer-to-peer network.

• No communication takes place without the master • Each device receives the packet information and
initiating it. uses it, if needed.
• Direct communication among slave devices is not • Any additional information that the node has will be
possible. sent in a new packet.
• Information to be transferred between slaves must There are two methods of transmitting PLC digital
be sent first to the network master unit, which will, data: parallel and serial transmission. In parallel data
in turn, retransmit the message to the designated transmission, all bits of the binary data are transmitted
slave device. simultaneously, as illustrated in Figure 14-34. Parallel
• Master/slave networks use two pairs of conductors. transmission of data can be summarized as follows:
One pair of wires is used for the master to transmit
data and the slave to receive them. On the other pair, • Eight transmission lines are required to transmit
the slaves transmit and the master receives. the 8-bit binary number.
• Each bit requires its own separate data path
A peer-to-peer network has a distributive means of and all bits of a word are transmitted at the
control, as opposed to a master/slave network in which same time.
one node controls all communications originating from
other nodes. The Allen-Bradley Data Highway, shown in • Parallel data transmission is less common but faster
Figure 14-33, is an example of a peer-to-peer network of than serial transmission.
programmable controllers and computers linked together In serial transmission one bit of the binary data is
to form a data communication system. The operation of transferred at a time, as illustrated in Figure 14-35. Serial
the network can be summarized as follows: transmission of data can be summarized as follows:
• Peer-to-peer networks can use token passing or • In serial transmission, bits are sent sequentially
Ethernet collision detection. on the same channel (wire) which reduces costs
• Each device has the ability to request use of, and for wire but also slows the speed of transmission.
then take control of, the network for the purpose of
transmitting information to or requesting informa- 1
tion from other network devices. 0
• Each device is identified by an address. 1
Receiving 0 Transmitting
• When the network is operating, the token passes side 1 side
from one device to the next sequentially. 1
0
• The device that is transmitting the token also knows 1
the address of the next station that will receive
the token. Figure 14-34 Parallel data transmission.

Process Control, Network Systems, and SCADA Chapter 14 321

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