Page 336 - Programmable Logic Controllers, Fifth Edition - Mobile version

Programmable logic controller

Printer

Serial cable
Point-to-point Serial Local area network (LAN)
communications messages
Serial data
communications

PLC

10:31 4801

Operator interface
Figure 14-24 Point-to-point serial communications link. Figure 14-25 Local area network (LAN) communication link.

Transmission media are the cable through which data
Figure 14-24 illustrates a point-to-point serial communi- and control signals flow on a network. The transmission
cations link. Serial communications is used with devices media used in data communications systems include
such as printers, operator workstations, motor drives, bar coaxial cable, twisted pair, or fiber optics (Figure 14-26).
code readers, computers, or another PLC. Serial communi- Each cable has different electrical capabilities and may be
cations interfaces are either built into the processor module more or less suitable to a specific environment or network
or come as separate modules. A serial module installed in type. Not all networks transmit information through
each controller is normally all that is required for two PLCs cable. Wireless Wi-Fi Ethernet networks, such as the DF1
of the same manufacturer to establish a point-to-point link. Radio Modem, communicate through radio waves, which
As control systems become more complex, they require are transmitted through the air. Fiber optic transmission
more effective communications schemes between the medium is becoming increasingly popular in PLC based
system components. A local area network or LAN is a applications. Compared with twisted pairs of wire or
system that interconnects data communications compo- coaxial cables, fiber optic cables have advantages such
nents within a limited geographical area, typically no more as their small size, no electromagnetic interference, and
than one or two miles. Essentially, a LAN is a private, long transmission ranges. Typically, a fiber optic cable
on-site communications system that allows communica- can carry a 1,000-megabaud signal 10 km. These types of
tion between computers, PLCs, robots, and the like. The cables have very large bandwidths so they are well-suited
rate at which characters can be transmitted along a com- to high-speed transmission in noisy environments.
munications line is dependent on the number of bits of In industrial applications, LANs have most often
binary information that can be sent at a given time. This been used as the communication system for distrib-
transfer of information is measured in bits per second, or uted control systems (DCS). Recall that a DCS system
baud. Figure 14-25 illustrates a LAN communication link.
Network communications supports communication among
multiple PLCs and other devices. PLC networks allow:
• Sharing of information such as the current state
of status bits among PLCs that may determine the Coaxial
action of one another.
• Monitoring of information from a central location.
Twisted pair
• Programs to be uploaded or downloaded from a
central location. Wireless system
• Several PLCs to operate in unison to accomplish a Fiber optic
common goal. Figure 14-26 Transmission media.

Process Control, Network Systems, and SCADA Chapter 14 317

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