Page 341 - Programmable Logic Controllers, Fifth Edition - Mobile version

SLC 5/04 CPU
RUN FORCE
FLT DH+
BATT RS232
Receiving 1 0 1 0 1 1 0 1 Transmitting RUNREM PROG
side side
Communication
DH+ ports
network
Figure 14-35 Serial data transmission.

• Serial data can be transmitted effectively over much Figure 14-36 Data Highway network connection.
greater distances than can parallel data.
• Each data word in the serial transmission must be Serial Communication
denoted with a known start bit sequence followed
by the data bits that contain the intelligence of the Serial data communication is implemented using stan-
data transmission and a stop bit. dards such as RS-232, RS-422, and RS-485. The RS in
the standard’s name means recommended standard that
• An extra bit, termed a parity bit, may be used to specifies the electrical, mechanical, and functional char-
provide some error-detecting ability. acteristics for serial communications. Serial communica-

A duplex communication system is a system com- tion interfaces are either built into the processor module
posed of two connected devices that can communicate or come as a separate communications interface module,
with one another in both directions at the same time. A as illustrated in Figure 14-37. The simplest type of con-
half-duplex system provides for communication in both nection is the RS-232 serial port. The RS interfaces are
directions, but only one direction at a time (not simul- used to connect to devices such as vision systems, barcode
taneously). Half-duplex transmission is used for master/ readers, and operator terminals that must transfer quanti-
slave communications. Full-duplex transmission allows ties of data at a reasonably high rate between the remote
the transmission of data in both directions simultaneously device and the PLC. The RS-232 type of serial transmis-
and can be used for peer-to-peer communications. sion is designed to communicate between one computer
The different networking schemes replace traditional and one controller and is usually limited to lengths up
point-to-point hardwiring. Network control of systems to 50 feet. RS-422 and RS-485 serial transmission types
minimizes the amount of wiring that needs to be done. are designed to communicate between one computer and
With traditional wiring multiple wires from each device, multiple controllers, have a high level of noise immunity,
fed through control cabinets, often result in large wire and are usually limited to lengths of 650 feet (for RS-485)
bundles running through the system. Due to the sheer vol- or 1650 feet (for RS-422).
ume of wires, installation time is considerable and trouble-
shooting is complex. If a network is used all devices can DeviceNet
be directly connected to a single transmission media cable. DeviceNet is an open device-level network. It is relatively
High-speed industrial networking technologies offer a low speed but efficient at handling the short messages
variety of methods for connecting devices. PLC network
configurations may be either open or proprietary (vendor-
unique). Following is an overview of some of the indus-
trial communication technologies that play a critical role
in today’s control systems. RS-232 module Vision Barcode
Data Highway Processor system reader Operator
terminal
The Allen-Bradley Data Highway networks, Data Highway
Plus (DH+) and DH-485, are proprietary communica-
tions networks. They use peer-to-peer communication
implementing token passing. The medium is shielded
twisted pair cable. Figure 14-36 shows the DH+ network
connection for an SLC 5/04 controller. The three-pin
Phoenix connector is used to form the network transmis- Figure 14-37 Serial communication interface.
sion media. Source: Courtesy Siemens.

322 Chapter 14 Process Control, Network Systems, and SCADA

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