N ETWORK I N T E R FA C E M ODULES
N ETWORK I N T E R FA C E M ODULES
Network interface modules (see Figure 8-34) allow a number of PLCs and other intelligent devices to communicate and pass PLC data over a high- speed local area communication network (see Chapter 18). Any device may interface with the network, because the network is not restricted to only products designed by the network’s manufacturer.
Nowadays, many third-party suppliers manufacture products that are com- patible with different PLC network environments. Among the most popular networks are:
• device-level bus networks (e.g., CANbus, Seriplex, etc.), which are
used by discrete devices • process field networks (e.g., Fieldbus and Profibus), which are used
by analog devices • Ethernet/IEEE 802.3 networks, used by PLC CPUs and computers • proprietary networks, which are widely used by large PLC manu-
facturers
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C HAPTER 8
Special Function I/O and Serial Communication Interfacing
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S ECTION 2
Components and Systems
Figure 8-34. (a) Mitsubishi’s MELSECNET/B interface and (b) Allen-Bradley’s CANbus
network interface.
A network interface module implements all of the necessary communication connections and protocols to ensure that a message is accurately passed along the network. In general, when a processor or other network device sends a message, its network interface transmits the message over the network at the network’s baud rate speed. The receiving network interface accepts the transmission, passes the information to the CPU, and if necessary, sends a command to the intended field device. As you will see in Chapter 18, the speed and protocol for the communication link varies depending on the network.
Depending on the network type and configuration, a network module can
be connected, at a distance of up to 10,000 feet, with 100 to 1000 devices (nodes). The communication media—twinaxial, coaxial, or twisted-pair— varies depending on the type of network. The different types of networks also utilize specific network interfaces. For example, a device-level CANbus network uses a CANbus-type interface. Chapter 19 provides more informa- tion on I/O bus networks. Figure 8-35 illustrates a typical configuration of
a PLC network using the different types of network interface modules.
C o u rt e s y o f M it s u b is h i E le c tr o n ic s , M o u n t P ro s p e c t, I L
C o u rt e s y o f A lle
n -B
ra
d le y , H ig h la n d H
e ig h ts , O H
(a)
(b)
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Serial Communication Interfacing 8
Network Module
Local Area Network
Interface Fieldbus Interface
LAN
CANbus Network
Smart Discrete I/O Devices
Fieldbus Network
Smart Process Field Devices
(b)
Figure 8-35. (a)
A standard PLC local area network and (b) a PLC local area network with CANbus (device bus) and Fieldbus (process bus) subnetworks.
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8 -6 F UZZY L OGIC I N T E R FA C E S
Fuzzy logic interfaces , which are offered by a few PLC manufacturers, provide a way of implementing fuzzy logic algorithms in PLCs. Fuzzy logic
algorithms analyze input data to provide control of a process. As shown in Figure 8-36, fuzzy logic modules do not function as actual input and output interfaces per se. Rather, they work with other input and output interfaces, providing an intelligent link between the two.
Decision making
Interfaces
based on fuzzy logic Fuzzy
Module Fuzzy implementation
in module
Figure 8-36. Fuzzy logic interface application.
Fuzzy logic modules are an integral part of the advanced capabilities of today’s programmable controllers. They help to bridge the gap between the discrete and analog decision-making functions of a PLC. In essence, fuzzy logic modules allow PLCs to “reason,” letting them interpret data in an analog-type form instead of just as ON or OFF. For example, a typical PLC connected to a temperature-sensing device can only sense whether a temp- erature is acceptable or unacceptable (see Figure 8-37a). That is, the temp- eratures between 60 ° F and 80 ° F are acceptable (logic 1); all other tempera- tures are unacceptable (logic 0). A PLC with fuzzy logic capabilities, however, can discern between the ranges of acceptable and unacceptable temperatures, judging a temperature to be either more acceptable or less acceptable (see Figure 8-37b). Thus, a fuzzy logic module can determine that
62 ° F is an acceptable temperature, but that it is not as acceptable as 70 ° F. The “reasoning” capabilities of fuzzy modules allow them to provide fine-
tuned control of analog processes, as well as nonlinear and time-variant processes, like tension and position control. These types of hard-to-control
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Serial Communication Interfacing 8
systems usually provide gross input deviations or insufficient input resolu- tion, which often require human intuition and judgment. Fuzzy logic modules can provide this type of human-like judgment.
1 70 ˚F (most acceptable)
62˚F (less acceptable)
(b)
Graphic Function
80˚F Figure 8-37. Temperature sensing in (a) a normal PLC and (b) a PLC with fuzzy
60˚F
logic capabilities.