C HAPTER E IGHT S PECI AL F U N CT I ON I /O AN D S ERI AL C OM M U N I CAT I ON I N T ERFACI N G

C HAPTER E IGHT S PECI AL F U N CT I ON I /O AN D S ERI AL C OM M U N I CAT I ON I N T ERFACI N G

No rule is so general, which admits not some exception.

—Robert Burton

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S ECTION Components Special Function I/O and C HAPTER 2 and Systems

Serial Communication Interfacing 8

C HAPTER In previous chapters, we discussed analog and digital I/O interfaces.

H IGHLIGHTS Although these types of interfaces allow control implementation in most types of applications, some processes require special types of signals. In this chapter, we will introduce special function I/O interfaces, which uniquely process analog and digital signals. We will also take a look at intelligent positioning, data-processing, and communication modules that expand the capabilities of PLCs. We will conclude with a discussion of peripheral interfacing and communication standards. When you finish this chapter, you will have learned about all the major components of programmable control- lers—from processors to intelligent interfaces—and you will be ready to explore PLC programming.

8 -1 I NTRODUCTION TO S PECIAL

I /O M ODULES

Special function I/O interfaces provide the link between programmable controllers and devices that require special types of signals. These special signals, which differ from standard analog and digital signals, are not very common, occurring in only 5–10% of PLC applications. However, without special interfaces, processors would not be able to interpret these signals and implement control programs.

Special I/O interfaces can be divided into two categories:

• direct action interfaces • intelligent interfaces

Direct action I/O interfaces are modules that connect directly to input and output field devices. These modules preprocess input and output signals and provide this preprocessed information directly to the PLC’s processor (see Figure 8-1). All of the discrete and analog I/O modules discussed in Chapters

6 and 7, along with many special I/O interfaces, fall into this category. Special direct action I/O interfaces include modules that preprocess low-level and fast-input signals, which standard I/O modules can not read.

Special function intelligent I/O interfaces incorporate on-board micro- processors to add intelligence to the interface. These intelligent modules can perform complete processing tasks independent of the PLC’s processor and program scan. They can also have digital, as well as analog, control inputs and outputs. Figure 8-2 illustrates an application of intelligent I/O interfaces. The method of allocating various control tasks to intelligent I/O interfaces is known as distributed I/O processing.

Special input/output modules are available along the whole spectrum of programmable controller sizes, from small controllers to very large PLCs. In general, special I/O modules are compatible throughout a family of PLCs.

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S ECTION Components Special Function I/O and C HAPTER 2 and Systems

Serial Communication Interfacing 8

data to Direct

Action

to PLC

module Action

Data Path

Connections

Figure 8-1. Direct action I/O interface application.

intelligent I/O

Intelligent I/O Module

Module controls actuator according to its input data

Input data to

from process

Control

intelligent I/O

Data Path

Connections

Figure 8-2. Intelligent I/O interface application.

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S ECTION Components Special Function I/O and C HAPTER 2 and Systems

Serial Communication Interfacing 8

In the next sections, we will discuss the most commonly found special I/O interfaces:

• special discrete • special analog • positioning • communication/computer/network • fuzzy logic