CH02B PLC Hardware Components

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CHAPTER 2B: PLC HARDWARE

COMPONENTS


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CHAPTER OBJECTIVES

1. List and describe the function of thehardware components used in PLC systems.

2. Describe the basic circuitry andapplications for discrete and analog I/Omodules, and interpret typical I/O and CPUspecifications.3. Explain I/O addressing.4. Describe the general classes and typesof PLC memory devices.

5. List and describe the different types ofPLC peripheral support device available.

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ANALOG I/O MODULES

1.Discrete devices are inputs and outputsthat have 2 static states: ON and OFF.

2.Analog devices are inputs and outputsthat can have an infinite number of states.3. Not only can these devices be ON andOFF, but they can also be barely ON,

almost totally ON, not quite OFF.4. These devices send/receive complexsignals to/from a PLC.

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ANALOG I/O MODULES

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1.Analog input interface modules containthe circuitry necessary to accept analog

voltage or current signals from analog fielddevices.2.These inputs are converted from analogto digital value by an analog-to-digital

(A/D) converter circuit.3. The conversion value which isproportional to the analog signal, is

expressed as a 12bit binary or as a 3bitbinary coded decimal (BCD) for use by theprocessor.


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ANALOG I/O MODULES

1.Analog input sensing devices includetemperature, light, speed, pressure, and

position transducers. See Figure 2-12 foran example.2.A varying dc voltage in millivolt range, isproportional to the temperature being

monitored is produced by thethermocouple.3. Voltage produced by thermocouple is

amplified and digitized by the analog inputmodule and then sent to the processor oncommand from a program instruction.

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ANALOG I/O MODULES


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1.There 2 basic types of analog inputmodules available:

a) Current Sensingb) Voltage Sensing

2.Voltage input modules are available in 2types:

a) Unipolar-Can except 1 polarity inputb) Bipolar- Can except split polarity input

3. The analog output interface modulereceives from the processor digital data.4. The digital data is passed through adigital-to-analog (D/A) converter circuit toproduce the necessary along form.

ANALOG I/O MODULES

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ANALOG I/O MODULES

Figure 2-13 illustrates the use of analogI/O modules in a typical PLC control

system.

The value is initially open 100%

Fluid is placed in the holding

tank

As the fluid level in the tanka roaches the reset oint

The processor modifies the

output, which adjusts the valve

to maintain a setp point


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SPECIAL I/O MODULES

Special I/O modules have been developedto meet several needs. Seepages 33 34

in the textbook for a complete list them.

NOTE:New modules continue to be developed to

meet specific application demands.At the same time, some modules, such asthe PID module are no longer necessarybecause the newer PLC models include PIDin their instruction set.


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I/O SPECIFICATIONS

1.Manufacturers specifications providemuch information about how an interface is

correctly and safely used.2. The specifications place certainlimitations not only on the module but onthe field equipment as well.

Seepages 34-35for the list of typicalManufacturers I/O specifications.

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1.The CPU houses the processor-memory

modules, communications circuitry, andpower supply. See Figure 2-142. CPU architectures may differ from onemanufacturer to another, but generally

most of the follow this sameinfrastructure.

THE CPU

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THE CPU


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THE CPU


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1.Many electronic components found in the

processors and other types of modules aresensitive to electrostaticvoltages.2. Voltages can be as low as 1mV (not ableto feel )

3. ESD can be created based on materialsin the surrounding lab or manufacturingenvironment.4. ESD can reach as high as 35kV.

See page 38 for ESD Safety Precautions.

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THE CPU

Electrostatic Discharge (ESD)


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THE CPU

Electrostatic Discharge (ESD)


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MEMORY DESIGN


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MEMORY DESIGN

1.Memory is a physical space inside the CPUwhere the program files (control plan) anddata files are stored and manipulated.2.Data is typically stored in a file by address.3.The information stored in the memory

relates to the how the input and output datashould be processed.4.The memory organization of typical PLCbased on I/O image tables. See Figure 2-17.

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MEMORY TYPES

1.PLCs have programmable memory thatallows users to develop and modify control

programs.2.This memory is made nonvolatile so if thepower is lost, the PLC holds it processorprogramming. See Figure 2-18.

3.Data stored in memory locations is by aprocess called writing.4.Data retrieved from memory locations is byreading.

See pages 40 -41 for several types ofmemory ICs.


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MEMORY TYPES


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PROGRAMMING DEVICES

1.Easy to use programming equipment is one ofthe important features of programmable

controllers.2.The programming device provides theprimary means by which the user cancommunicate with the circuits of the

controller. See Figure 2-19.3.The programming device is used to input thedesired instructions. See Figure 2-20 for anexample of Programming Device.

4. Desktop PCs and notebook computers canalso be used to communicate with a PLC. SeeFigure 2-21, page 43 in text book


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PROGRAMMING DEVICES


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PROGRAMMING DEVICES


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1. Printers are used to provide hard-copyprintouts of the processors memory in ladderprogram format.

2. Hardcopy format of large programs caneasily be viewed instead on a screen

3.Some older PLC systems use a magneticcassette recorder to record and store the userprogram. See Figure 2-22.

4. Some PLCs use internal EEPROMs orEPROM memory modules that can store abackup to the program entered in the PLC. SeeFigure 2-23.

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RECORDING AND RETRIEVING DATA


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RECORDING AND RETRIEVING DATA


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PLC WORKSTATIONS

1.A PLC workstation or operator interface canbe connected to communicate with a PLC.

2. Replaces pushbuttons, pilot lights,thumbwheels , and other operator controls.

3. Luminescent touch-screen keypads providean operator interface that operates like atraditional hard-wired control panels.

4. The PLC workstation is part of the Human-Machine-Interface (HMI) being studied atUniversity and Manufacturing CorporationsResearch Labs.

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PLC WORKSTATIONS

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CH02B PLC Hardware Components

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