I-4. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM Line I: Install Control Circuits and Devices. LEARNING GUIDE I-4 INSTALL PLCs

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1 I-4 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM Level 4 Line I: Install Control Circuits and Devices LEARNING GUIDE I-4 INSTALL PLCs

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3 Foreword The Industry Training Authority (ITA) is pleased to release this major update of learning resources to support the delivery of the BC Electrician Apprenticeship Program. It was made possible by the dedicated efforts of the Electrical Articulation Committee of BC (EAC). The EAC is a working group of electrical instructors from institutions across the province and is one of the key stakeholder groups that supports and strengthens industry training in BC. It was the driving force behind the update of the Electrician Apprenticeship Program Learning Guides, supplying the specialized expertise required to incorporate technological, procedural and industry-driven changes. The EAC plays an important role in the province s post-secondary public institutions. As discipline specialists the committee s members share information and engage in discussions of curriculum matters, particularly those affecting student mobility. ITA would also like to acknowledge the Construction Industry Training Organization (CITO) which provides direction for improving industry training in the construction sector. CITO is responsible for organizing industry and instructor representatives within BC to consult and provide changes related to the BC Construction Electrician Training Program. We are grateful to EAC for their contributions to the ongoing development of BC Construction Electrician Training Program Learning Guides (materials whose ownership and copyright are maintained by the Province of British Columbia through ITA). Industry Training Authority January 2011 Disclaimer The materials in these Learning Guides are for use by students and instructional staff and have been compiled from sources believed to be reliable and to represent best current opinions on these subjects. These manuals are intended to serve as a starting point for good practices and may not specify all minimum legal standards. No warranty, guarantee or representation is made by the British Columbia Electrical Articulation Committee, the British Columbia Industry Training Authority or the Queen s Printer of British Columbia as to the accuracy or sufficiency of the information contained in these publications. These manuals are intended to provide basic guidelines for electrical trade practices. Do not assume, therefore, that all necessary warnings and safety precautionary measures are contained in this module and that other or additional measures may not be required.

4 Acknowledgements and Copyright Copyright 2011, 2014 Industry Training Authority All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or digital, without written permission from Industry Training Authority (ITA). Reproducing passages from this publication by photographic, electrostatic, mechanical, or digital means without permission is an infringement of copyright law. The issuing/publishing body is: Crown Publications, Queen s Printer, Ministry of Citizens Services The Industry Training Authority of British Columbia would like to acknowledge the Electrical Articulation Committee and Open School BC, the Ministry of Education, as well as the following individuals and organizations for their contributions in updating the Electrician Apprenticeship Program Learning Guides: Electrical Articulation Committee (EAC) Curriculum Subcommittee Peter Poeschek (Thompson Rivers University) Ken Holland (Camosun College) Alain Lavoie (College of New Caledonia) Don Gillingham (North Island University) Jim Gamble (Okanagan College) John Todrick (University of the Fraser Valley) Ted Simmons (British Columbia Institute of Technology) Members of the Curriculum Subcommittee have assumed roles as writers, reviewers, and subject matter experts throughout the development and revision of materials for the Electrician Apprenticeship Program. Open School BC Open School BC provided project management and design expertise in updating the Electrician Apprenticeship Program print materials: Adrian Hill, Project Manager Eleanor Liddy, Director/Supervisor Beverly Carstensen, Dennis Evans, Laurie Lozoway, Production Technician (print layout, graphics) Christine Ramkeesoon, Graphics Media Coordinator Keith Learmonth, Editor Margaret Kernaghan, Graphic Artist Publishing Services, Queen s Printer Sherry Brown, Director of QP Publishing Services Intellectual Property Program Ilona Ugro, Copyright Officer, Ministry of Citizens Services, Province of British Columbia To order copies of any of the Electrician Apprenticeship Program Learning Guide, please contact us: Crown Publications, Queen s Printer PO Box 9452 Stn Prov Govt 563 Superior Street 2nd Flr Victoria, BC V8W 9V7 Phone: Toll Free: Fax: crownpub@gov.bc.ca Website: Version 1 Corrected, June 2016 Revised, April 2014 Corrected, January 2014 New, October 2012

5 LEVEL 4, LEARNING GUIDE I-4: INSTALL PLCs Learning Objectives Learning Task 1: Describe the features of programmable logic controllers Self-Test Learning Task 2: Describe the memory system of the processor Self-Test Learning Task 3: Describe input and output (I/O) types Self-Test Learning Task 4: Describe basic installation procedures Self-Test Learning Task 5: Describe the operating cycle of the PLC processor Self-Test Learning Task 6: Describe basic programming instructions Self-Test Learning Task 7: Describe the interaction of hardware and software Self-Test Learning Task 8: Write basic PLC programs Self-Test Learning Task 9: Use a programming terminal Self-Test Learning Task 10: Describe PLC operating modes Self-Test Learning Task 11: Connect and maintain PLC systems Self-Test Answer Key CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 5

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7 Learning Objectives I-4 Learning Objectives The learner will be able to describe the operating principles of PLCs. The learner will be able to describe PLC installation procedures and requirements. The learner will be able to write basic PLC programs and use a programming terminal. The learner will be able to connect and maintain PLCs. Activities Read and study the topics of Learning Guide I-4: Install PLCs. Complete Self-Tests 1 through 11. Check your answers with the Answer Key provided at the end of this Learning Guide. Resources All the resources you need are included in this text. However, you are encouraged to obtain the following to provide supplemental learning information: Programmable Logic Controllers, by Colin D. Simpson, Prentice Hall. Technician s Guide to Programmable Controllers, by Richard A. Cox, Delmar Publishers. Canadian Electrical Code, Part 1 (latest edition), by Canadian Standards Association. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 7

8 Learning Objectives I-4 BC Trades Modules We want your feedback! Please go the BC Trades Modules website to enter comments about specific section(s) that require correction or modification. All submissions will be reviewed and considered for inclusion in the next revision. SAFETY ADVISORY Be advised that references to the Workers Compensation Board of British Columbia safety regulations contained within these materials do not/may not reflect the most recent Occupational Health and Safety Regulation. The current Standards and Regulation in BC can be obtained at the following website: Please note that it is always the responsibility of any person using these materials to inform him/herself about the Occupational Health and Safety Regulation pertaining to his/her area of work. Industry Training Authority January CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

9 Learning Task 1: Describe the features of programmable logic controllers A programmable controller is a digital electronic apparatus with a programmable memory. Usually called a PLC for programmable logic controller, it is most commonly used to control a machine or automated process. Elements of PLCs All programmable controllers consist of the essential elements shown in Figure 1. Figure 1 Essential elements of programmable controllers Input section The input section is designed to sense the state of the field devices that are wired to the input terminals. The devices may be of various types and have various voltage ratings, but they mainly fall into three categories: Discrete input devices such as contacts and electronic switches Analogue input devices such as potentiometers and thermistors Pulse-generating devices such as shaft encoders CPU section The CPU (central processing unit) section is the brain of the PLC. This section includes the PLC s power supplies, memory and microprocessor. It also has the ability to communicate with a programming device via a programming port or with other PLCs via a communication port. The CPU power supply is usually fed from a clean AC power source. Although an uninterruptible power supply (UPS) system is the preferred source, many installations use ferro-resonant regulating transformers or even raw AC line power with only a slight reduction in reliability. CPUs rated for DC are available, and are useful where an AC source is not available. The PLC uses the power supplied to its line terminals as the input for its own internal power supply. The internal power supply includes circuitry to reduce, filter and regulate the incoming power for the PLC s integrated circuits. Like any computer, the PLC has a microprocessor, memory and communication chip(s), which use voltages in the ±15 V DC range. To improve CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 9

10 Learning Task 1 I-4 noise immunity, only these ICs are connected to this supply. Where remote modules are required, a second, internally-generated supply may be used. The CPU section also includes battery, used to support essential parts of the volatile memory. Battery types range from non-rechargeable D cells, which last a little over a year, to lithium batteries which can be expected to last up to 15 years. Output section The output section provides power to loads, usually by switching an external AC or DC source. The loads are relatively small (10 A maximum). Loads switched by the PLC output section fall into three main categories: Discrete output devices, such as contactor and solenoid valve coils and pilot lights Analogue output devices, such as servo-valve coils and variable-speed drive input references Pulse-driven devices, such as stepper motors Programming devices and peripherals Every PLC has a communication port to which a programming device (terminal) can be connected. The programming device will have a display of some sort and a keyboard. It will be able to perform the following functions: Program entry A PLC program must be entered into some form of storage, keystroke by keystroke, prior to being run and tested. Documentation Text descriptions for ladder diagrams and reports are usually entered. Also, a link is provided to a printer for user program printouts. Run mode Giving the processor in the PLC the go-ahead to solve the program and cause outputs to be energized is called putting the PLC in run mode. Monitoring The terminal monitors the running program for the purpose of troubleshooting. Besides allowing the running program to be watched, the terminal allows outputs to be forced on or off. PLC programming can be done using terminals that contain their own microprocessor and memory. As such, they are computers themselves and are called intelligent terminals. A personal computer is one type of intelligent terminal. Some hand-held terminals have small LCD or LED displays and are microprocessor-based, which qualifies them as intelligent terminals. An intelligent terminal will allow a PLC program to be developed without being connected to the PLC itself. 10 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

11 Learning Task 1 I-4 A dumb terminal is one that must be connected to the PLC to operate. It has no standalone capabilities. Hand-held programmers Hand-held programmers, unlike personal computers, are always designed to be used with a specific PLC. They are a type of dedicated terminal (a computer terminal connected to only a single data processing system that provides a direct communication link). Hand-held programmers vary in size from that of a digital multimeter to the size of a cereal box. They are ruggedly built, making them ideal for use where they must be frequently moved from place to place, as in troubleshooting or making minor program changes. While most hand-held programmers are dumb, some are smart and can be used for program development and contain a program as a backup, if desired. Hand-held programmers have one serious drawback: they cannot display much of the program at once. Sometimes only one instruction can be shown at a time. Trying to monitor a program of any size can be like watching a ball game through a crack in a fence. Full-size dedicated terminals Most dedicated terminals are full-size, which means they have a CRT and a specialized keyboard (with keys similar to the hand-held versions) that is of the same size as those found on personal computers. In fact, some dedicated programming terminals are personal computers that have been mounted in a more durable enclosure. Figure 2 shows a typical full-size dedicated terminal. Personal computer terminals Most program development is done on a personal computer with no connection to the PLC; that is, it is done offline. Besides making the program much more visible, the computer allows for boilerplating: using a developed part of a program or a template to speed programming at subsequent rungs. Figure 2 Typical full-size dedicated terminal CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 11

12 Learning Task 1 I-4 Many motors are controlled from a single, stop-start, push-button station without any interlocking. A template for this type of control, once built, can be used many times over. The operator can then copy or move blocks of programming with a few keystrokes, which saves many hours when entering or editing a large program. Aside from the program instructions that control the outputs, descriptions for instructions, rungs and page titling are also entered. Some software allows wire labels to be generated with the help of a printer. Emulation software may be available that allows a program to be run without the need for a connection to a PLC. However, this amount of testing prior to going live is rarely necessary. A nuclear power plant is an example of where emulation software would be required. Once a program has been developed it is stored in memory and then downloaded into the PLC. It may also be stored on a network or portable memory device for transferring to a laptop or a programming terminal. The next step is to put the PLC in run mode. Once in run mode, the process of de-bugging the program begins. The user puts the computer on-line to allow the running program to be monitored on the CRT. Here again, the ability of the computer to display many program instructions at once is very useful. On-screen highlighting of inputs and outputs indicates to the user which devices are actually on in the field. This is very convenient for tracking down faulty input and output devices. Printers A printer is an essential peripheral that is used to print out listings (programs) and reports. It may be connected to a programming terminal or to a special module called an ASCII module, which fits into the PLC I/O rack. (ASCII is an acronym for American Standard Code for Information Interchange, standard 8-bit code for representing characters as binary numbers.) LCD or LED readouts Where a brief message regarding the status of a machine is required by an operator, an LCD or LED display can be used to good advantage. The message might be OUTFEED CONVEYOR OVERLOAD TRIP or LUBRICATION LEVEL LOW or DESPAIR NOT. This peripheral is most often not produced by the PLC manufacturer but is purchased from an independent vendor. Advantages of PLCs PLCs were originally created in response to demand by the automotive industry for a controller that met the following requirements (which were not being adequately met by the hard-wired or computer systems of the day): Reliability Flexibility Expandability Accuracy 12 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

13 Learning Task 1 I-4 Ease of installation Maintainability Reliability Reliability is achieved in several ways: The improved reliability of semiconductors greatly improved component life expectancies, sometimes by a factor of a hundred or more. Reduction in the number of parts also played a role. Using purpose-built power supplies that could handle brownouts and spikes helped, too. Rugged packaging that could tolerate rough handling and dirty or intemperate environments was a great improvement. Previous attempts at computerizing industrial processes using off-the-shelf computers often failed because they could not handle vibration, had poor power supplies and were too sensitive to heat. Flexibility Inherent in the programmability of PLCs is their flexibility. Enormous gains were made because of reduced start-up time on new installations and in refinements to process-controlling programs on the fly, with little or no downtime. Changed programs could be quickly reprinted and documentation became virtually instantaneous. Expandability PLCs that were part of processes that required wholesale changes (due to model year changes in the automotive industry, for example) could be expanded by increasing memory size or I/O (input/output) capacity as required. Or, as sometimes happened, the PLC would be reused for an entirely different purpose. Accuracy PLC timers compared with thermal, pneumatic and oil-dashpot hardware timers are infinitely more accurate. A timing tolerance of ±0.01 seconds is common for PLC timers. Ease of installation Compared with a hard-wired installation doing the same job, wiring required for PLC installations is much simpler. Maintainability Of all the advantages that PLCs have over hard-wired systems, the most important to an electrician is maintainability. To troubleshoot a complex system, an electrician must have access to the current status of all field devices and to the program that controls the process. The status of a field device is immediately apparent by indicating lights; this allows device faults to be pinpointed quickly, usually without using a tester of any sort. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 13

14 Learning Task 1 I-4 PLCs tend to be modular in nature. A faulty module can be quickly replaced, usually without having to disconnect and reconnect any wires. Gaining access to programs for interpretation and editing is an essential part of dayto-day maintenance. Relay ladder logic programming is usually used to put computer programs within quick reach of electricians accustomed to relay controls. It bears a strong resemblance to schematic wiring diagrams and is much easier to learn than other programming languages. Now do Self-Test 1 and check your answers. 14 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

15 Learning Task 1 I-4 Self-Test 1 1. Give three reasons why PLCs are more reliable than the hard-wired controls they replaced. 2. Why is a PLC system program printout more convenient than a schematic diagram for a hard-wired system? 3. What does every PLC have that allows much diagnosis to be done without using a tester? 4. What language is usually used to program PLCs? 5. What type of control installation would not benefit from the use of a PLC? 6. Draw lines to match the term in the left-hand column with the term in the right-hand column. discrete input 0 to 10 V discrete output pulse generator analogue input pulse driven analogue output servo-valve coils proximity switch magnetic starter shaft encoder (roto-pulser) stepper motor CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 15

16 Learning Task 1 I-4 7. What type of long-lasting battery may be used to back up volatile PLC memory? 8. What is the term for the connector into which a programming terminal is plugged? 9. What is the main difference between an intelligent terminal and a dumb terminal? 10. Where may a hand-held programming terminal be preferred over a personal computer for PLC application? 11. Where may a personal computer be preferred over a hand-held programming terminal for PLC application? 12. Downloading is a process that uses a programming terminal to copy a user program into the memory. 13. A computer that is on-line monitoring a running program allows the user to determine which outputs are on and which are off. How is this achieved? 14. Why print out a PLC program if it can be viewed on a CRT? Go to the Answer Key at the end of the Learning Guide to check your answers. 16 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

17 Learning Task 2: Describe the memory system of the processor A digital memory stores single bits of data in groups of 8, called bytes (Figure 1). - - Figure 1 Digital memory data storage Most PLC programs run on 16-bit machines, which means that the processor handles 2 bytes at a time. On a 16-bit machine, a 2-byte word is called a word length or just a word of memory. The memory of a PLC is divided into two main sections, each section containing thousands of bytes of memory: The executive memory The user memory Executive memory The executive memory is shipped from the factory as part of the PLC and is never changed by the user. It contains program instructions that allow the PLC to communicate, perform diagnostic functions and run the user program. Because it never has to be changed, it is stored in some type of non-volatile memory. User memory The user memory contains the user program and data to do with program instructions and the status of field devices. Figure 2 shows a user program with only two instructions. Figure 2 User program containing one input and one output instruction The memory size required is determined by the number of inputs and outputs (I/O) and the number and type of program instructions. Memory size for a single PLC can range from 1 kilobyte (1 Kb) for a mini-plc to 32 kilobytes (32 Kb) or more for a PLC that might have more than a thousand I/O points. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 17

18 Learning Task 2 I-4 The user memory (in contrast to the executive memory) is constantly undergoing change, particularly in sections to do with the inputs, the outputs and the running program. The program itself (the ladder diagram), once debugged, may not be changed for months or even years. However, for a large program controlling a complex process, the user memory may be refined every few weeks. This need for ongoing change suggests that a type of memory should be used that can be readily changed. A read/write, volatile memory is usually used, with steps taken to ensure that the user program and essential program data are not lost in a power failure. A small battery supports any volatile memory used for the user program and essential data. Occasionally, a non-volatile backup for the user program is provided. If the user memory contents are lost, the program in the backup memory is loaded into the PLC user memory and the process being controlled may be quickly restarted without having to key in the program again. The backup memory will be EEPROM, UVPROM or NOVRAM. EEPROM (Electrically-Erasable Programmable Read-Only Memory) Being electrically erasable, this PROM offers a great deal of flexibility. Program changes can be made quickly without requiring special equipment. UVPROM (Ultra-Violet Programmable Read Only Memory) The inside of the memory chip is accessible through a window by which light-sensitive material is made conductive using ultra-violet light. Once the chip has been burned the window is taped over. Processes which will rarely ever be changed can be backed up using UVPROM. A disadvantage is that the program must be burned into the memory chip using a PROM burner which is a special piece of equipment. This process takes time and handling. NOVRAM (Non-Volatile Random Access Memory) As well as becoming popular in PLCs, this type of memory is now widely used for memory in modems. Its bit locations are set up to parallel those in the RAM memory for the user program. Once the user program has been debugged, the program may be stored in NOVRAM quickly with a single keyboard command. It can be loaded just as easily to replace the RAM memory contents, if necessary, or even to autoload. Nowadays, most programs are developed using a personal computer that has disk storage. A program can be reloaded from a portable computer in a couple of minutes once the connection to the PLC is made. The backup memory types described above are usually not required. I/O addressing The one-rung program in Figure 2 would not work as shown because the program instructions have not been told which I/O terminal they are to respond to or control. To direct the program instructions to the appropriate memory location, an address for that location is required. If a particular PLC has 32 inputs and 16 outputs, the addresses might be I/0 (i.e., input zero ) to I/31 for the inputs and O/0 to O/15 for the outputs. Addressing schemes vary greatly: some use X as the first character in an input address and Y as the first character in an output address. Most start from 0, but some start numbering from CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

19 Learning Task 2 I-4 Figure 3 shows a more realistic addressing for a typical PLC. I Figure 3 Addressing applied to program instructions With some addressing schemes where a large number of I/O are involved, I/O addresses can be 8 or more characters long. Addresses will be reflected in wire numbers for inputs and outputs. Now do Self-Test 2 and check your answers. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 19

20 Learning Task 2 I-4 Self-Test 2 1. What are the two main sections of PLC memory? 2. A PLC with a 64 K memory would be considered to be: a. relatively small b. relatively large c. about average 3. A ladder diagram program is always backed up with some sort of PROM or NOVRAM memory. a. True b. False 4. What type of memory would be used to store a program that monitors battery status, and in which section of a memory map would the program reside? Questions 5 to 8 relate to the 4-word section of memory, shown in Figure 1, which is used for 32 input devices and 32 output devices. Figure 1 5. How many inputs are: a. on? b. off? 20 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

21 Learning Task 2 I-4 6. How many outputs are: a. on? b. off? 7. Give the address of the? input address indicated. 8. Give the address of the? output address indicated. Go to the Answer Key at the end of the Learning Guide to check your answers. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 21

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23 Learning Task 3: Describe input and output (I/O) types The input/output (I/O) modules provide the physical link between the actual field equipment and the central processing unit. The I/O modules provide the interface circuitry that allows the PLC to interpret the incoming signals and respond accordingly. Types of I/O modules The I/O modules are normally placed in racks and are slotted so that they can be addressed according to their position on the rack. Their racks may or may not contain the main processor. If they are in a rack other than the one with the main processor, they are referred to as remote I/O, and communication with these modules is conducted through a separate processor over a serial connection. Typically, I/O modules will have 8, 16 or 32 points (inputs or outputs). Discrete The most common type of I/O interface is the discrete type. Discrete I/O can be in either of two possible states: on or off. Discrete inputs are essentially switches or contacts that are either open or closed. Loads that can be connected to discrete outputs are devices such as relays, magnetic starters, solenoid valves and pilot lights. The most common types of discrete I/O cards are as follows: Inputs Outputs 24 volts AC/DC volts AC 48 volts AC/DC 120 volts AC/DC 120 volts AC 230 volts AC/DC 230 volts AC TTL level TTL level 120 volts DC Non-voltage 230 volts DC Isolated inputs TTL level Isolated outputs Contact (relay) All types of I/O provide electrical isolation between the field devices and the processor. The circuit of either type of module will generally consist of a power section and a logic section separated by the isolation circuit. The DC module converts the input to low level of DC which is passed through a filter delay. AC voltages are reduced to a low level, rectified through a bridge network, and isolated by means of an opto-isolator. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 23

24 Learning Task 3 I-4 TTL (5 volts DC) allows interfacing with solid-state devices such as thumbwheel switches, binary coded decimal (BCD) displays, solid-state controls and sensing devices. Non-voltage interfaces allow the sensing of the closure of dry contacts. The switching of an AC output generally is through the use of an SCR with protection at the output provided by the use of an RC snubber and a MOV (metal oxide varistor). The outputs are always fused, either individually or in groups, and when replacing these fuses caution should be taken to use the exact type of fuse as a replacement. The fuses used are engineered to provide maximum protection and any compromising will only result in a defective output. DC outputs generally use power transistors switched by the processor with protection provided by a free-wheeling diode. Sinking and sourcing DC output and input devices are available to match corresponding modules in the field. The other type of output of note is the relay contact type. The relay contact type of output switches its contacts to provide voltage at the output. The isolated output switch is a separate source of control voltage through the output point. Analogue Analogue I/O modules allow the monitoring and control of devices that require an analogue signal to control them or that produce an analogue signal as an input to the PLC. These cards are smart cards, meaning they have their own processor that handles the task of converting analogue signals to digital or digital values to analogue signals. Typical analogue devices include: Analogue Inputs Temperature transducers Pressure transducers Load cell transducers Humidity transducers Flow transducers Potentiometers Analogue Outputs Analogue control valves Actuators Chart recorders Electric motor drives Analogue meters Pressure transducers 24 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

25 Learning Task 3 I-4 Analogue I/O are available in unipolar (positive) and bipolar (positive and negative) ratings. Standard ratings are as follows: Analogue Inputs 4 20 ma 0 to +1 volts DC 0 to +5 volts DC 0 to +10 volts DC 1 to +5 volts DC 5 volts DC +10 volts DC Analogue Outputs 4 20 ma ma 0 to +5 volts DC 0 to +10 volts DC 2.5 volts DC 5 volts DC +10 volts DC In most PLCs the transfer of information between the analogue card and the processor is accomplished through the use of some sort of block transfer instruction which causes the values to be transmitted to the processor when the program allows. Connection to analogue devices should be done through shielded cable to provide better noise immunity. Discrete input cards (modules) The input module monitors the ON/OFF status of the user s input devices connected to it. Input signals can originate from limit, float, pressure, selector switches, push-buttons, transducers or many other sensing and switching devices. As can be seen in the table below, modules are available for devices of different voltage levels and characteristics. Many input modules have filtering circuitry to suppress contact bounce and to guard against recognition of transients as data. Typically there are 8, 16 or 32 inputs on a single module. DC VDC 120 VDC 240 VDC TTL AC 24 VAC 48 VAC 120 VAC 240 VAC A DC input module converts the DC to a lower level and isolates it before the signal is sent to the processor. An AC module rectifies the input, drops the voltage to a low level and isolates it before the signal is sent to the processor. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 25

26 Learning Task 3 I-4 Every input module has four basic functions, as follows: 1. Termination provides a terminal for the attaching of field wiring. 2. Signal conditioning incoming signals are rectified and their level is reduced. As well, any electrical noise is removed. 3. Isolation incoming signals are electrically and physically isolated by opto-isolators. Normal isolation ratings are 1500 V RMS. 4. Indication for convenience purposes. An indicator is placed between the input terminal and common. This allows for faster diagnostic functions. (This indicator is driven by machine power.) Figure 1 is a block diagram of a typical AC input module. Figure 1 Block diagram of an AC input module Although modules today are designed to be removed or replaced under power, exceptional cautions must be observed when doing so. Generally, I/O modules are provided with the means of replacing them without having to disconnect the field wiring. The exception to this may be found on some of the shoebox size controllers. See Figure 2. Figure 2 Generalized schematic of an input module 26 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

27 Learning Task 3 I-4 The input module in Figure 2 provides the following: 1. Bridge rectifier used on AC or AC/DC modules only. Converts an AC signal to a DC signal. 2. Noise debounce filter and level changing removes unwanted AC noise from the incoming signal. Provides a short delay to eliminate the contact bounce as the switch closes, via capacitor and resistors. 3. Threshold detection ensures that the incoming signal is at the rated level or above. A zener diode will not allow signal until voltage reaches adequate level. 4. Optical isolation optically isolates the input signal from the PLC s processor signals. The input signal is converted to a 0 V or 5 V signal for the processor. Some input cards will accept either AC or DC signals. The generalized schematics will accept either. DC input modules (sink/source) There are DC only modules available as well. They are either sinking or sourcing types. A sinking type module would receive or sink a signal from the field. In other words, the DC positive line would be switched in the field and the switched leg would be the input to the module. A sourcing type module would source the positive voltage and the field device would switch to negative. Sourcing input/sinking module A sourcing type input would be wired so that the module would be the source of the negative voltage through the optical isolation LED and the field device would switch to the positive. A sourcing field device (Figure 3) is used with a sinking module. The field device sources the current; the module sinks the current. DC +24 supply 24 Sourcing field device (limit switch) +V I0 I1 I2 Electronic circuitry inside the PLC module. A simplified optical isolation LED is shown. I3 I4 I5 The module needs power (+V and V) to operate. I6 I7 V DC sinking module Figure 3 Sourcing input/sinking-type module CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 27

28 Learning Task 3 I-4 Sinking input/sourcing module A sinking type input would be wired so that the module would be the source of the positive voltage through the optical isolation LED and the field device would switch to the negative. A sinking field device (Figure 4) is used with a sourcing module. The field device sinks the current; the module sources the current. +V DC supply Sinking field device (limit switch) I0 I1 I2 I3 I4 I5 I6 Electronic circuitry inside the PLC module. A simplified optical isolation LED is shown. Figure 4 Sinking input/sourcing-type module I7 V The module needs power (+V and V) to operate. DC sourcing module Discrete input devices Also available are field devices that use three wires. Many DC photoelectric and proximity switches fall into this category. They will be labelled as being either NPN or PNP, which refers to the type of transistor internally used to switch the current. Be careful with the connections, as most will use the International Electrotechnical Commission (IEC) standard colors for DC: so the positive (+V) will be brown, the negative (-V) will be blue, and switched output will be black. A three-terminal proximity sensor is shown below (Figure 5). Switched output - black Positive DC voltage - brown Negative DC voltage - blue Threaded body for mounting Indication LEDs one is for power and the other lights any time the sensor is triggered. Figure 5 Three-terminal sensor with IEC colored wires When activated, an NPN type of three-terminal sensor (Figure 6) will output a negative on its switched line. In order to get current to flow through the optical isolation LED, it must be used with a DC sourcing module. Note the complete path for current flow through the optical isolation LED and NPN transistor once it is gated by the sensor head. 28 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

29 Learning Task 3 I-4 DC supply NPN sensor Sensor head and amplifier +V I0 I1 I2 I3 I4 I5 I6 I7 V Electronic circuitry inside the PLC module. A simplified optical isolation LED is shown. The module needs power (+V and V) to operate. DC sourcing module Figure 6 Three-terminal NPN sensing connected to a DC sourcing input module When activated, a PNP type of three-terminal sensor (Figure 7) will output a positive on its switched line. In order to get current to flow through the optical isolation LED, it must be used with a DC sinking module. Note the complete path for current flow through the optical isolation LED and PNP transistor once it is gated by the sensor head. DC supply PNP sensor Sensor head and amplifier +V I0 I1 I2 I3 I4 I5 Electronic circuitry inside the PLC module. A simplified optical isolation LED is shown. The module needs power (+V and -V) to operate. I6 I7 V DC sinking module Figure 7 Three-terminal PNP sensing connected to a DC sinking input module Important considerations for the correct operation of input cards Most input cards have a fairly high input impedance. Some sensors have Triac or SCR outputs. These outputs often leak a small current. It is possible for an input card to see this leakage current as a true input even though the field device is off. In these cases, a resistor is placed between the input and the negative to reduce the net impedance, shunting the leakage current by the input. The value of R should be low enough to make the input reliable without adding too much load to the sensor. Remember to calculate the power dissipation! CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 29

30 Learning Task 3 I-4 Discrete output cards (modules) A discrete output card receives a signal from the processor and turns on an output device such as a coil, an indicating lamp or a solenoid valve. There are cards available in 8-bit to 32-bit configurations. Common output voltage levels DC AC VDC 120 VDC 240 VDC TTL 24 VAC 48 VAC 120 VAC 240 VAC A DC output module also provides isolation and conditioning to prevent electrical noise being generated or transferred throughout a system. Every output module has four basic functions: 1. Termination provides a terminal to attach field wiring. 2. Noise suppression by the use of MOVs (metal oxide varistors) and an RC snubbing network, electrical noise caused by switching inductive loads is suppressed. 3. Isolation output signals from the processor are electrically and physically isolated by optoisolators. Normal isolation ratings are 1500 V RMS. 4. Indication a LED or a neon lamp is used to indicate that an output is ON. Although modules today are designed to be removed or replaced under power, exceptional cautions must be observed when doing so. Generally, I/O modules are provided with the means of replacing them without having to disconnect the field wiring. The exception to this may be found on some of the shoebox size controllers. AC outputs An AC output module (Figure 8) provides an interface through which power is provided according to the program stored in the processor. Figure 8 Block diagram of an AC output module 30 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

31 Learning Task 3 I-4 AC output circuits, like input circuits, vary widely among PLC manufacturers and in general can be depicted by the block diagram configuration shown in Figure 8. This block configuration describes the main sections that form an output module and are used here to illustrate the module s operation. The circuit consists primarily of the logic and power sections, coupled by an isolation circuit. The output consists primarily of the logic and power sections, coupled by an isolation circuit. The output interface can be thought of as a simple switch through which power can be provided to control the output device in the sequence shown below. 1. Logic the processor sends a logical 1 or 0 to the isolation circuit. 2. Isolation the 1 or 0 is passed to the switch. 0 turns the switch OFF. 1 turns the switch ON. 3. Switch the switch selection is a Triac that turns ON with a 1 and OFF with a Filter an MOV and an RC snubber are used to suppress electrical noise. The typical schematic of an AC output circuit (Figure 9) shows that the switching circuit in the power section generally uses a Triac to switch the power to the load. Figure 9 Generalized schematic of an AC output circuit Some modules will use a silicon-controlled rectifier (SCR), a power transistor or small relays to switch power to the load, but the Triac is the most common. The AC switch is normally protected by an RC snubber and often a metal oxide varistor (MOV), which is used to limit the peak voltage to some value below the maximum rating and also to prevent electrical noise from affecting the circuit operation. A fuse may be provided in the output circuit to prevent excessive current from damaging the AC switch. If the fuse is not provided in the circuit, it should be supplied by the user and should conform to the manufacturer s specifications. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 31

32 Learning Task 3 I-4 As with input circuits, the output module has LEDs or neon indicators to indicate status. If the circuit contains a fuse, a fuse status indicator may also be incorporated. An AC output connection diagram is illustrated in Figure 10. Note that the switching voltage, which is used to turn on the output device, is supplied to the module from the field (L 1 ) A well-designed installation should have fuses installed so that a field wiring fault will not shut down the complete process. If in doubt, add fuses to each external device to properly protect the wiring and to speed troubleshooting. DC output modules (sink/source) DC output modules are considerably different from AC output modules. DC modules can be specified in two ways: sourcing or sinking. The output module can source the current to turn on the output device. This method requires that a supply is taken to the output module. Figure 10 AC output module connection diagram In Figure 11, the output module switches the positive line of the supply. All of the field devices are wired with the negative lead common. This output module is said to be sourcing the current to drive the output device. 1 2 Figure 11 DC sourcing-type output module/sinking load DC output modules employ the same general functions as AC output modules. The AC module uses a Triac in the final stage. The DC module uses a power transistor. A typical schematic for a sourcing DC module is shown in Figure CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

33 Learning Task 3 I-4 Figure 12 Sourcing-type output card DC output modules can also sink currents. This configuration requires that the positive lead is common to all field devices and the negative side is switched through the module. See Figures 13 and Figure 13 DC sinking output module-type/sourcing load Figure 14 Sinking-type output card CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 33

34 Learning Task 3 I-4 Isolated AC and DC outputs Isolated outputs operate in the same manner as the standard AC and DC output interfaces. The only difference is that each output has its own return line circuit (common) independent or isolated from the other outputs. This configuration allows the control of output devices powered by different sources that may also be at different ground (common) levels. The standard modules have one return connection common to all outputs; however, some standard modules may provide one return line per four outputs if the interface has eight or more outputs available. In general, the isolated interfaces provide less modularity (i.e., fewer points per module) than their standard counterpart because extra terminal connections are necessary for the independent return line. Connections for this type of output interface are illustrated in Figure Figure 15 Isolated outputs S2 and MS are supplied by the same control circuit. S1 and L are fed from independent control circuits. Remember that an AC isolated output cannot work on DC and a DC isolated output does not work on AC. Contact outputs The contact output interface allows output devices to be switched by a NO or NC relay contact. Electrical isolation between the power output signal and the logic signal is provided by the separation not only between the contacts, but also between the coil and the contacts. Filtering, suppression and fuses are also generally incorporated. The basic operation of the module is the same as the standard AC or DC output module. When the processor sends the status (1 or 0) to the module during the output update, the state of the contacts will change. If a 1 is sent to the module from the processor, a normally open contact will close and a normally closed contact will change to an open condition. If a 0 is sent, no change occurs to the normal state of the contacts. 34 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

35 Learning Task 3 I-4 The contact output can be used to switch either AC or DC loads, but it is normally used in applications such as multiplexing analogue signals switching small currents at low voltage, and interfacing with DC drives for controlling different voltage levels. High-power contact outputs are also available for applications that require switching of high currents. A contact output circuit is shown in Figure 16. The device connection for this output module is similar to the AC output. Figure 16 Contact outputs Some important considerations when using output modules Output modules that use Triacs will leak a small amount of current. If the field device is low impedance, the leakage current is of little consequence. If you disconnect a field device without removing the power from the output module, there could be a shock hazard. Do not assume that you are safe. Lockout! This leakage current can also make some outputs appear to be on after the output is off. The current is enough to keep neon lamps lit dimly. This effect can be stopped by connecting a resistor across the load (neon lamp). The resistor lowers the output impedance and the lamp will go out. Typical values for the resistor would be ohms, 10 watts. Interposing relays may be required if the loads to be switched are of higher voltage or current. I/O module installation Placement of modules must conform to the documentation provided. The layout of the rack should be provided by the programmer. The slots of the rack conform to the addressing used in the program. Voltage ratings of the modules should be double-checked. Modules are expensive and the installer will feel very foolish putting low-voltage modules in a slot meant for 120 V modules. Wiring of I/O Wiring of I/O must conform to the requirement of the Canadian Electrical Code. The size of wire must be sufficient to carry the current involved. The voltage rating of all the wiring in the PLC cabinet must be the voltage rating of the conductors requiring the highest voltage rating. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 35

36 Learning Task 3 I-4 Table 1: Types of I/O modules Module type Voltage rating Typical power supply requirement AC/DC Input AC/DC Input Module Isolated AC/DC Input DC Input DC Input DC Input DC Selectable Input DC Input Fast Response DC Driver Logic Input Analogue Input (8 bit) Analogue Input (12 bit) TTL Input Encoder/Counter Encoder/Counter Thermocouple Input Thermocouple Input Expander 120 V 220/240 V 120 V V 48 V V 5 30 V V V 5 V V 75 ma 75 ma 50 ma 75 ma 75 ma 75 ma 150 ma 75 ma 75 ma 400 ma 1.3 A 125 ma 1.5 A 1.5 A 2 A 500 ma AC Output AC Output Isolated AC Output Contact Output DC Output DC Output Analogue Output (12 bit) Grey Encoder Output (8 bit) 120 V 220/240 V 120 V V 48 V 210 ma 225 ma 225 ma 425 ma 165 ma 165 ma 1.5 A 120 ma The PLC panel enclosure is usually located as close to the operator s console as practicable to make troubleshooting easier. Now do Self-Test 3 and check your answers. 36 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4