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All rights reserved The right of John Ridley to be identified as the author of this work has been asserted in accordance with the Copyright, Designs and Patents Act No part of this publication may be reproduced in any material form including photocopying or storing in any medium by electronic means and whether or not transiently or incidentally to some other use of this publication without the written permission of the copyright holder except in accordance with the provisions of the Copyright, Designs and Patents Act or under the terms of a licence issued by the Copyright Licensing Agency Ltd, 90 Tottenham Court Road, London, England W1T 4LP.

Ltd, Pondicherry, India www. Swindon, who died aged 42, June His teaching of PLC fault-finding techniques, I and many others will never forget. For example the FX1S programmable controller cannot be used for: 1. The latest version of their Windows-based software is Gx-Developer Version 8 on which this book is based and included with this software is the Gx-Simulator software known as Ladder Logic Tester.

The advantage of this simulator software is that it enables ladder diagram programs to be tested without the use of a PLC. This book is intended for both students and engineers who wish to become competent in programming PLCs to meet the requirements of a wide variety of applications.

Students who are undertaking engineering courses will find the text covers most of the requirements for the following Edexcel Units. Manager, Mitsubishi Customer Technology Centre. Manager, Mitsubishi Automation Training Department. In addition, I would like to acknowledge with grateful thanks the support, encouragement and patience of my editors Rachel Hudson and Doris Funke at Elsevier, whilst writing this book. Boolean algebra A mathematical method for simplifying logic gate circuits and PLC ladder diagrams.

Copying Used to make a copy of a ladder diagram but having a different filename. COM1 Communications port no. A serial communications port, which is usually a 9-pin plug situated at the rear of a computer.

It enables communications between the computer and the PLC. CPU Central processor unit. DAC Digital to analogue converter.

DOS Disk operating system. A Microsoft computer operating system, which was a forerunner to Windows. Find A Gx-Developer function used for finding a device, an instruction or the start of a required ladder diagram line. Gx-Developer The Mitsubishi Electric ladder diagram software, whose use is described in this book. Also known as Logic Ladder Tester. It is similar to a low-level assembly type microprocessor program. It was, at one time, the only method available for producing PLC programs before the availability of ladder diagram software.

All ladder diagram programs though have to be converted to Instruction Programs for downloading to the PLC. A PCMCIA card is also available as an interface for an RS connection with laptop computers, which do not have this type of serial output connection. It is used in the automatic control of machinery and plant equipment. The great advantage of a PLC is that it can be programmed using software, i. Gx-Developer, to carry out a wide variety of tasks.

RS A serial communications system used for transferring information. Used in a PLC system for downloading and uploading instruction programs. Safety Relay A Safety Relay is used in control systems to ensure that that the system will fail safe whenever an Emergency Stop condition occurs. It connects the computer to the PLC to enable programs to be downloaded, uploaded and monitored.

Sink A term, which describes the direction of current flow into or out of either an input or an output terminal of a PLC. Source A term, which describes the direction of current flow into or out of either an input or an output terminal of a PLC. The FX2N range of PLCs includes a transistor source output type, which means that current will flow out the Y output terminal through the output load and to a 0 V connection.

A modern type of communications system for connecting peripherals, i. There are adapters available, which enable a Mitsubishi Electric SC09 communication cable to be connected to the USB port of a computer. The most powerful facility which PLCs have, is that they can be easily programmed to produce their control function, instead of having to be laboriously hard-wired, as is required in relay control systems.

However, the method of programming a PLC control system can nevertheless use relay ladder diagram techniques, which therefore enables the skills of an outdated technology to be still viable with that of the new. The PLC was initially designed by General Motors of America in , who were interested in producing a control system for their assembly plants and which did not have to be replaced every time a new model of car was manufactured.

The initial specification for the PLC was: 1. Easily programmed and reprogrammed, preferably in plant, to enable its sequence of operations to be altered. Easily maintained and repaired.

More reliable in a plant environment. Smaller than its relay equivalent. Cost-effective in comparison with solid-state and relay systems, then in use. The central processor unit CPU This is the main control unit for the PLC system, which carries out the following: a Downloads and uploads ladder diagram programs via a serial communications link.

This gives the impression that a real hardwired electrical control system is being monitored. Input unit The input unit enables external input signals, i. Output unit The output unit is connected to its externally operated devices, i. Each time the program is executed, i. Programs can be designed using conventional relay ladder diagram techniques. Test if the program is valid for use on the chosen PLC. Programs can be permanently saved either on a computer's hard disk or on floppy disks.

Programs can be re-loaded from either the hard disk or the floppy disk. Ladder diagram contacts and coils can be annotated with suitable comments. Hard copy printouts can be obtained. The program can be transferred to the PLC, via a serial link. The program within the PLC can be transferred back to the computer. Introduction to PLCs 3 9. The ladder diagram control system can be monitored in 'real time'.

Modifications can take place, whilst the PLC is online. As the software uses drop-down menus, there is no need to remember keypress characters. The drop-down menus are selected using a mouse. All of the functions can be accessed using an icon, instead of the drop-down menus. Ladder diagrams can be entered more quickly. Modifications can be easily carried out. Improved monitoring facilities, i. Fault-finding diagnostics.

Improved documentation, i. The main considerations that must be taken into account when configuring a system are: 1. External devices, inputs and outputs. Power supply requirements. Special function units. It is very important that confusion is avoided when these units are discussed. The basic way to describe the difference between a base unit, an extension unit and an extension block is as follows: 1. A base unit is made up of four components, i.

An extension unit is made up of three components, i. An extension block is made up of one or two components, i. It can be seen that the extension block does not have a power supply. It therefore obtains its power requirement from either the base unit or an extension unit. Hence it is necessary to determine how many of these un-powered units can be connected before the 'On Board' power supply capacity is exceeded. The tables in Sections 1. From the smaller FX2N table below, it can be seen that if no un-powered extension blocks have been used, then the maximum available current from the 24 V supply is mA.

However, if one input and one output extension block were fitted, then the available current falls to 0 mA and a separate 24 V power supply would then be required for supplying the input switches and any sensors. Its function is to supply, via the ribbon cable bus connections, any special units connected to the system. The table below details the current available from this supply. Introduction to PLCs No. The part number can be broken down as in Figure 1. Serial no.

This time should be taken into account when programming, as it will have a direct effect on the way the program will operate. Anything less than 4 mA, will result in the PLC input not turning on. The current into a PLC input must not exceed 7 mA; anything in excess of this could result in the input being damaged.

The input signals can come from a wide variety of devices, i. Push buttons. Rotary switches.


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