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PLC PLC -- BASIC COURSEBASIC COURSE

by:by:MOHAMED HUSSEINMOHAMED HUSSEIN

A PLC is a device which processes input signals top p gproduce output signals that is directly used to control aprocess or an operation. PLC make use ofmicroprocessor technology as its core but it is not acomputer which needs some high level skills orknowledge to assemble and manipulate. PLC can beregarded as a specially built computer which can beprogrammed easily to do control tasks without havingany superior skills or knowledge.

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• PLC was first introduced in the late. The primary reason fordesigning such a device was eliminating the large costdesigning such a device was eliminating the large costinvolved in replacing the complicated relay based machinecontrol systems.

• first used in the automotive industry (in United States)

• the main drawback of adopting PLC during the early yearsis the programming procedureis the programming procedure

• The arrival of microprocessor in 1978 indirectly changesthe PLC structure which make it easier to understand andthe PLC itself becomes cheaper

Among the advantages of using PLC are:

• Operational Speed –

• Documentation –

• Security –

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Among the PLCmanufacturers are:

• Allen Bradley• Siemens• Mitsubishi• Matsushita (NAIS)• OMRON• GE‐Fanuc• FESTO

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PLC architecture:

• CPU (Central Processisng Unit)

• Memory

• Programming Interface

• Communication Interface

• Power Supply

• Communication Interface

• Input/Output (I/O)

Basic components for a PLC to operate:

•

••

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PLC pheripherals:

••

•

•

•

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PLC can be programmed using several methods:

• Ladder logic diagram

• Function Block Diagram (FBD)

• StructuredText (ST)

I i Li (IL)• Instruction List (IL)

• Sequential Function Chart

In general, PLC acts as control device that actuatesall devices connected to itsOUTPUT channels

PLCmakes decision based on:

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Among the devices which can be connected to thePLCOUTPUT channels are :PLCOUTPUT channels are :

Among the devices which can be connected to thePLC INPUT channels are :PLC INPUT channels are :

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COUNTERs and TIMERs are built inside the PLC andare activated via PLC program. They cannot be seenphysically.

PLC is a mediator between the INPUT and OUTPUT.The use of PLC provides flexibility to the generatedp y gOUTPUT via the written program.

INPUT

OUTPUT

OUTPUTINPUT

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• PLC programs are written using the softwaresupplied by the PLCmanufacturer.

PLC PLC t th ti hi h• PLC program PLC executes three operations whichare:

– the input signals at the INPUT terminalsare read and registered into the PLCmemory

– the INPUT information are transferred tothe program Based on the program requirements allthe program. Based on the program requirements allthe OUTPUTs to be generated are registered into thememory

– the OUTPUT information are transferredto the respectiveOUTPUT terminals

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• Ladder Diagram is the most popular method toprogram a PLCp g

• Each INPUT and OUTPUT in Ladder Diagram arerepresented in form of symbols

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• Two most commonly used contact in the programare:

• All input devices (input signals) are wired to thePLC’s INPUT terminalsPLC s INPUT terminals

+ve

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• All output devices are wired to the PLC’s OUTPUTterminals

240V AC

• Each INPUT and OUTPUT terminals are associatedwith a specific identity/label known as address

• INPUT and OUTPUT terminals are divided intoseveral block with each block consist of eight (8)addresses (maximum)

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• For SIEMENS S7‐200 PLC , it consist 16 INPUTi l d OUTPUT i l di id d iterminals and 10 OUTPUT terminals divided into

two (2) blocks each

• The addresses for INPUT blocks are:

• The addresses forOUTPUT blocks are: