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Ladder Diagram Format-Digital Logic Design And Programming-Lecture Slides, Slides of Digital Logic Design and Programming

Aliah University Digital Logic Design and Programming

Prof. Prasanna Singh delivered this lecture at Aliah University for Digital Logic Design and Programming course. It includes: Ladder, Diagram, Format, Block, Type, Instructions, Ladder, Matrix

Typology: Slides

2011/2012

Uploaded on 07/20/2012

shally_866 🇺🇸

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Ladder Diagram Format

In PLCs with

enhanced ladder

format functional

instructions instead

of block-type

instructions, the

ladder matrix may

use one or more

contact symbol

spaces to represent

the instruction in

the programming

device Figure b.

docsity.com

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Ladder Diagram Format  In PLCs^ withenhanced^ ladderformat^ functionalinstructions^ insteadof block-typeinstructions,^ theladder^ matrix^ mayuse one^ or^ morecontact^ symbolspaces to representthe instruction^ inthe programmingdevice Figure b.

Ladder Diagram Format^ ^ A ladder matrix represents all the possible locations where a contactsymbol instruction can be placed.^ ^ The programming device usually displays all of these possible locationson^ the^ screen,^ allowing^ the^

user^ to^ place^ contact^ symbols^ in

the desired locations.  However, according to the maker of the PLC, certain rules apply tocontact placement.  One rule, which is present in almost all PLCs, prevents reverse (i.e.,right-to-left) power flow in a ladder rung.  PLC logic does not allow reverse power to avoid sneak paths.  Sneak paths occur when power flows in a reverse direction through anundesired field device, thus completing a continuity path.  If a PLC’s logic requires reverse power flow, the user must reprogramthe rung with forward power flow to all contact elements.

Reverse Power Flow atContact D^ ^ Solve the logic rung shown in previous figure so that no reversepower flow condition exists.^ ^ The reverse condition is not part of the required logic for theoutput to be energized.^ ^ SOLUTION^ ^ The forward power flow of the logic determines output Y.^ ^ Using logic concepts, the output Y is defined, using forwardpaths only, as:Y=(A٠B٠C)+(A

٠D٠E)+(F٠E)

^ can be minimized, using Boolean algebra’s distributed rule, to:Y=A٠(B٠C+D٠

E)+(F٠E)

Logic Solution and LadderDiagram Representation

Solution^ ^ Output Y, including the reverse powerflow logic, is represented by: ^ The term F • D • B • C implements thereverse^ power^

flow^ sequence^ thatoutput Y requires

Ladder Diagram Implementationof the solution

Ladder Relay Instructions^ ^ A contact, regardless of whether it represents an input/outputconnection or an internal output, can be used throughout thecontrol program whenever the condition it represents must beevaluated.^ ^ The format of the rung contacts in a PLC program depends onthe desired control logic.^ ^ Contacts may be placed in whatever series, parallel, or series/parallel configuration is required to control a given output.^ ^ When logic continuity exists in at least one left-to-right contactpath, the rung condition is TRUE; that is, the rung controls thegiven output.^ ^ The rung condition is FALSE if no path has continuity

Contacts:Contacts:Normally open^

-|^ |-

Normally closed^

Positive^ transition sensing^

-|P|-

Negative transition sensing^

-|N|- docsity.com

Examine-on/Normally Open^ ^ An examine-ONinstruction with a logic0 reference address ^ An examine-ONinstruction with a logic 1reference address.

Examine-off/Normally Closed^ ^ An examine-OFF instruction, also called a normally closed (NC)contact instruction, tests for an OFF condition in the referenceaddress.^ ^ Like an examine-ON instruction, the address can reference theinput table, the output table, or the internal bit storage sectionof the output table.^ ^ During^ the^ execution^ of

an^ examine-OFF^ instruction,

the processor examines the reference address for an OFF condition.  If^ the^ reference^ address^ has

a^ logic^0 status^ (OFF),^ the instruction will continue to provide power (continuity) throughthe normally closed contacts.  If^ the^ reference^ address^ has

a^ logic^1 status^ (ON),^ the instruction will open the normally closed contact, thus breakingcontinuity to the rung.  An examine-OFF instruction can be associated with a logic NOTfunction,^ so^ that^ if^ the^ reference

address^ is^ NOT^ ON,^ logic continuity will be provided.

Coil:Coil:Coil^

-( )- negative coil^

-(/)- Set Coil^

-(S)- Reset Coil^

-(R)- Retentive memory Coil^

-(M)- Set retentive memory Coil^

-(SM)- Reset retentive memory Coil^

-(RM)- Positive Transition-sensing^ Coil^

-(P)- Negative Transition-sensing^ Coil^

-(N)- ^ Set coil latches the state, reset coil de energize the set coil. retentivecoil retain the state after power failure.

Output Coil^ ^ An^ output^ coil^ instruction

controls^ either^ a^ real^ output(connected to the PLC via output^ interfaces)^ or^ an^ internaloutput (control relay). ^ This instruction uses an output coil address bit in the internalstorage area as its reference address. ^ The^ —(^ )—^ symbol^ may

also^ represent^ an^ output^

coil instruction.  During the execution of an output coil instruction, the processorevaluates all the input conditions in the ladder rung.  If no continuity exists, the processor places a 0 in the outputcoil address bit, indicating an OFF condition to the output coilinstruction.  However, if the processor detects continuity in any path, theprocessor^ places^ a^ logic^1

in^ the^ output^ coil^ address^

bit referenced by the instruction.