Programmable Logic Controllers

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Dr. D. J. Jackson Lecture 3-1 Electrical & Computer Engineering

Programmable Logic

Controllers

PLC Addressing and Basic

Instructions

Basic addressing

• For the Allen-Bradley PLCs and the simulator

used, the input and output image areas (in

addition to other data) are held in “data files”

• Each “data file” holds information such as

– Discrete input values – Discrete output values

– Values associated with timers – Values associated with counters – Status information

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PLC data files

Simulator data files AB PLC data files

I/O memory mapping

• Basic AB PLC addressing

X1:x.0/0

Data file type I – input O – output S – status B – binary T- timer C-counter

Data file number (can be omitted for the

basic data files)

Slot number (not used with the simulator)

Word number: selects a specific word in a data file

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Example Addressing

CPU Module (slot 0) Digital Input Module (slot 1) 16 inputs Digital Output Module (slot 2) 8 outputs Would occupy addresses I1:1.0/0 through I1:1.0/15 Would occupy addresses O0:2.0/0 through O0:2.0/7

Basic instructions

• Examine If Closed (XIC)

• Examine If Open (XIO)

• Output Energize (OTE)

• Output Latch (OTL)

• Output Unlatch (OTU)

• One-shot Rising (OSR)

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Examine if closed (XIC)

• Alternate name: Normally open contact

• Use the XIC instruction in your ladder program to determine if a bit is on.

– When the instruction is executed, if the bit addressed is on (1), then the instruction is evaluated as TRUE.

– When the instruction is executed, if the bit addressed is off (0), then the instruction is evaluated as FALSE.

Instruction symbol

Normally open switch is open

Normally open switch is closed

Examine if open (XIO)

• Alternate name: Normally closed contact

• Use the XIO instruction in your ladder program to determine if a bit is off.

– When the instruction is executed, if the bit addressed is on (1), then the instruction is evaluated as FALSE.

– When the instruction is executed, if the bit addressed is off (0), then the instruction is evaluated as TRUE.

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Output energize (OTE)

• Alternate name: coil

• This instruction is usually used in conjunction with XIC or XIO or any other input instruction.

• If the logic preceding the OTE instruction is true (1), the OTE instruction will be energized

Instruction symbol Not energized

Energized

An OTE instruction can only be the last

instruction on a rung.

Output latch (OTL)

• The OTL instruction is used only to turn

a bit on and latch it on

L

Initially not energized Energized Latched

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Output unlatch (OTU)

• The OTU instruction is used only to turn

a bit off and latch it off

• OTL and OTU are generally used in pairs

U

One-shot rising (OSR)

• When the rung conditions preceding the OSR

go from false-to-true, the OSR instruction

goes true also but for only one scan

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Branches

• Branches can be inserted in ladder logic programs to create parallel combinations of various ladder logic elements

Multiple Series/Parallel Branches

• Multiple series/parallel branches can be utilized with multiple inputs and also affecting multiple outputs in parallel

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PLC simulator usage

• Ladder logic elements (instructions) can be dragged and dropped onto the ladder window to create a ladder logic program

• The ladder logic program is executed by going online, ‘downloading’ the PLC program, and switching to run mode

OFFLINE/ONLINE control Download PLC run mode

ProSim simulations

• The Logixpro simulator provides a set of built-in simulations • The I/O simulation initially will be of most use since it provides

access to simulated discrete inputs (switches) and outputs (lamps)

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Other Prosim simulations

• Available simulators

– Door simulator

– Silo simulator – Traffic simulator – Batch simulator

– Dual Compressor simulator – Bottle line simulator

– Four floor elevator simulator

Programmable Logic Controllers