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IE 262

Programmable Logic Controllers andPLC Programming

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PLC System

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input output

Ladder Diagram

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Example 1

Example 2

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SensorSensor

Output

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accumulated value

done bit

The number of time units or counts that have accrued. When the accumulated valueequals the preset value, the done bit is enabled. .

A bit that ends the time delay or counter count when the accumulated value is equalto the preset value. The done bit is one of the most frequently used status bits.

preset value

A value set in advance that triggers a timer or counter action. When the accumulatedvalue equals the preset value, the done bit is enabled.

enable bit

A bit that begins timer timing when the bit is true (1). The enable bit is one of themost frequently used status bits.

PLC timer

PLC counter

A control device that automatically starts or stops machines and other devices when

a preset time period has been exceeded. PLC timers do not exist in the real world,but rather as a set of instructions inside the PLC microprocessor.

A device that counts, calculates, or keeps a record of the number of times somethinghappens. PLC counters do not exist in the real world, but rather as a set of instructionsinside the PLC microprocessor.

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PLC TIMERS PERFORMS VARIOUS FUNCTIONS

They delay an action They cause an operation to run a predetermined period of time They record the total accumulated time of continuous or intermidiate events

ACTIVATING A TIMER

A timer is activated by a change in the logic continuity of its rung

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1

2

3

5

7 END

T4:2/ENEN

DN

DN

EN

EN

DN

6

O:2/0

T4:3/EN T4:1/DNTON

T4:1/EN

TON

TON

T4:1/EN T4:3/EN T4:2/DN

T4:1/EN T4:2/EN T4:3/DN

T4:2/EN

T4:3/EN

O:2/1

O:2/2

T4:1Pre:30 Acc:0

T4:2Pre:10 Acc:0

T4:3Pre:20 Acc:0

RED

YELLOW

GREEN

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An ON-delay timer is enabled when its rung is true. A rung is truewhen there is at least one path made by the instructions that are truefrom the left rail to the timer instruction.

An OFF-delay timer is enabled when its rung is false. When a timer isenabled, its Accumulate value shows how many Time Base intervals

have elapsed since the timer was activated.A timer is done when its Accumulate value reaches its Preset value.

When an ON-delay timer or an OFF-delay timer is timing, its rungcondition change, i.e. a rung changes from true to false for an ON-delay timer or a rung changes from false to true for an OFF-delaytimer, will cause the timer to stop and its Accumulate value to bereset to zero.

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Each timer instruction has three very useful status bits. These bits are Timer Enable (TE), Timer Timing(TT) and Timer Done (TN). Each of these bits has one bit of memory and the memory is affected by thecorresponding bit status. For an ON-delay timer and a retentive timer,

The Timer Enable bit is high when the timers rung is true; it is low when the rung is false.

The Timer Timing bit is high when the timers rung is true and the Accumulate value is less than the Preset

value. This bit is low when the rung is false or after the Accumulate value equals the Preset value.

The Timer Done bit is high when the rung is true and the timer is done. It is low when the rung is false orbefore the timer is done.

For an OFF-delay timer,

The Timer Enable bit is high when the timers rung is false; it is low when the rung is true.

The Timer Timing bit is high when the timers rung is false and the Accumulate value is less than thePreset value. This bit is low when the rung is true or after the Accumulate value equals the Preset value.

The Timer Done bit is high when the rung is false and the timer is done. It is low when the rung is true orbefore the timer is done.

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I:1/0

I:1/1B:3/0

B:3/0

TON

T4:0EN

DN

T4:0/EN

T4:0/TT

T4:0/DN

B:3/0

O:2/0

O:2/1

O:2/2

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PLC COUNTER INSTRUCTIONS

Counters are output instructions that are internal to a PLC

Counters simply count and store the number of events that occur

The two common type of counters are up-counters and down-counters

Up-counters increment and down counters decrement each time thecounters rung goes from FALSE to TRUE

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I:1/0

I:1/2

I:1/1

C5:1/CU

C5:1/CD

C5:1/DN

C5:1

O:2/0

O:2/1

O:2/2

CU

CD

DN

DN

RES

l

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Example:1

When the start pushbutton is pressed, the red pilot light turns on and the conveyor

starts running. Light bulbs travel along a conveyor and when the 6th light bulb issensed via proximity sensor, the conveyor stops. An operator picks up the light bulbsand restarts the machine by pressing Start button. Pressing Stop shuts down thewhole process.

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STARTI: 1/0 STOP

I: 1/1 B3:O

O:2/0

O:2/1C5:1/DN

I:1/2

B3:O

B3:O

O:2/1CTU

C5:1Preset:6 Acc. DN

CU

RESETC5:1

I:1/1

1

2

3

5

6 ENDC5:1/DN

B3:O (internal bit)

Red Pilot

Conveyor

Count Up

Reset the counterif/when STOP is

pressed

If START is pressed andcounter preset is notequal to 6

DN:Done bit. BecomesTRUE (1) when Accumulator value isequal to preset value.FALSE(0) otherwise.

E l 2

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Example:2

I: 1/0

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I: 1/1 B3:O

O:2/0

O:2/1

I:1/5

T4:1/DN

B3:O

O:2/1CTU

C5:1Preset:6 Acc. DN

CU

RESETC5:1

I:1/1

1

2

3

5

6 END

T4:1/DN

B3:O

C5:1/DN

C5:1/DN

T4:1

TCNP:5 sec

EN

DN