Copyright © 2002 Delmar Thomson Learning Chapter 15 Timer and Counter Instructions

Chapter 15

Timer and Counter Instructions

Objectives

Describe the function of an on-delay timer.

Describe the function of an off-delay timer.

Describe in what instances one would use a retentive timer.

Describe the function of an up counter.Describe the function of a down counter.

Objectives (cont’d.)

Describe in what instances one would use an up counter versus a down counter.

Define preset, accumulative value, and the timer or counter address.

Explain how the various timers and counters are reset.

T4, Timer FileThe timer file stores only timer

elements.An element is a word or group of words

that work together as a unit.A timer is made of of three pieces or

words:Preset valueAccumulated valueStatus bits

T4, Timer File (cont’d.)

The preset value and accumulated value are 16-bit signed integers.

Status bits are single bits that make up one 16-bit word.

These three words work together as a unit.

One Timer Element Is Made of Three 16-Bit Words

Timer Addressing

Sample timer element addressT4:2T4 = timer file 4:2 = timer element #2 (0-255

timer elements per file)

Sub-Element

A sub-element is part of an element addressable as a unit.

The preset value and accumulated value are sub-elements of a timer:T4:0.PRET4:0.ACC

Timer Status Bits

Timers have three status bits.Done bit (DN) is true when the

accumulated value and preset are equal.

Timer timing bit (TT) is true when the timer is timing.

Enable bit (EN) is true when the timer instruction is enabled or true.

Timer Bit Addressing

Status bit addresses for timer file 4, timer element 2 (T4:2) are listed below:T4:2/DN is the address for the done bit.T4:2/EN is the address for the enable

bit.T4:2/TT is the address for the timer

timing bit.

Timer File T4

C5, Counter FileThe counter file stores only counter

elements.An element is a word or group of

words that work together as a unit.A counter is made of of three pieces

or words:Preset valueAccumulated valueStatus bits

C5, Counter File (cont’d.)

The preset value and accumulated value are 16-bit signed integers.

Status bits are single bits that make up one 16-bit word.

These three words work together as a unit.

One Counter Element Is Made of Three 16-Bit Words

Counter Addressing

Sample counter element address C5:2C5 = timer file 5:2 = counter element #2 (0-255

timer elements per file)

Sub-Element

A sub-element is part of an element addressable as a unit.

The preset value and accumulated value are sub-elements of a counter:C5:0.PREC5:0.ACC

Counter Status Bits

Counters have five status bits.Done bit (DN) is true when the

accumulated value and preset are equal.

Count up enable bit (CU) is true when the up counter is true or enabled.

Count down enable bit (CD) is true when the count down counter is enabled or true.

Counter Status Bits (cont’d.)

The overflow bit (OV) is true when the up counter has overflowed above +32767.

The underflow bit (UN) is true when the down counter has underflowed below -32768.

The update accumulator bit (UA) is a high-speed counter status bit for fixed SLC 500 PLCs.

Counter Status Bit Addressing

Status bit addresses for counter file 5, counter element 0 (C5:0) are listed below:

C5:0/DN is the address for the done bit.

C5:0/CU is the address for the count up enable bit.

Counter Status Bit Addressing (cont’d.)

C5:0/CD is the address for the count down enable bit.

C5:0/OV is the address for the count up overflow bit.

C5:0/UN is the address for the count down underflow bit.

Counter File C5

SLC 500 On-Delay Timer

SLC 500 Timer Instructions

SLC 500 On-Delay Timer and Associated Status Bits

SLC 500 Off-Delay Timer

SLC 500 Retentive Timer

Retentive Timer and Its Reset Instruction

SLC 500 Counters

SLC 500 Count Up Counter

SLC 500 Count Down Counter

SLC 500 Count Down Counter Instruction

Reset Instruction to Reset Counter C20:7

Using the Clear Instruction to Clear C5:0.ACC and

C5:1.Acc

Copyright © 2002 Delmar Thomson Learning Chapter 15 Timer and Counter Instructions

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