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1 timer instructions – Rockwell Automation 1772-LP3 PLC - 2/30 Programmable Controller Programming and Operations Manual User Manual

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Timer and Counter Instructions

Chapter 5

5Ć2

The remaining 4 bits in a word (bits 14-17) are not used to form a BCD
number. In the accumulated value word, they are used as status bits. In the
preset value word, they are not used and are available for internal storage
provided data is not transferred to the preset word by a Get/Put transfer.
With .01 sec timers these bits are used for internal timing functions and
cannot be used for storage.

The processor requires time to monitor the status of the I/O image tables
and execute instructions in the users program. Every instruction requires
execution time each scan whether the rungs condition instructions are true
or false unless the instruction is skipped by a Jump instruction.

A timer counts elapsed time-base intervals and stores this count in its
accumulated value word. When timing is complete (when AC = PR), bit 15
is either set on or off depending on the type of timer instruction. For all
timers, bit 17 is set on when rung conditions are true and is set off when
they are false. Both status bits are located in the accumulated value word
(Figure 5.2).

Figure 5.2

Timer Accumulated Value Word

17

16

15

14

13

12

11

10

07

06

05

04

03

02

01

00

Most

Significant

Digit

Middle

Digit

Least

Significant

Digit

Goes ON and OFF at

Selected Time Base Rate

of 1.0 or 0.1 second.

Accumulated Value

in BCD Form

Timed Bit.

This Bit is set to 1 or 0

When the Timer has

Timed Out, that is AC=PR.

Enabled Bit.

This Bit is Set to 1

When Timer Rung

Conditions are True.

The three types of timers available with the PLC-2/30 processor are:

Timer On-Delay –(TON)–
Timer

Off-Delay

–(TOF)–

Retentive

Timer

–(RTO)–

5.1
Timer Instructions