Rockwell Automation 1772-LS_LSP,D17726.8.6 PROG/OPER MANUAL-MINI PLC-2/05 - Manual (Page 68)

Chapter
8
8Ć1
Timers and Counters
This chapter describes two instructions that keep track of timed intervals or
counted events:
timers
counters
Timer and counter instructions are output instructions internal to the processor.
They provide many of the capabilities available with timing relays and solid
state timing/counting devices. Usually conditioned by examine instructions,
timers and counters keep track of timed intervals or counted events according to
the logic continuity of the rung. You can program up to 488 internal timers.
The last valid timer address is 1677.
Each timer or counter instruction has two 3-digit values. Each value requires
one word of data table memory. These 3-digit values are:
Accumulated Value (AC)
Storage: Begins at word address 030.
Function: Timers - number of elapsed timed intervals
Counters - number of counted events
Both - upper 4 bits of accumulated word (14-17)
are the status bits.
Preset Value (PR)
Storage: Always at address 100 words greater than its corresponding AC value.
Function: Denotes the number of timed intervals or events to be counted. When
the accumulated value equals the preset value, AC = PR, a status bit is set and
can be examined to turn an output device on or off.
A timer counts the elapsed time-base intervals and stores this count in the
accumulated value word. All timers must be placed within the first eight data
table blocks. Timer instructions have three time bases: 1.0 second, 0.1 second,
or 0.01 second.
Chapter Objectives
Introduction
Timer Instructions