Rockwell Automation 1761-HHP-B30 MicroLogix 1000 with Hand-Held Programmer (HHP) - Manual (Page 133)

Chapter 8
Using Basic Instructions
8–14
Each timer address is made of a 3-word element. Word 0 is the control word,
word 1 stores the preset value, and word 2 stores the accumulator value.
EN TT DN Internal Use
15 14 13
Preset Value
Accumulator Value
EN = Timer Enable Bit
TT = Timer Timing Bit
DN = Timer Done Bit
Word 0
Word 1
Word 2
Entering Parameters
Accumulator Value (ACC)
This is the time elapsed since the timer was last reset. When enabled, the
timer updates this continually.
Preset Value (PRE)
Specifies the value which the timer must reach before the controller sets the
done bit. When the accumulated value becomes equal to or greater than the
preset value, the done bit is set. You can use this bit to control an output
device.
Preset and accumulated values for timers range from 0 to +32,767. If a timer
preset or accumulated value is a negative number, a runtime error occurs.
Timebase
The timebase determines the duration of each timebase interval. The
timebase is selectable as 0.01 (10 ms) second or 1.0 second.
Timer Accuracy
Timer accuracy refers to the length of time between the moment a timer
instruction is enabled and the moment the timed interval is complete.
Timing accuracy is –0.01 to
+
0 seconds, with a program scan of up to 2.5
seconds. The 1-second timer maintains accuracy with a program scan of up
to 1.5 seconds. If your programs can exceed 1.5 or 2.5 seconds, repeat the
timer instruction rung so that the rung is scanned within these limits.
Important: Timing could be inaccurate if Jump (JMP), Label (LBL), Jump
to Subroutine (JSR), or Subroutine (SBR) instructions skip over
the rung containing a timer instruction while the timer is timing.
If the skip duration is within 2.5 seconds, no time will be lost; if
the skip duration exceeds 2.5 seconds, an undetectable timing
error occurs. When using subroutines, a timer must be executed
at least every 2.5 seconds to prevent a timing error.
Timer Instructions Overview