Rockwell Automation 1772-LP3 PLC - 2/30 Programmable Controller Programming and Operations Manual - Manual (Page 333)

Programming .01ĆSecond Timers
Appendix C
CĆ2
Given any preset value, a Mini-PLC-2 controller timer is accurate to within
one interval of its time base (and this is generally true for any type of
timer). Specifically, the timed interval does not exceed the preset interval,
but it may be as much as 1 time-base unit shorter than the preset. Let’s
illustrate this with the following examples:
TON: Time base = 1.0 second; preset value = 100. This time interval
will be greater than 99 seconds and less than or equal to 100 seconds, as
shown below:
99 seconds < TON timed out < = 100 seconds
TON: Time base = 0.1 second; preset value = 100. This time interval
will be greater than 9.9 seconds and less than or equal to 10 seconds, as
shown below:
9.9 seconds < TON timed out < = 10.0 seconds
TON: Time base = 0.01 second; preset value = 100. This time interval
will be greater than 0.99 seconds and less than or equal to 1 second, as
shown below:
0.99 seconds < TON timed out < = 1.0 second
Note that special programming techniques are required to use the 10-msec
timer in a program. These techniques are discussed later.
Programmed timers examine internal pulses of the Mini-Processor (refer to
figure C.1). A change in the state of this internal clock causes the timer to
increment its accumulated value. Note, however, that the timing pulses are
continuous and are only examined by the Mini-Processor when a timer
instruction is being executed in the program. As Figure C.1 shows, when
the Mini-Processor initially examines this internal clock, the clock may
have just changed state or may be just about to change state. It is this
variable that makes possible the inaccuracy of up to 1 time-base increment.
C.2
Timer Accuracy