Rockwell Automation 1772-LP3 PLC - 2/30 Programmable Controller Programming and Operations Manual - Manual (Page 338)

Programming .01ĆSecond Timers
Appendix C
CĆ7
The processor can also update a timer only at the instant it is executing
that timer instruction. Remember that an integral timing clock (see the
preceding section, Timer Accuracy, on the previous page) puts out pulses
for the 1.0-, 0.1- and 0.01-second timers. When the 1.0- and 0.1-second
timers are used in a program, the timing pulses are always longer than the
process or scan time. No special programming is required; these timers will
not miss a timing pulse.
Timing pulses for the 10-msec time base, however, are usually shorter
than the program scan time. Since the processor can only increment a timer
while it is executing that instruction, the 10-msec timer could miss one or
more timing pulses on each program scan. The solution is to instruct the
processor to execute the timer instruction often enough that it will not miss
a pulse.
In order to compensate for the length of the scan time and to assure
accurate timing, 10-msec timer programming must be repeated several
places in the program.
A typical program using the total memory can nominally be assumed
to have a scan time of less than 30 msec. (See Scan Time Computation,
below.) In such a program, enter the same timer rung at 3 different
places in the program: once near the beginning, once near the middle, and
once near the end of the program. The processor will update the timer
accumulated value each time it scans that timer instruction. Refer to
Figure C.2 and note the following:
The rung must be identical each time it is used: the same Examine
instructions to condition the rung, the same timer word address, the
same time base, and the same preset value.
Again, this technique is required only for 0.01-second timers. (The
program scan is fast enough to assure accurate operation of the 1.0- and
0.1-second timers with only one timer rung per program.)
C.5.3
Programming
Compensation