Below describes the accuracy of a timer, from the coil is triggered to the contact is turned ON:
α： 0.001 sec. for the 1 ms timer; 0.01 sec. for the 10 ms timer; 0.1 sec. for the 100 ms timer
T ： Set time of timer (sec.)
Ts ： scan time (sec.)
If in the program, a timer contact is written in front of its coil, the maximum timing error would be extra 2 Ts.
If the set time of timer is “0”, the output contact will action in the next scan.
For the T246~T249, which is using the system's 1ms interrupt to increase the current value if the coil of
the timer has been triggered.
+Ts
－α
input processing
˙˙˙˙˙˙˙˙˙˙
X0
T0
T0
Y1
K120
When the PLC scans to the coil of an general non-retentive timer (not includs T192~T199), it accumulates or clears the
present value of the timer then controls its contact.
When the PLC scans to the END instruction, it accumulates or clears the current value of each T192~T199 timer then
controls its contact.
At the 1 ms system interrupt, it accumulates the present value of activated T246~T249 timers. Then when the PLC
scans to the timer's coil, it controls the contact of the timer.
To conclude, reminders for using timers in subroutine are shown below:
(1) A subroutine which will be executed once during each scan time can adopt all types of timers.
(2) A subroutine which will be executed several times during each scan time can adopt T192~T199 and T246~T249
timers.
(3) The subroutine which may not be executed at every scan time can adopt T192~T199 timers.
(4) An interrupt subroutine (which is only executed in interruption) must adopt T192~T199 timers.
Direct setting by using a constant number K
T200 is a timer using a 10 ms as the time unit.
When the setting constant K is 150, the set value of T200 is 1.5 Sec.
(150×10 ms=1500 ms=1.5 s).
T200 is a timer using a 10 ms as the time unit.
If D0=200 , T200 = 2 Sec. timer.
If D0=1000 , T200 =10 Sec. timer.
The set value of T200 can be modified by to change the content value
of D0.
Indirect setting by using a Data Register D
Below shows the action procedures of a general non-retentive timer:
During the timer T0 is started and the CPU repeatedly to
execute its “OUT” coil. That may increase the present value
of the timer by a few numbers (not only 1) if the period of
scan time is more than the timer's resolution unit.
T0
contact turned
to ON
Y1
ON
X0＝OFF→ON
Timer T0 is started
X0
T200
K150
MOV K200 D0
X1
X0
T200
D0
21
2-5-3 Using a Timer in a Subroutine
2-5-4 Methods to Appoint the Set Value of a Timer
2-5-5 Detailed Description about the Output Action and Accuracy of a Timer
st
1 scan
nd
2 scan
th
n scan
th
n+1 scan
At the moment, the accumulated time is reaching to 12 Sec.
Since the scan of CPU already passed the coil of T0, so the contact of
T0 would not be activated.
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