Automating Manufacturing Systems - Process Control and ...

plc timers - 9.8
A
TON
Timer T4:0
Time Base 1.0
Preset 4
Accumulator 0
(DN)
(EN)
T4:0/DN
B
A
T4:0/EN
T4:0/DN
T4:0/TT
B
4
3
2
0
T4:0 Accum.
0 3 6 9 13 14 17 19
Figure 9.8
An Allen-Bradley TON Timer
The timing diagram in Figure 9.8 illustrates the operation of the TON timer with a
4 second on-delay. A is the input to the timer, and whenever the timer input is true the EN
enabled bit for the timer will also be true. If the accumulator value is equal to the preset
value the DN bit will be set. Otherwise, the TT bit will be set and the accumulator value
will begin increasing. The first time A is true, it is only true for 3 seconds before turning
off, after this the value resets to zero. (Note: in a retentive time the value would remain at
3 seconds.) The second time A is true, it is on more than 4 seconds. After 4 seconds the TT
bit turns off, and the DN bit turns on. But, when A is released the accumulator resets to
zero, and the DN bit is turned off.
A value can be entered for the accumulator while programming. When the program
is downloaded this value will be in the timer for the first scan. If the TON timer is
not enabled the value will be set back to zero. Normally zero will be entered for the preset