Page 160 - Programmable Logic Controllers, Fifth Edition

Input
I:1/0 Ladder logic program Output
L1 TOF L2
TIMER OFF DELAY EN
S1 Timer T4:3
I:1/0 Time base 1.0 DN O:2/1 PL
Preset 15
Accumulated 0
T4:3/DN O:2/1

True
S1 input
enable bit (EN) False

Timed period timing bit (TT)
15 s
O delay
timed duration
True (logic 1)
Timed output
done bit (DN) False (logic 0)
O:2/1
Preset value = accumulated value
Figure 7-21 Off-delay programmed timer.

the instruction goes from ON to OFF or from true to to 1, immediately turning on motors M1, M2,
false. The operation of the circuit can be summarized and M3.
as follows: • If SW is then opened, logic continuity to all three

• When the switch connected to input I:1/0 is first timers is lost and each timer begins counting.
closed, timed output O:2/1 is set to 1 immediately • Timer T4:1 times out after 5 s resetting its done bit
and the lamp is switched on. to zero to de-energize motor M1.
• If this switch is now opened, logic continuity is lost
and the timer begins counting.
• After 15 s, when the accumulated time equals the Timer OFF DELAY (TOF)
preset time, the output is reset to 0 and the lamp Enable Bit (EN) 1
switches off. TOF Timer Timing Bit (TT) 0
Instruction
• If logic continuity is gained before the timer is timed ON Done Bit (DN) 1
Accumulating
out, the accumulated time is reset to 0. For this rea- Enable Bit (EN) NO
0
son, this timer is also classified as nonretentive. TOF Timer Timing Bit (TT) 1
Instruction Done Bit (DN) 1
Figure 7-22 illustrates the use of an off-delay timer OFF Accumulating YES
instruction used to switch motors off sequentially at Enable Bit (EN) 0
5 second intervals. The operation of the program can be Timed Out Timer Timing Bit (TT) 0
summarized as follows: Accum = Preset Done Bit (DN) 0
Accumulating NO
• Timer preset values for T4:1, T4:2, and T4:3 are set Enable Bit (EN) 1
for 5, 10, and 15 s, respectively. Instruction OFF Timer Timing Bit (TT) 0 1
Done Bit (DN)
after timed out
• Closing the input switch SW immediately sets Accumulating Reset
the done bit of each of the three off-delay timers Table showing how each bit is e ected during the program operation.

Programming Timers Chapter 7 141

pet73842_ch07_131-155.indd 141 05/11/15 4:21 PM