Page 164 - Programmable Logic Controllers, Fifth Edition

True
Time input PB1
False
EN (enable) bit On Enable bit is reset
O when input pushbutton
Accum = Preset PB1 is opened.
8 9
7
Accumulated value 5 6
retained when rung 4
condition goes false 2 3
1
0
Accumulated value
On
DN (done) bit
O
On
PL output
O
On
Reset input PB2
O
01 23 45 67 89 10 11 12
Time in seconds

Figure 7-28 Retentive on-delay timer timing chart.

• When the accumulated value (9) equals the preset • If the timing rung goes false, the timer will stop
value (9), the timer done bit T4:2/DN is set to 1 and timing but will recommence timing for the stored
the pilot light output PL is switched on. accumulated value each time the rung goes true.
• Whenever the momentary reset pushbutton is • When the reset PB2 is closed, the T4:2/DN bit is
closed, the timer accumulated value is reset to 0. reset to 0 and turns the pilot light output off. The ac-
cumulated value is also reset and held at zero until
Figure 7-28 shows a timing chart for the retentive on-
delay timer program. The timing operation can be sum- the reset pushbutton is opened.
marized as follows: The program drawn in Figure 7-29 illustrates a prac-
tical application for an RTO. The purpose of the RTO
• When the timing rung is true (PB1 closed), the
timer will commence timing. timer is to detect whenever a piping system has sustained

Inputs Ladder logic program Output
L1 L2
S1 T4:1
Reset
RES
S1 Horn
Key switch PS
RTO
RETENTIVE TIMER ON EN
Pressure Timer T4:1
switch Preset 60 DN
PS
Accumulated 0
T4:1 Horn

DN
Figure 7-29 Retentive on-delay timer alarm program.

Programming Timers Chapter 7 145

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