Page 167 - Programmable Logic Controllers, Fifth Edition

L1 Inputs Ladder logic program Outputs L2
TS1 T4:6 TON
TIMER ON DELAY EN G
DN Timer T4:5
PS1 Time base 1.0
Preset 1 DN
Accumulated 0
LS1 T4:5 R
TON
TIMER ON DELAY EN
DN
Timer T4:6
Time base 1.0 DN Y
Preset 1
Accumulated 0
TS1 T4:5 G

DN
PS1 T4:5 R

DN
LS1 T4:5 Y

DN
Figure 7-33 Annunciator flasher program.

of an output is required. For example, if a flashing light • The first timer, T4:1, is programmed for a preset
is required in the event of a control system failure, a pro- time of 30,000 s and begins timing when input SW
gram with reciprocating timers could be used to create the is closed.
flashing output function. • When T4:1 completes its time-delay period 30,000 s
Two timers can be interconnected to form an later, the T4:1/DN bit will be set to 1.
oscillator or reciprocating circuit. The oscillator logic • This in turn activates the second timer, T4:2, which
is basically a timing circuit programmed to generate is preset for the remaining 12,000 s of the total
periodic output pulses of any duration. Figure 7-33 42,000-s time delay.
shows the program for an annunciator flasher circuit. • Once T4:2 reaches its preset time, the T4:2/DN bit
Two internal timers form the oscillator circuit, which will be set to 1, which switches on the output PL,
generates a timed, pulsed output. The oscillator circuit the pilot light, to indicate the completion of the full
output is programmed in series with the alarm condi- 42,000-s time delay.
tion. If the alarm condition (temperature, pressure, or
limit switch) is true, the appropriate output indicating • Opening input SW at any time will reset both timers
light will flash. Note that any number of alarm con- and switch output PL off.
ditions could be programmed using the same flasher A typical application for a PLC circuit combining
circuit. both cascading and reciprocating functions would be
At times you may require a time-delay period longer the control of traffic signals. The ladder logic circuit of
than the maximum preset time allowed for the single timer Figure 7-35 illustrates a control of a set of traffic lights in
instruction of the PLC being used. When this is the case, one direction. The operation of the program can be sum-
the problem can be solved by simply cascading timers, as marized as follows:
illustrated in Figure 7-34. The operation of the program
can be summarized as follows: • Transition from red light to green light to amber
light is accomplished by the interconnection of the
• The total time-delay period required is 42,000 s. three TON timer instructions.

148 Chapter 7 Programming Timers

pet73842_ch07_131-155.indd 148 05/11/15 4:22 PM