Page 159 - Programmable Logic Controllers, Fifth Edition

Start
L1 Hardwired relay circuit L2
Stop PB2 Lube oil
PB1 pump motor OL
M1

M1-1 Main drive
PS1 motor OL
M2

(Lube oil
pressure switch) TD
OL
M3
TD-1 Feed
(15 s) motor

Ladder logic program Outputs
Inputs
L1
PB1 PB2 M1 L2

PB1 OL
M1 M1
PB2
OL
PS1 M2 M2
PS1
OL
M3
TON
TIMER ON DELAY EN
Timer T4:0
Time base 1.0 DN
Preset 15
Accumulated 0

T4:0 M3

DN

Figure 7-20 Automatic sequential control system.

• When the lube-oil pump builds up sufficient oil to time until its preset value equals the accumulated
pressure, the lube-oil pressure switch PS1 closes. value. The program sequence is reset by actuating
• This in turn energizes coil M2 to start the main the reset button.
drive motor and energizes coil TD to begin the time-
delay period. 7.4 Off-Delay Timer Instruction
• After the preset time-delay period of 15 s, TD-1 con- The off-delay timer (TOF) operation will keep the
tact closes to energize coil M3 and start the feed motor. output energized for a time period after the rung con-
• The ladder logic program shows how an equivalent taining the timer has gone false. Figure 7-21 illustrates
circuit could be programmed using a PLC. The en- the programming of an off-delay timer that uses the
able bit is used to seal in the timer so it continues SLC 500 TOF timer instruction. TOF starts timing when

140 Chapter 7 Programming Timers

pet73842_ch07_131-155.indd 140 05/11/15 4:21 PM