Page 163 - Programmable Logic Controllers, Fifth Edition

involves pumping fluid from tank A to tank B. The opera-
tion of the process can be summarized as follows:
• Before starting, PS1 must be closed. Cam-operated
contact
• When the start button is pushed, the pump starts.
The button can then be released and the pump con-
tinues to operate.
• When the stop button is pushed, the pump stops. Motor-driven
cam
• PS2 and PS3 must be closed 5 s after the pump starts.
If either PS2 or PS3 opens, the pump will shut off
and will not be able to start again for another 14 s. Figure 7-26 Electromechanical retentive timer.

7.5 Retentive Timer
similar to the nonretentive on-delay timer (TON), with one
A retentive timer accumulates time whenever the device major exception—a retentive timer reset (RES) instruction.
receives power, and it maintains the current time should Unlike the TON, the RTO will hold its accumulated value
power be removed from the device. When the timer ac- when the timer rung goes false and will continue timing
cumulates time equal to its preset value, the contacts of the where it left off when the timer rung goes true again. This
device change state. Loss of power to the timer after reach- timer must be accompanied by a timer reset instruction to
ing its preset value does not affect the state of the contacts. reset the accumulated value of the timer to 0. The RES in-
The retentive timer must be intentionally reset with a sepa- struction is the only automatic means of resetting the accu-
rate signal for the accumulated time to be reset and for the mulated value of a retentive timer. The RES instruction has
contacts of the device to return to its nonenergized state. the same address as the timer it is to reset. Whenever the
Figure 7-26 illustrates the action of a motor-driven, RES instruction is true, both the timer accumulated value
electromechanical retentive timer used in some appliances. and the timer done bit (DN) are reset to 0. Figure 7-27
The shaft-mounted cam is driven by a motor. Once power shows a PLC program for a retentive on-delay timer. The
is applied, the motor starts turning the shaft and cam. The operation of the program can be summarized as follows:
positioning of the lobes of the cam and the gear reduction
of the motor determine the time it takes for the motor to • The timer will start to time when time pushbutton
turn the cam far enough to activate the contacts. If power is PB1 is closed.
removed from the motor, the shaft stops but does not reset. • If the pushbutton is closed for 3 s and then opened
A PLC retentive timer is used when you want to retain for 3 s, the timer accumulated value will remain
accumulated time values through power loss or the change at 3 s.
in the rung state from true to false. The PLC-programmed • When the time pushbutton is closed again, the timer
retentive on-delay timer (RTO) is programmed in a manner picks up the time at 3 s and continues timing.

Inputs Ladder logic program Output
L1 L2
Reset
PB2 T4:2
PB2 RES
Time PL
PB1
PB1 RTO
RETENTIVE TIMER ON EN
Timer T4:2
Time Base 1.0 DN
Preset 9
Accumulated 0

T4:2 PL

DN
Figure 7-27 Retentive on-delay timer program.

144 Chapter 7 Programming Timers

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