Page 157 - Programmable Logic Controllers, Fifth Edition

Timers may or may not have an instantaneous output
Input condition A On (also known as the enable bit) signal associated with
O
On them. If an instantaneous output signal is required from
Timer-enable bit O a timer and it is not provided as part of the timer in-
On struction, an equivalent instantaneous contact instruc-
Timer-timing bit O 4 s 10 s tion can be programmed using an internally referenced
On relay coil. Figure 7-18 shows an application of this
Timer-done bit O
technique. The operation of the program can be sum-
marized as follows:
Timer
accumulated 0 • According to the hardwired relay circuit diagram,
value coil M is to be energized 5 s after the start pushbut-
Figure 7-16 Timing diagram for an on-delay timer. ton is pressed.
• Contact TD-1 is the instantaneous contact, and con-
tact TD-2 is the timed contact.
• The ladder logic program shows that a contact in-
• When the accumulated value reaches 10 s, the done struction referenced to an internal relay is now used
bit (DN) goes from false to true and the timer- to operate the timer.
timing bit (TT) goes from true to false. • The instantaneous contact is referenced to the in-
• When input A goes false, the timer instruction ternal relay coil, whereas the time-delay contact is
goes false and also resets, at which time the con- referenced to the timer output coil.
trol bits are all reset and the accumulated value Figure 7-19 shows an application for an on-delay timer
resets to 0. that uses an NCTO contact. This circuit is used as a warn-

The timer table for an Allen-Bradley SLC 500 is ing signal when moving equipment, such as a conveyor
shown in Figure 7-17. Addressing is done at three dif- motor, is about to be started. The operation of the circuit
ferent levels: the element level, the word level, and the can be summarized as follows:
bit level. The timer uses three words per element. Each • According to the hardwired relay circuit diagram,
element consists of a control word, a preset word, and coil CR is energized when the start pushbutton PB1
an accumulated word. Each word has 16 bits, which is momentarily actuated.
are numbered from 0 to 15. When addressing to the bit
level, the address always refers to the bit within the • As a result, contact CR-1 closes to seal in CR coil,
word: contact CR-2 closes to energize timer coil TD,
and contact CR-3 closes to sound the horn.
EN = Bit 15 enable • After a 10-s time-delay period, timer contact TD-1
TT = Bit 14 timer timing opens to automatically switch the horn off.
DN = Bit 13 done • The ladder logic program shows how an equivalent
circuit could be programmed using a PLC.
• The logic on the last rung is the same as the timer-
timing bit and as such can be used with timers that
do not have a timer-timing output.
Timer Table Timers are often used as part of automatic sequential
/EN /TT /DN .PRE .ACC control systems. Figure 7-20 shows how a series of mo-
T4:0 0 0 0 10 0 tors can be started automatically with only one start/stop
T4:1 0 0 0 0 0 control station. The operation of the circuit can be sum-
T4:2 0 0 0 0 0 marized as follows:
T4:3 0 0 0 0 0
T4:4 0 0 0 0 0 • According to the relay ladder schematic, lube-oil
T4:5 0 0 0 0 0
pump motor starter coil M1 is energized when the
Address T4:0 Table: T4: Timer start pushbutton PB2 is momentarily actuated.
• As a result, M1-1 control contact closes to seal in
Figure 7-17 SLC 500 timer table. M1, and the lube-oil pump motor starts.

138 Chapter 7 Programming Timers

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