Page 196 - Programmable Logic Controllers, Fifth Edition - Mobile version

Inputs Ladder logic program 1 min timer
L1 Start SW
TON
TIMER ON DELAY EN
O On
Timer T4:1 DN
Time base 1.0
Start SW Preset 60
Accumulated 0

Total
Start SW T4:1 Sensor parts
Sensor CTU CU
COUNT-UP COUNTER
DN Counter C5:1 DN
Preset 0
Accumulated 0
Start SW C5:1
RES

Figure 8-36 Product flow rate program.
Source: Photo courtesy Omron Industrial Automation, www.ia.omron.com.

• Every time the motor stops due to an overload A timer is sometimes used to drive a counter when
condition, the motor start circuit is locked out for an extremely long time-delay period is required. For
5 min. example, if you require a timer to time to 1,000,000 s,
• If the motor trips off more than 5 times in an you can achieve this by using a single timer and counter.
hour, the motor start circuit is permanently locked Figure 8-37 shows how the timer and counter would be
out and cannot be started until the reset button is programmed for such a purpose. The operation of the pro-
actuated. gram can be summarized as follows:
• The lock-out pilot light is switched on whenever a • Timer T4:0 has a preset value of 10,000, and coun-
permanent lock-out condition exists. ter C5:0 has a preset value of 100.
Figure 8-36 shows a product part flow rate program. • Each time the timer T4:0 input contact closes for
This program is designed to indicate how many parts pass 10,000 s, its done bit resets timer T4:0 and incre-
a given process point per minute. The operation of the ments counter C5:0 by 1.
program can be summarized as follows: • When the done bit of timer T4:0 has turned on
and off 100 times, the output light becomes
• When the start switch is closed, both the timer and
counter are enabled. energized.
• The counter is pulsed for each part that passes the • Therefore, the output light turns on after 10,000 ×
parts sensor. 100, or 1,000,000, seconds after the timer input
contact closes.
• The counting begins and the timer starts timing
through its 1-minute time interval.
• At the end of 1 minute, the timer done bit causes the 8-7 High-Speed Counters
counter rung to go false. The maximum counting frequency of a traditional PLC’s
• Sensor pulses continue but do not affect the PLC counter is limited by the scan time of the processor. When
counter. the frequency of the input signal is higher than that of
• The number of parts for the past minute is repre- the scan time, it is necessary to utilize a high-speed
sented by the accumulated value of the counter. counter (HSC), to avoid errors. For example, using an
• The sequence is reset by momentarily opening and incremental encoder in a length-measuring application
closing the start switch. generally requires the use of a high-speed counter. The

Programming Counters Chapter 8 177

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