Page 189 - Programmable Logic Controllers, Fifth Edition - Mobile version

Photoelectric
sensor

Material
Photoelectric processing system
sensor

Out-feed
(finished parts)

In-feed
(raw parts)
(a)

Inputs Ladder logic program
L1 IN-Feed
count
CTU
IN-Feed COUNT-UP COUNTER CU
count Counter C5:1
Preset 50 DN
Accumulated 0
OUT-Feed
count
OUT-Feed Accumulated = No. in-process parts
count
Reset to CTD
zero COUNT-DOWN COUNTER CD
Counter C5:1 DN
Preset 50
Accumulated 0
RESETReset
C5:1
RES

(b)
Figure 8-26 In-process monitoring program. (a) Process. (b) Program.

8.4 Cascading Counters • The status bits of both counters are programmed in
series to produce an output.
Depending on the application, it may be necessary to count
events that exceed the maximum number allowable per coun- • These two counters allow twice as many counts to
ter instruction. One way of accomplishing this count is by be measured.
interconnecting, or cascading, two counters. The program • A CTU instruction that is reset while the counter
of Figure 8-27 illustrates the application of the technique. logic remains true will result in an accumulated value
The operation of the program can be summarized as follows: of 1 instead of 0. Using the OSR instruction in the
counter enabling logic prevents this from happening.
• The output of the first counter is programmed into Another method of cascading counters is sometimes
the input of the second counter.
used when an extremely large number of counts must
• When the accumulated value of the second counter be stored. For example, if you require a counter to
is equal to its preset, the DN bit will be true, which count up to 250,000, it is possible to achieve this by
allows the first counter to count. using only two counters. Figure 8-28 shows how the

170 Chapter 8 Programming Counters

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