Page 184 - Programmable Logic Controllers, Fifth Edition

• The retentive OSR instruction is true for only
one scan and then false again, even if the trig- L1 Input LS1 OSR rung C5:1
B3:0/0
gering proximity switch signal stays true. This OSR RES
may be required for the count pulse to operate LS1
properly.
• After a count of 50, the done bit of the counter LS1
changes state to stop the conveyor motor automati-
cally and reset the counter’s accumulated value to B3:0/0
zero. One PLC scan
• The conveyor motor can be stopped and started Figure 8-19 One-shot rising (OSR) instruction.
manually at any time without loss of the accumu-
lated count.
• The accumulated count of the counter can be reset instruction conditions the rung so that the counter
manually at any time by means of the count reset C5:1 reset output instruction goes true for one pro-
button. gram scan.
The Allen-Bradley SLC 500 one-shot rising (OSR) • The output reset instruction goes false and remains
instruction is an input instruction that triggers an event false for successive scans until the input makes
to occur one time. The OSR instruction is placed in the another false-to-true transition.
ladder logic before the output instruction. When the • The OSR bit is set to 1 as long as the limit switch
rung conditions preceding the OSR instructions go from remains closed.
false-to-true, the OSR instruction goes true also but for • The OSR bit is reset to 0 when the limit switch is
only one scan. Figure 8-19 illustrates the operation of an opened.
OSR rung which can be summarized as follows:
Applications for the OSR instruction include freez-
• The OSR, one-shot rising instruction is used to ing rapidly displayed LED values. Figure 8-20 shows a
make the counter reset instruction (RES) true for one-shot instruction used to send data to an output LED
one scan when limit switch input LS1 goes from display. The one-shot allows the rapidly changing ac-
false to true. cumulated time from the timer to be frozen to ensure a
• The OSR is assigned a Boolean bit (B3:0/0) that is readable, stable display. The operation of the program is
not used anywhere else in the program. summarized as follows:
• The OSR instruction must immediately precede the • The accumulated value of timer T4:1 is converted
output instruction. to Binary Coded Decimal (BCD) and moved
• When the limit switch closes the LS1 and OSR, to output word O:6 where an LED display is
input instructions go from false to true. The OSR connected.

Inputs Output
L1 PB
I:1/0 B3:0/0 0
TOD 1
I:1/0 OSR To BCD 2
Source T4:1.ACC 3
SW Destination O:6 4
I:1/1 5
I:1/1 O:6 6 0000
TON 7
TIMER ON DELAY EN 8
Timer T4:1 DN 9
Time base 1.0 10
Preset 1000 11
Accumulated 0 12
13
14
15
Figure 8-20 OSR instruction used to freeze rapidly displayed LED values.

Programming Counters Chapter 8 165

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