Programmable Logic Controllers, 3rd Edition Page 291 (291 of 414)

Chapter 13 PLC Logic and Bit Shift Instructions 291
000
End
T4:0/DN
007
EN
DN
TON
Timer On Delay
Timer T4:0
Time Base 0.1
Preset 20
Accum 5
001
EN
DN
BSR
Bit Shift Right
File #B3:0
Control R6:0
Bit Address B3:0/3
Length 4
002
R6:0/UL
I:1/0
Start
T4:0/DN
B3:0/3
003
B3:0/0 O:2/0
GLT
004
B3:0/1 O:2/1
RLT
005
B3:0/2 O:2/2
WLT
Reset
006
I:1/1 R6:0
RES
Figure 13-17. PLC ladder logic diagram and binary table for Example 13-7. Using the bit shift right
function to turn on and off three lights in descending order.

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Chapter 13 PLC Logic and Bit Shift Instructions 291
000
End
T4:0/DN
007
EN
DN
TON
Timer On Delay
Timer T4:0
Time Base 0.1
Preset 20
Accum 5
001
EN
DN
BSR
Bit Shift Right
File #B3:0
Control R6:0
Bit Address B3:0/3
Length 4
002
R6:0/UL
I:1/0
Start
T4:0/DN
B3:0/3
003
B3:0/0 O:2/0
GLT
004
B3:0/1 O:2/1
RLT
005
B3:0/2 O:2/2
WLT
Reset
006
I:1/1 R6:0
RES
Figure 13-17. PLC ladder logic diagram and binary table for Example 13-7. Using the bit shift right
function to turn on and off three lights in descending order.

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292 Programmable Logic Controllers: Hardware and Programming
Summary
• The NOT instruction converts a bit file word’s bits from a logic
high (1) to a logic low (0), and vice versa.
• The AND, OR, and XOR instructions perform logic operations
on the bit of two data source locations and place the outcome in
a destination location.
• The bit shift left (BSL) instruction shifts left a specified number
of bits in a file; the bits are shifted one position every time the
instruction is energized.
• The bit shift right (BSR) instruction shifts right a specified
number of bits in a file; the bits are shifted one position every
time the instruction is energized.
• Bit shift instructions (BSL and BSR) are typically used to control
lights on a display board.
Review Questions
1. How many control registers are available in an Allen-Bradley
SLC 500 PLC? Name them.
2. Which coil is energized to indicate that a bit shift left (BSL) has
shifted the bits as many times as indicated in the length?
3. Which data file(s) are ordinarily used for bit shift instructions?
4. How can a bit shift instruction be brought to its beginning or
original position?
5. What instruction can be used to reset a bit shift instruction?
6. When is the control bit R6:0/EN in a bit shift instruction
energized?
7. What input transition causes the bit shift right (BSR) instruction
to shift the bits once?
Complete each of the following sentences with the correct word(s).
8. In bit shift instructions, data file ______ or ______ can hold the
bits, which are to be shifted.
9. The bit where the first shifted bit exits is called the ______.
10. The ______ determines the number of bits to be shifted.
11. Words ______ through ______ can be used for control registers in
the bit shift instructions.
12. For every low-to-high input switch transition on the bit shift left
(BSL) instruction, data bits will shift ______ once.
13. In a bit shift right (BSR) instruction that uses the R6:0 control
register, the enable bit is addressed as ______.

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290 Programmable Logic Controllers: Hardware and Programming
Example 13-7
The diagram in Figure 13-17 demonstrates how to use a timer on
delay (TON) function and a bit shift right (BSR) function to turn on and off
lights in descending order every two seconds. The timer on delay controls
the bit shift right function, which moves a bit through the positions of
B3:0. Lights connected to the outputs at addresses O:2/0, O:2/1, and
O:2/2 illuminate when a bit is rotated into their corresponding binary
position: B3:0/0, B3:0/1, and B3:0/2. Lights first turn on and then off
in descending order. This process continues until the start switch at
address I:1/0 is opened. The reset pushbutton at address I:1/1 is used
to reset the control register R6:0 and set the bit pointer to the begin-
ning location B3:0/7.
End
005
R6:0/UL
Reset will bring the data file #B3:0 to its original setting.
**Close input I:1/2 to re-enter the lost bit that comes out of the right
side (R6:0/UL). Therefore, creating Rotate Right.
003
I:1/2
Rotate Mode
Reset
B3:0/7
Rotate Entry Location
004
I:1/3 R6:0
RES
Figure 13-16. Continued.

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