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 ______.