The RSLogix basic Comparison Instructions
- From the LogixPro Simulations Menu, select the I/O Simulation.
- Clear out any existing program by selecting the "New" entry in the
File menu, and then select the "Clear Data Table" entry in the Simulations
menu.
- Now enter the following program being careful to enter the addresses
and values exactly as shown.
- Once you have completed entering your program, download
your program to the PLC.
- Ensure that SW0 and SW! are configured as Normally Open pushbuttons
then place the PLC into the Run mode.
- Toggle the state of switch SW0 (I:1/0) continuously while observing
the truth of each of rungs as indicated by the lamps.
- Once the count exceeds nine or ten, reset the counter and repeat the
above sequence. Keep doing this until you are convinced that the instructions
are operating as described in the RSLogix documentation.
- Finally, indicate the observed state of the lamps, by circling the
appropriate numbers below:
Lamp 0 is On during counts: 1...2...3...4...5...6...7...8...9...10
Lamp 1 is On during counts: 1...2...3...4...5...6...7...8...9...10
Lamp 2 is On during counts: 1...2...3...4...5...6...7...8...9...10
Lamp 3 is On during counts: 1...2...3...4...5...6...7...8...9...10
Conclusions:
The basic Comparison instructions compare the values stored in two memory
locations. These two values can be the data stored in two different word
locations, or one can be the data stored in a word and the other can be a
constant value. The basic comparison instructions are:
- EQU .... The Equal instruction goes true if the source A and B
values are Equal to each other
- NEQ .... The Not Equal instruction goes true if the source A and B
values are Not Equal to each other
- LES .... The Less Than instruction goes true if the value in
source A is Less Than the value in source B
- GRT .... The Greater Than instruction goes true if the value
in source A is Greater Than the value in source B
- LEQ .... The Less Than OR Equal instruction goes true if the value
in source A is Less Than or Equal to the value in source B
- GEQ .... The Greater Than OR Equal instruction goes true if the
value in source A is Greater Than or Equal to the value in source B
Since any PLC word including Timer and Counter accumulators and presets can be
used as the source value in any of the basic comparison instructions, these
instructions prove extremely versatile and are widely used in RSLogix programs.
The LIM Instruction .... Limit Comparison
- Modify or rewrite your existing program so that the first 3 rungs appear
as shown below.
- Ensure that addresses and values are exactly as shown.
- Once you have completed modifying your program, download
your program and place the PLC into the Run mode.
- Toggle the state of switch SW0 (I:1/0) continuously while observing
the truth of Lamp 4.
- Once the count exceeds ten, reset the counter and repeat the
above sequence. Keep doing this until you are convinced that the LIM
instruction is operating as described in the RSLogix documentation.
- Finally, indicate the observed state of Lamp 4, by circling the
appropriate numbers below:
Lamp 4 is On during counts: 1...2...3...4...5...6...7...8...9...10
Conclusions:
The LIM (Limit Comparison) instruction compares the Test value to the value
of the Low Limit and the value of the High Limit. The instruction goes true if
.. Test is Equal to or Greater than the Low Limit .. And .. Test is Less Than or
Equal to the High Limit. A logical equivalent to rung 2 is shown below:
The LIM instruction provides in a single package the same functionality that
would normally necessitate the utilization of 2 basic comparison instructions.
The MEQ Instruction .... Masked, Equal Comparison
The MEQ instruction is sometimes considered an advanced level instruction which
many might deem inappropriate for inclusion in a basic level exercise. The MEQ
is however listed with the other comparison instructions, so a quick peek at it's
functionality should not cause major harm. The subject of "Masking" relates to the
act of controlling which bits within a binary value or word are passed through to a
destination.
The following program is a very simple demonstration of how extraneous
information can be selectively ignored with the judicious use of masking.
There are several other instructions in the RSLogix instruction set that employ
masking and work similarly, but only the MEQ instruction will be reviewed here.
- Clear your existing program by selecting the "New" entry in the
File menu.
- Now enter the following program being careful to enter the addresses
and values exactly as shown.
- Note: address I:5 is the address of the I/O simulator's input card which
has the thumbwheel switches connected to it.
The MEQ and EQU instructions are almost identical in operation. The only
difference is that with the MEQ instruction, selected bits within the Source
value can be "masked out" or deleted prior to doing the comparison for
Equality. In the above MEQ example we are going to mask out all bits
other than the 4 which contain the data from the first thumbwheel switch.
- Once you have completed your program, download your program and place
the PLC into the Run mode.
- Starting with the right hand (units) thumbwheel only, increment the
displayed value up and down and note how both lamps energize when the value
is set to 4.
- Now set the first thumbwheel to 4 and start incrementing the second
(tens) thumbwheel. If your program is operating correctly only lamp 6
should remain lighted.
- Finally, set the thumbwheel values to match those listed below, and
circle the appropriate number if the corresponding lamp is On.
Lamp 5 is On when the thumbwheel value is: 1...4...14...34...54...94...104
Lamp 6 is On when the thumbwheel value is: 1...4...14...34...54...94...104
Conclusions:
The MEQ instruction has the same functionality of an EQU instruction, but it
allows you to first mask out any extraneous information or bits prior to doing the
actual test for equality.
Advanced level? Not really, but it does require a reasonably good understanding
of numbering systems and in particular binary.
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