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Programmable Logic
Controllers
Comparison and Math Operations
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 7-
Comparison Instructions
Instruction Purpose Mnemonic Name EQUQU Equalqua Test whether two values are equalest et e t o a ues a e equa NEQ Not Equal Test whether one value is not equal to a second value LES Less Than Test whether one value is less than a second value LEQ Less Than or Equal
Test whether one value is less than or equal to a second value GRT Greater Than Test whether one value is greater than another
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 7-
GEQ Greater Than or Equal
Test whether one value is greater than or equal to a second value MEQ Masked Comparison for Equal
Test portions of two values to see whether they are equal. Compares data through a mask LIM Limit Test Test whether one value is within the limit range of two other values
About the Comparison Instructions
• Comparison instructions are used to test
pairs of values to condition the logical
continuity of a rungf
– Thus, comparison instructions would seldom, if
ever, be the last instruction on a rung
• As an example, suppose a LES instruction is
presented with two values
If the fi st al e is less than the second then the
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 7-
– If the first value is less than the second, then the
comparison instruction is true
Equal (EQU) & Not Equal (NEQ)
- Use the EQU instruction to test whether two values are equal. - If source A and source B are equal, the instruction is logicallyq , g y true. If these values are not equal, the instruction is logically false.
- Use the NEQ instruction to test whether two values are not equal. - If source A and source B are not equal, the instruction is logically true. S A b dd
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 7-
- Source A must be an address. Source B can be either a program constant or an address.
- Negative integers are stored in two’s complementary form.
Masked Comparison for Equal (MEQ)
- Use the MEQ instruction to compare data at a source address with data at a compare address.
- Source is the address of the value you want to compare.
- Mask is the address of the mask through which the instruction moves data
p
- Use of this instruction allows portions of the data to be masked by a separate word.
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 7-
- The mask can be a hexadecimal value.
- Compare is an integer value or the address of the reference.
- If the 16 bits of data at the source address are equal to the 16 bits of data at the compare address (less masked bits), the instruction is true.
- The instruction becomes false as soon as it detects a mismatch.
Limit Test (LIM)
- Use the LIM instruction to
test for values within or
outside a specified range,
depending on how you setp^ g^ y
the limits.
High Limit values can be
word addresses or constants,
restricted to the following
combinations:
- If the Test parameter is a program constant, both the Low Limit and High Limit t t b d
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 7-
parameters must be word addresses.
- If the Test parameter is a word address, the Low Limit and High Limit parameters can be either a program constant or a word address.
True/False Status of the LIM
Instruction
- If the Low Limit has a value equal to or less than the High Limit, the instruction is true when the Test value is between the limits or is equal to either limitvalue is between the limits or is equal to either limit.
Example, low limit less than high limit:
False True False
Low Limit High Limit
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 7-
Low Limit High Limit Instruction is True when Test value is
Instruction is False when Test value is 5 8 5 through 8 -32768 through 4 and 9 through 32767
True/False Status of the LIM
Instruction (continued)
- If the Low Limit has a value greater than the High Limit, the instruction is false when the Test value is between the limitsbetween the limits.
Example, high limit less than low limit:
True False True
High Limit Low Limit
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 7-
Low Limit High Limit Instruction is True when Test value is
Instruction is False when Test value is 8 5 -32768 through 5 and 8 through 32767
6 through 7
Updates to Arithmetic Status Bits
• The arithmetic status bits are found in Word 0, bits
0–3 in the controller status file.
• After an instruction is executed, the arithmetic statusAfter an instruction is executed the arithmetic status
bits in the status file are updated:
With this bit: The Controller:
S:0/0 Carry (C) sets if carry is generated; otherwise cleared
S:0/1 Overflow (V) indicates that the actual result of a math instruction does not fit in the designated d ti ti
Electrical & Computer Engineering Dr. D. J. Jackson Lecture 7-
destination
S:0/2 Zero (Z) indicates a 0 value after a math, move, or logic instruction S:0/3 Sign (S) indicates a negative (less than 0) value after a math, move, or logic instruction
