Page 395 - Programmable Logic Controllers, Fifth Edition - Mobile version

• Both counters can also be reset manually at any
Ladder logic program
time by actuation of the Manual_Conveyor_Reset
Calculate button.
DIV
Divide
Source A 5
Source B 3 Comparison Instructions
Dest Answer_Real Compare instructions are used to compare two values.
1.6666666 They can be used to see if two values are equal, if one
value is greater or less than the other, and so on. In
Tag Name Value Style Data Type ControlLogix controllers, compare instructions are input
Calculate 1 Decimal BOOL instructions that do comparisons by either using an ex-
Answer_Real 1.6666666 Float REAL pression or doing the comparison indicated by the specific
instruction. Figure 15-83 shows the Compare toolbar for
Figure 15-81 DIV instruction rung and its Monitor Tags the ControlLogix controller.
window.
The equal (EQU) instruction is used to test if two
values are equal. Values compared can be actual values
The DIV instruction is used to divide two numbers. or tags that contain values. Figure 15-84 shows an exam-
Figure 15-81 shows an example of a DIV instruction rung ple of an EQU instruction rung along with its Monitor
along with its Monitor Tags window. The operation of the Tags window. The operation of the rung can be summa-
rung can be summarized as follows: rized as follows:

• A constant (5) is used for Source A and a con- • The value stored at Source A is compared to the
stant (3) for Source B. Note that tags could have value stored at Source B.
been used for Source A or Source B. • If the values are equal, the instruction is
• When the Calculate tag is true the DIV instruction logically true.
is executed. • If the values are unequal, the instruction is
• Source A (5) is divided by Source B (3) and the logically false.
result (1.6666666) is stored in the Dest tag Answer_ • In this example Source A (25) is equal to Source B
Real. Note that in this example a Real-type tag has (25) so the instruction is true and output Equal_PL
been used for its destination. is on.
• Source A and Source B may be SINT, INT, DINT,
The program of Figure 15-82 is used as part of a parts
tracking system with three conveyors. The number of or REAL data types.
parts in conveyor 1 and the number of parts in conveyor The not equal (NEQ) instruction is used to test two
2 are added to get the number of parts on conveyor 3. values for inequality. Figure 15-85 shows an example of
The operation of the program can be summarized as an NEQ instruction rung. When Source A is not equal to
follows: Source B, the instruction is logically true; otherwise, it
is logically false. In this example the two values are not
• Each time Conveyor_1_Sensor is actuated
the accumulated value of Counter_1_Parts is equal so the Not_Equal_PL is energized.
The less than (LES) instruction is used to check if a
incremented by 1. value from one source is less than the value from a sec-
• Each time Conveyor_2_Sensor is actuated ond source. Figure 15-86 shows an example of an LES
the accumulated value of Counter_2_Parts is instruction rung. When Source A is less than Source B,
incremented by 1. the instruction is logically true; otherwise, it is logically
• The addition in the ADD instruction places the sum false. In this example Value_1 (100) is less than Value_2
of the accumulated values of the two counters in the (300) so the Less_Than_PL is energized.
Conveyor_3_Parts tag. The greater than (GRT) instruction is used to check
• When the accumulated value for either counter is if a value from one source is greater than the value from
equal to 150 the reset (RES) instructions for both a second source. Figure 15-87 shows an example of a
counters are enabled to automatically reset both GRT instruction rung. When Source A is greater than
counter ACC values to zero. Source B, the instruction is logically true; otherwise,

376 Part 5 Math, Comparison, and Move Instructions

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