Page 239 - Programmable Logic Controllers, Fifth Edition - Mobile version

Ladder logic program
Output
L2
LIM PL1
LIMIT TEST
Low limit N7:28 PL1
100
Test N7:29
125
High limit N7:27
50
True ( 50) 50 100 ( 100) True
False

Figure 10-29 LIM instruction where the low limit value is
greater than the high limit value. Figure 10-31 MEQ instruction can be used to monitor the
state of limit switches.
Source: Courtesy Jayashree Electrodevices.
• Instruction is false for test values greater than 50
and less than 100. source contains the limit switch address and the compare
• Instruction is true because the test value is 125. stores their desired states. The mask can block out the
switches you don’t want to compare (Figure 10-31).
The program of Figure 10-30 shows a practical ex- Figure 10-32 shows an example of an MEQ instruc-
ample of the ControlLogix program with the Limit Test tion. The operation of the logic rung can be summarized
(LIM) instruction. In this program, the LIM instruction as follows:
will energize the Count_Within_Range output when the
counter is within the range of 6 to 12 counts. Note that the • When the data at the source address match the data
range includes the values set as the low and high limits. at the compare address bit-by-bit (less masked bits),
The masked comparison for equal (MEQ) instruction the instruction is true.
compares a value from a source address with data at a com- • The instruction goes false as soon as it detects a
pare address and allows portions of the data to be masked. mismatch.
One application for the MEQ instruction is to compare • A mask passes data when the mask bits are set (1); a
the correct position of up to 16 limit switches when the mask blocks data when the mask bits are reset (0).

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CMP LIM MEQ EQU NEQ LES GRT
Bit Timer/Counter Input/Output Compare

Inputs Ladder logic program Outputs
L1 Counter_Input L2
<Local:1:I.Data.1>
CTU Count_Within_Range_Output
Counter_Input Count Up Counter_Range CU
Counter
Preset 20 DN
Accum 0
Counter_Reset Count_Within_Range_Output
LIM <Local:2:O.Data.1>
Limit Test (CIRC)
Low Limit 6
Test Counter_Range.ACC
0
High Limit 12
Counter_Reset
<Local:1:I.Data.2> Counter_Range
RES

Figure 10-30 ControlLogix program with the Limit Test (LIM) instruction.

220 Chapter 10 Data Manipulation Instructions

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