RSLogix 5000 Math Instructions – CPT, ADD, SUB, MUL, DIV, MOD, SQRT, NEG, ABS
Published on Aug 05, 2025 | Category: MathsShare this Page:
RSLogix 5000 provides powerful math instructions to perform arithmetic and advanced calculations within ladder logic programming. These instructions include basic operations such as ADD, SUB, MUL, and DIV, as well as functions like MOD (modulus), SQRT (square root), NEG (negate), and ABS (absolute value).
The CPT (Compute) instruction allows for complex mathematical expressions to be written in a single block, making it ideal for combining multiple operations efficiently. This guide explains each instruction, its syntax, parameters, and usage with examples for Allen Bradley ControlLogix and CompactLogix PLCs.
What is a Math Instruction in RSLogix 5000?
Math instructions in RSLogix 5000 are used to perform arithmetic and numerical operations within ladder logic programs. These instructions operate on tag values and support common data types such as INT (Integer), DINT (Double Integer), and REAL (Floating Point). Math instructions are essential for tasks like analog scaling, counter calculations, totalization, and data comparison in control systems.
You can use individual math instructions for simple operations or combine them inside the CPT (Compute) instruction for advanced expressions. All math instructions require valid source and destination tags. The source tags provide input values, and the destination tag stores the result.
Types of Math Instructions in RSLogix 5000
- ADD – Adds two source values and stores the result in a destination tag. Supports INT, DINT, or REAL data types.
- SUB – Subtracts the second source tag from the first. Ideal for calculating differences like setpoint–feedback. Works with numeric data types.
- MUL – Multiplies two numeric values. Commonly used for scaling factors or unit conversions.
- DIV – Divides one tag value by another. Be cautious of divide-by-zero errors. Data types must match.
- MOD – Returns the remainder after division. Useful in batch or periodic logic (e.g., every nth cycle).
- SQRT – Calculates the square root of a value. Input must be positive and typically of type REAL.
- NEG – Converts a positive value to negative and vice versa. Useful in reversing direction or polarity.
- ABS – Returns the absolute value (positive). Useful in magnitude comparison, regardless of sign.
- CPT (Compute) – Evaluates a full arithmetic expression using multiple tags and operators. Requires the destination tag and expression in text format.
ADD Instruction in RSLogix 5000
The ADD instruction performs addition by summing two values: Source A and Source B. The result is stored in the Destination tag.
Each of these fields – Source A, Source B, and Destination – can be tags or constants. The data types commonly used are INT, DINT, or REAL.
Operands:
- Source A: First value (can be a tag or a constant like 10, 5.5, etc.)
- Source B: Second value (tag or constant)
- Destination: Tag where the result is stored (must be of compatible data type)
For example, if:
- Source A = Input1 (REAL)
- Source B = 25.5 (constant)
- Destination = tag_OutputSum (REAL)
The result of tag_Input1 + 25.5 will be saved in tag_OutputSum.
This instruction is commonly used for analog signal adjustments, scaled calculations, and process totalization.
SUB Instruction in RSLogix 5000
The SUB instruction performs subtraction by subtracting Source B from Source A. The result is stored in the Destination tag.
All three fields – Source A, Source B, and Destination – can be tags or constants. The most commonly used data types are INT, DINT, or REAL.
Operands:
- Source A: Minuend value (tag or constant)
- Source B: Subtrahend value (tag or constant)
- Destination: Tag where the result is stored (must match or support the data type of the sources)
For example, if:
- Source A = Tank_Level (REAL)
- Source B = 5.0 (constant)
- Destination = Adjusted_Level (REAL)
The result of Tank_Level - 5.0 will be stored in Adjusted_Level.
This instruction is useful in applications like setpoint deviation, level correction, or adjusting sensor offsets.
MUL Instruction in RSLogix 5000
The MUL instruction multiplies two values: Source A and Source B, and stores the result in the Destination.
Both sources can be tag values or constants, and the destination must support the result's data type. Common data types used are INT, DINT, or REAL.
Operands:
- Source A: First value (tag or constant)
- Source B: Second value (tag or constant)
- Destination: Result tag
Example:
- Source A = Motor_RPM (DINT)
- Source B = 60 (constant)
- Destination = RPM_Per_Minute (DINT)
This instruction is widely used for scaling, power calculation (e.g., V × I), and time conversion.
DIV Instruction in RSLogix 5000
The DIV instruction divides Source A by Source B and stores the result in the Destination tag.
Ensure the data types support the division result. Use REAL data type for fractional or floating-point results to avoid truncation.
Operands:
- Source A: Numerator (tag or constant)
- Source B: Denominator (tag or constant)
- Destination: Result tag
Example:
- Source A = Total_Weight (REAL)
- Source B = Bag_Count (REAL)
- Destination = Weight_Per_Bag (REAL)
This instruction is commonly used for average calculation, ratio control, and time-based rate measurements.
RSLogix 5000 MOD (Modulo) Instruction
The MOD instruction in RSLogix 5000 is used to find the remainder after division. It is helpful in applications where a value must wrap around after reaching a certain limit, such as indexing, blinking operations, or alternating between states.
Parameters:
- Source A: Dividend (the number to be divided)
- Source B: Divisor (must not be zero)
- Destination: The remainder after division (Source A MOD Source B)
Example:
If Source A = 17 and Source B = 5, then the result stored in Destination = 2 (because 17 ÷ 5 = 3 remainder 2).
RSLogix SQRT (Square root) Instruction
The SQRT instruction calculates the square root of a given non-negative number. It takes one SOURCE value and stores the result in the DEST tag.
Usage Format:
- Source: A REAL tag or numeric constant (must be ≥ 0)
- Dest: A REAL tag to store the result
Example:
- Source: Motor_Power (Type: REAL, Value: 144.0)
- Dest: Root_Result (Type: REAL)
Result: Root_Result = 12.0
Application: Calculating the magnitude of voltage or current in vector-based systems, or in RMS formula like √(A² + B²).
RSLogix NEG (Negate) Instruction
The NEG instruction changes the sign of a number. It multiplies the input by -1. This is useful when dealing with direction reversal, signed values, or correcting values in control logic.
Parameters:
- Source: Any numeric value
- Destination: Stores the negative equivalent of the source
Example:
If Source = 25, then Destination = -25. Similarly, if Source = -10, then Destination = 10.
RSLogix 5000 CPT Instruction
The CPT (Compute) instruction in RSLogix 5000 evaluates a complete mathematical expression and stores the result in a specified Dest tag.
This instruction is used when complex arithmetic operations involving multiple operands and mathematical functions are required. The CPT instruction enables expressions like (A + B) * C / D in a single rung.
- All operands must be numerical types such as DINT, REAL, or LREAL.
- If any operand or the destination is LREAL, all other operands are internally converted to LREAL before execution.
- If the destination or any operand is REAL, and no LREAL is involved, all operands are converted to REAL.
- The result of the expression is stored in the Dest tag.
Imagine a vertical cylindrical tank. The volume of a cylinder is calculated using the formula:
Volume = π × r² × h
In the PLC:
- Radius is the radius of the tank (in meters), stored in tag: Radius
- Height is the current water level in the tank (in meters), stored in tag: Level
- FilledVolume is the result tag (in cubic meters)
CPT instruction expression:
CPT Dest: FilledVolume Expression: 3.1416 * Radius * Radius * Level
This computes the volume of water based on the current level in the tank.Advantages of CPT Instruction
- Allows a single instruction to perform complex calculations.
- Reduces the need for multiple rungs and MOV/ADD/MUL instructions.
- Supports a wide range of math functions including trigonometric and logarithmic operations.
RSLogix 5000 ABS (Absolute Value) Instruction
The ABS instruction in RSLogix 5000 calculates the absolute value of a number. It removes the negative sign, if any, and returns only the positive magnitude of the input.
This instruction is useful when you want to ensure a value is always positive, such as when dealing with direction-independent measurements like speed, temperature deviations, or error values.
How It Works:
- Source: The input tag or numeric value (can be negative or positive).
- Dest: The output tag that stores the absolute (positive) result.
Example:
If Source = -45.7, then after ABS instruction executes, Dest = 45.7.
Use this to avoid errors in calculations that require non-negative values.
