Comparison, Data Move, and Math Instructions

Comparison, Data Move, and Math Instructions in Ladder Logic

Modern PLCs do far more than switch outputs on and off. They compare numbers, move data between registers, and perform arithmetic — all inside ladder logic rungs. This lesson covers the instructions you will use most in industrial maintenance: GRT, LES, EQU, MOV, ADD, SUB, and MUL. Mastering these lets you read, write, and troubleshoot the control programs running the machinery you maintain.

How Comparison Instructions Work

A comparison instruction sits on a rung like a normally open contact — it passes power (evaluates TRUE) when its condition is met, and blocks power (evaluates FALSE) when it is not. The PLC re-evaluates every rung every scan, so a comparison updates continuously while the program runs.

GRT (Greater Than): Passes power when the Source A value is greater than the Source B value. Example: if the process temperature tag reads 215 and the setpoint tag holds 200, GRT is TRUE and any output on that rung energizes.

LES (Less Than): Passes power when Source A is less than Source B — the logical opposite of GRT.

EQU (Equal): Passes power only when Source A exactly equals Source B. EQU is used most often with integer counters and timers, where exact equality is realistic. On analog values that change continuously, EQU can be difficult to trigger reliably; use a deadband or a combination of GRT and LES instead.

In Allen-Bradley ControlLogix / CompactLogix notation, you specify a tag (a named memory location) for each source rather than a raw address. Other manufacturers use similar function blocks under slightly different names, but the logic is the same.

The MOV (Move) Instruction

MOV copies a value from a Source to a Destination. It is an output instruction — it executes when the rung is TRUE. A common use is loading a setpoint at startup:

• Source: a constant (for example, 200) or a tag holding an operator-entered value
• Destination: the setpoint tag your comparison instruction reads

Because MOV executes every scan the rung is TRUE, you typically control it with a one-shot contact so it fires once, or you leave it always TRUE if you want the destination to track the source continuously.

Math Instructions: ADD, SUB, MUL

Math instructions are also output instructions — they execute when the rung is energized.

ADD: Adds Source A and Source B and stores the result in the Destination tag. In this lesson you will use ADD to accumulate flow: each time a flow pulse counter increments, ADD adds the new count to a running total stored in a DINT (32-bit signed integer) tag.

SUB (Subtract): Subtracts Source B from Source A and writes the result to Destination.

MUL (Multiply): Multiplies Source A by Source B and writes the result to Destination. Useful for unit conversions — for example, converting pulse counts to engineering units.

Integer vs. Floating-Point Data Types

Choosing the right data type prevents subtle errors.

• INT (16-bit integer): Holds whole numbers from −32,768 to 32,767. Adequate for small counters and setpoints in °F.
• DINT (32-bit integer): Holds whole numbers from roughly −2.1 billion to +2.1 billion. Use for totalizers where counts accumulate over long periods.
• REAL (32-bit floating point): Holds decimal values. Use when you need fractional precision — for example, 147.3 gallons. Floating-point math can introduce small rounding errors; never use EQU to compare two REAL tags directly.

In this lab the accumulated flow tag is a DINT because pulses are whole numbers. If you later convert to gallons with a decimal factor, move the result to a REAL tag.

Analog Scaling and the Temperature Input

The temperature transmitter in this lab produces a 4–20 mA signal representing 0–250 °F. The PLC analog input module converts this to a raw integer count. A scaling function (often a built-in SCL or CPT instruction, or a manual formula) maps that count to engineering units stored in a REAL or INT tag. In this lab the scaling is already configured; your comparison reads the scaled tag directly. Always verify the scaled value against a known reference before relying on a comparison for process control.

Safety Rules for This Lab

Lockout/Tagout: The PLC trainer is low-voltage, but the principles you practice here apply directly to live equipment. Never modify a program on live production equipment without following your facility's Management of Change and LOTO procedures per OSHA lockout/tagout requirements.

Online Edits: When monitoring a running program, changes take effect immediately. Confirm with your instructor before going online. Never make online edits to unfamiliar programs on actual machinery — an unintended output could move actuators and cause injury.

PPE: Safety glasses are required in the lab at all times. Closed-toe shoes are required.

Static Sensitive Devices: Handle PLC modules by the edges; ground yourself before touching terminals.

Connecting It to the HMI

An HMI (Human-Machine Interface) reads tag values directly from the PLC data table. When you write the accumulated gallons to a DINT tag and the setpoint to an INT tag, the HMI display updates automatically each scan. This is why naming tags clearly and choosing the correct data type matter — the HMI inherits whatever the PLC tag holds.