PLC Data Types: Introduction
When we write a PLC program, we are not only writing logic but we are managing memory, speed, accuracy, and reliability. Every signal coming from the field, whether it is a push button, flow transmitter, temperature sensor, or HMI input, must be stored in PLC memory using a correct data type.
Many beginners think that all numbers are the same. But in PLC programming, choosing the wrong data type can create serious problems such as overflow errors, wrong scaling, slow scan time, communication mismatch, and incorrect display on HMI or SCADA.
Understanding PLC Data Types helps you write cleaner programs, troubleshoot faster, and build stable automation systems.
In this article, we will clearly explain 9 powerful PLC Data Types used in industrial automation with simple examples and practical usage.
Bit (BOOL)
A Bit is the smallest data unit in PLC memory. It can store only two values: ON or OFF, TRUE or FALSE, 1 or 0.
A Bit occupies only one memory position and is extremely fast to process.
It is mainly used for digital signals such as push buttons, proximity sensors, limit switches, interlocks, alarms, and motor status feedback.
Example: A start push button gives ON when pressed and OFF when released. The PLC stores this signal as a Bit.
Bit is not suitable for storing numeric values, counts, or analog data.
In real plants, thousands of Bits are used to represent field digital signals and logic conditions.
Byte
A Byte consists of 8 Bits grouped together. Instead of handling 8 individual bits separately, they can be stored and processed as one Byte.
A Byte can represent values from 0 to 255.
It is often used in communication data, device status registers, and bit-mapped signals.
Example: A valve manifold may send 8 digital status bits in one Byte through Modbus communication.
Byte allows compact data transfer and reduces communication overhead.
Bytes are useful when multiple small digital states must be transmitted together efficiently.
Word
A Word is a 16-bit data type. It stores larger numeric values than a Byte.
A Word can represent values from 0 to 65,535 (unsigned).
Many PLCs use Word registers to store raw analog input values.
Example: A pressure transmitter gives a 4–20 mA signal converted into a digital value like 0–27648 inside the PLC, stored as a Word.
Word is suitable for counters, preset values, and raw sensor data.
Word is commonly used in IO modules and register-based communication protocols.
Integer (INT)
An Integer stores signed whole numbers, meaning it can represent both positive and negative values.
Typical range is –32,768 to +32,767.
It is ideal for calculations, counters, indexing, and scaled engineering values.
Example: Temperature scaled from 0–100°C may be stored as 0–100 INT.
Integer cannot store decimal values.
If decimal accuracy is required, Integer should not be used.
Double Integer (DINT)
A Double Integer is a 32-bit signed number designed for large numeric values.
It supports a much larger range compared to INT.
Used for production counters, flow totalizers, energy accumulation, and batch quantities.
Example: A water plant totalizer accumulating millions of liters requires DINT to avoid overflow.
DINT consumes more memory but ensures safety for long-term counting.
For totalizers and long-running processes, DINT is always recommended.
Real (FLOAT)
Real data type stores decimal numbers using floating-point format.
It allows fractional values such as 12.45, 0.876, or 125.75.
Used for flow, temperature, pressure, density, and PID calculations.
Example: Flow rate displayed as 145.32 m³/hr must be stored as Real.
Real calculations are slightly slower than Integer calculations but provide high accuracy.
Real is essential for engineering calculations and precise control.
String
String data type stores text characters instead of numbers.
Used for alarm messages, batch names, operator inputs, product codes, and recipes.
Each character occupies memory, so length should be controlled.
Example: HMI displays “Pump Running” or “Tank Level High” using String variables.
Strings are rarely used in control logic but heavily used in visualization systems.
Strings improve operator interaction and system readability.
Double Word (DWORD)
A Double Word is a 32-bit unsigned value.
Used mainly in bit manipulation, communication registers, and status flags.
Stores values from 0 to very large unsigned range.
Example: Industrial communication protocols pack multiple status bits inside a DWORD.
Preferred when signed values are not required.
DWORD is common in PLC communication mapping and diagnostics.
Time
Time data type stores time-based values such as milliseconds, seconds, or minutes.
Used in timers, delays, sequencing, watchdogs, and runtime tracking.
Helps create accurate process timing.
Example: Motor delay start of 5 seconds stored as Time.
Time resolution depends on PLC scan cycle and timer accuracy.
Time improves process synchronization and safety.
Comparision Table of Different PLC Data Types
| Data Type | Size | Value Range | Stores | Typical Applications | Example |
|---|---|---|---|---|---|
| Bit (BOOL) | 1 Bit | 0 or 1 | ON / OFF status | Push buttons, limit switches, alarms, interlocks | Motor Start = TRUE |
| Byte | 8 Bits | 0 to 255 | Group of bits | Device status, communication bytes | Valve status byte |
| Word | 16 Bits | 0 to 65,535 | Unsigned numeric value | Analog raw values, registers, counters | AI raw = 27648 |
| Integer (INT) | 16 Bits | -32,768 to +32,767 | Signed whole numbers | Scaling, indexing, counters | Temperature = 85 |
| Double Integer (DINT) | 32 Bits | Very large signed range | Large numeric values | Totalizers, production counts | Total = 2,500,000 |
| Real (FLOAT) | 32 Bits | Decimal numbers | Floating-point values | Flow, pressure, PID calculations | Flow = 125.75 |
| String | Variable | Text length limited | Characters / text | HMI messages, recipes, batch names | “Pump Running” |
| Double Word (DWORD) | 32 Bits | Large unsigned range | Bit-mapped registers | Communication status mapping | Status Register |
| Time | Variable | ms to hours | Time values | Timers, delays, sequencing | T#5s |
Common Mistakes While Selecting PLC Data Types
Using Integer instead of Real for analog scaling causes loss of decimal accuracy.
Using INT for large counters leads to overflow.
Using Real for simple ON/OFF logic wastes memory and slows scan time.
Communication mismatch occurs when PLC and HMI use different data types.
Excessive String usage increases memory load unnecessarily.
Correct data type selection avoids these issues.
What we learn today?
Understanding PLC Data Types is a fundamental skill for every automation engineer, technician, and student. It directly impacts system accuracy, memory usage, processing speed, and troubleshooting efficiency.
By selecting the right data type for each signal, you create stable PLC programs, reliable control systems, and scalable automation projects. Whether you are handling a simple digital input or a complex flow calculation, the data type always matters.
Mastering PLC Data Types will make you a better programmer and a stronger automation professional.
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