Data Types in PLC

In this post, we will understand the various PLC data types like a bit, byte, integer, real, string, etc.

Contents

Various new programmers face issues in understanding a data type of PLC. Data type means the format of the variable or constant used in a program.

Without understanding a data type and how its format works, a PLC programmer cannot develop and define a program properly.

Bit

The format of a variable starts with a single bit. Its value is defined as 0 or 1.

In simpler terms, compare it with the Indian currency. It starts with a single paisa and multiple paisas make a rupee. Similarly, the format of any data type starts with a bit.

Its value is only true or false. The “bit” is also called as Boolean.

Byte

Now, 8 bits make a byte. The first start bit means LSB (least significant bit) and the last bit means MSB (most significant bit).

Refer to the below image. It comprises 8 number of bits, with each bit having its individual value. The range of a format is defined by 2^n.

Here, n is the number of bits used in a data type.

So, as per the format used here in byte, the value for 2⁸will be 256. That means 256 numbers of combinations or values will be possible in a byte.

Any format with multiple bits can be classified into two types – signed and unsigned. Signed means the range has both positive and negative numbers.

Unsigned means the range has only positive numbers. So, here in a byte, an unsigned byte means 0 to 25; and a signed byte means -128 to 127.

A byte is also called a short integer.

Integer

Now, 16 bits make an integer. The first start byte means LSB (least significant byte) and the last byte means MSB (most significant byte).

The value for 2¹⁶will be 65536. That means 65536 numbers of combinations or values will be possible in an integer.

An integer is also called as a word. But, a word cannot be signed. It will be unsigned only.

An unsigned integer means 0 to 65535, and a signed integer means -32768 to 32767.

Double Integer

Now, 32 bits make a double integer. The value for 2³²will be 4294967296. That means 4294967296 numbers of combinations or values will be possible in a double integer.

An unsigned double integer means 0 to 4294967296, and a signed double integer means -2147483648 to 2147483647.

In a PLC memory, two words make a double integer. So, for example, consider a Schneider PLC. %MW0 and %MW1 both will make a double integer; whereas only a single %MW0 or %MW1 will make an integer.

So, if you use %MD0 in this PLC, the next available address will be %MD2; as %MD0 comprises of %MW0 and %MW1.

Real

Here too, 32 bits makes a real format. But, the major difference between real and double integers is that a real variable can directly be assigned a decimal value.

The value will be from -3.4028235E+38 to 3.4028235E+38.

Like a double integer, a real format also uses two memory words.

String

A string format can contain any string of characters – numeric or alphanumeric. Every PLC has its own reserved memory for string variables.

The memory space needed for a string is 1 byte per character + 1 additional byte.

So, say a value – WELCOME. It will require 7 bytes for storing this value plus one additional byte; which means a total of 8 bytes will be required.

These are some of the generally used data types in a PLC.