Modbus RTU Data Tables and Formats

Modbus Message Examples

Follow the links in this table to see examples of Modbus requests and responses.

Table Name	Data size	Read Function codes	Write Single Function codes	Write Multiple Function codes
Discrete Inputs	1 bit	FC02	read only	read only
Coils	1 bit	FC01	FC05	FC15
Input Registers	16 bit	FC04	read only	read only
Holding Registers	16 bit	FC03	FC06	FC16

Coils and Registers

In Modbus protocol, data is organized into coils and registers, each accessed using specific function codes as shown above.

Coils represent single-bit values (ON/OFF), zero or one.

Registers store 16-bit values.

When reading or writing coils, up to 8 coils can be included in one data byte in the message. When a message is reading or writing any amount of coils not a multiple of 8, unused bits are padded with zeroes.

Data Types

In the example above for Function code 03, the master requested 3 registers, 40108 through 40110.
The slave's response shows 40108 contains AE41 in hexadecimal which is equal to these 16 bits 1010 1110 0100 0001 in binary. Great! But what does it mean?

Well, it depends on the data type assigned to the register.

Data Type	Possible values	Value
16-bit unsigned integer	whole number 0 to 65535	AE41 = 44,609
16-bit signed integer	whole number -32768 to 32767	AE41 = -20,927
two character ASCII string	any 2 ASCII characters	AE41 = ® A
discrete on/off value	0 or 1	can only be 0000 or 0001

Register 40108 can also be combined with 40109 to form any of these 32-bit data types:

Data Type	Possible values	Value
32-bit unsigned integer	whole number 0 to 4,294,967,295	AE41 5652 = 2,923,517,522
32-bit signed integer	whole number -2,147,483,648 to 2,147,483,647	AE41 5652 = -1,371,449,774
32-bit IEEE Single precision Floating point number	any real number	AE41 5652 = -4.395978 E-11
four character ASCII string	any 4 ASCII characters	AE41 5652 = ® A V R
discrete on/off value	0 or 1	can only be 0000 0000 or 0000 0001

More registers can be combined to form longer ASCII strings. Each 16 bit register being used to store two ASCII characters (two bytes). This can be useful for storing model numbers or serial numbers

Floating Point

The IEEE 754 Floating Point Standard is a mathematical formula that allows a real number (a number with decimal points) to be represented by 32 bits with an accuracy of about seven digits. A 32-bit Single precision floating point number allows for a wide range of values, from very large to very small. For even higher resolution, a 64-bit Double precision floating point number is possible using four 16 bit registers.

Here is a spreadsheet for for inputs of 4 bytes or 2 words. Download IEEE float calculator

Byte and Word order

The Modbus specification doesn't define exactly how the data is stored in the registers. Therefore, some manufacturers implemented modbus in their devices to store and transmit the more significant byte (high byte) first followed by the less significant low byte. ( AE before 41). Alternatively, others store and transmit the low byte first ( 41 before AE).

Similarly, when registers are combined to represent 32-bit data types, Some devices store the more significant 16 bits (high word) in the first register and the remaining less significant low word in the second register( AE41 before 5652)
while others do the opposite ( 5652 before AE41).9:38 PM 2026-04-09

In summary, there are four possibilities of 32-bit unsigned integers.

Byte order	Word order	also known as	Value
High byte first	High word first	"big endian"	AE41 5652 = 2,923,517,522
High byte first	Low word first		5652 AE41 = 1,448,259,137
Low byte first	High word first		41AE 5256 = 1,101,943,382
Low byte first	Low word first	"little endian"	5256 41AE = 1,381,384,622

It doesn't matter which order the bytes or words are sent in, as long as the receiving device knows which way to expect it. You can see the error is signicant if it's not correct.

What is a Modbus Map?

A modbus map is simply a document or list for a specific Modbus slave device that defines how its data is organized, incuding:
            - what the data is (eg. pressure or temperature readings)
            - where the data is stored (which coils and register tables and the data addresses)
            - how the data is stored (data types, byte and word ordering)

Some devices are built with a fixed map that is defined by the manufacturer. While other devices allow the operator to configure or program a custom map to fit their needs.

What else?

If you read this page, We would love to hear your comments. Please send an email to info@simplymodbus.ca and let us know what you found helpful and what topics we could add, expand or clarify.