Byte order and word order pitfalls

Sometimes Modbus communication works, but the value still looks wrong. This often happens because of byte order or word order.

Start with the basics

One Modbus register is 16 bits. That means some values fit into one register, but larger values need two or more registers.

Examples:

• a small integer may fit in one register
• a 32-bit integer uses two registers
• a float usually uses two registers

What byte order means

Byte order is the order of bytes inside a value.

If the byte order is not what the client expects, the final number can be wrong.

What word order means

Word order matters when a value uses more than one 16-bit register.

If the two registers are combined in the wrong order, the value may still be read, but it will not make sense.

What this looks like in practice

Common symptoms are:

• very large numbers
• impossible negative values
• values that jump strangely
• correct-looking communication but wrong readings

A simple example

A temperature may be stored across two registers as a float. If the registers are combined in the wrong order, the result may be a meaningless number instead of a real temperature.

How to solve it

Check the device documentation.

Make sure it clearly states:

• data type
• byte order
• word order

If the manual is unclear, test the possible combinations and compare the result with a known real value.

What manufacturers should do

Do not just say “float” or “32-bit value.” Also state how the bytes and words are arranged.

Final note

If the values look wrong but communication is fine, do not only check the register address. Also check byte order and word order.

Consulting

Manufacturers can hire me for help with Modbus device documentation, testing, and troubleshooting.