Endianness

Endianness in computing refers to the order in which bytes are stored in a multi-byte value, such as a 16-bit or 32-bit integer. There are two main types of endianness: big-endian and little-endian.

Big-endian refers to the order in which the most significant bytes are stored first, followed by the second most significant bytes, and so on. This is the order in which human beings typically read and write numbers.

Little-endian refers to the order in which the least significant bytes are stored first, followed by the second least significant bytes, and so on. This is the order in which many computer systems store multi-byte values, as it can be more efficient for certain types of operations.

Endianness is important in computing because it affects how multi-byte values are stored and interpreted by different devices and systems. Suppose different devices or systems use different endianness conventions. In that case, they may be unable to correctly interpret the values being transmitted between them, which can lead to errors or incorrect data being processed.

Modbus Endianness

When reading multiple registers from a Modbus server, the endianness of the registers can affect how the values stored in the registers are interpreted by the device reading the values.

For example, suppose a Modbus server stores 16-bit registers in Big-Endian format, and a device communicates with the server using Little-Endian format. When the device reads two 16-bit registers from the server, it will expect the least significant bytes to be stored in the first register and the second least significant bytes to be stored in the second register. However, the server is actually storing the most significant bytes in the first register and the second most significant bytes in the second register. As a result, the device will not be able to correctly interpret the values of the registers, resulting in an incorrect 32-bit value being read.

On the other hand, if the device is communicating with the server using the same endianness as the server, it will be able to correctly interpret the values of the registers and read the correct 32-bit value.

It is important for devices communicating with Modbus servers to correctly interpret the endianness of the registers being read or written, as this ensures that the correct values are transmitted between the devices and the server.

Big- and Little-Endian

The Modbus memory can be thought of as a table of 16-bit registers. Each register is 16 bits long and can hold a 16-bit value. The registers are numbered from 0 to 65535. The first register is address 0; the second is address 1, and so on. The last register is address 65535.

In the table above, "Modbus Address" represents the address of the first register in the Modbus protocol. The "Storage Format" column indicates whether the registers are stored in Big-Endian or Little-Endian format. "Register 1" and "Register 2" represent two 16-bit registers that store 32-bit values.

For each 32-bit value, the most significant bytes are stored in the first register, and the second most significant bytes are stored in the second register in the Big-Endian format. In the Little-Endian format, the least significant bytes are stored in the first register, and the second least significant bytes are stored in the second register.

It is important to note that in Modbus, the bytes within a register are rarely swapped. Instead, the order in which the bytes are stored is used to determine the value of the registers as a whole. For example, if a 32-bit value is stored as 0x1234 0x5678 in Big-Endian order, the value of the registers would be 0x12345678. If the same value was stored as 0x5678 0x1234 in Little-Endian order, the value of the records would also be 0x12345678.

The choice of Big-Endian or Little-Endian storage for registers in Modbus is determined by the specific function being used and the device that is accessing the registers. However, both formats are commonly used in the Modbus protocol, and it is important for devices to correctly interpret the data in registers regardless of the storage format being used.

For example, suppose a Modbus client communicates with a Modbus server using the Big-Endian format. In that case, it will expect the most significant bytes of a 32-bit value to be stored in the first register and the second most significant bytes to be stored in the second register. On the other hand, suppose the server is using the Little-Endian format. In that case, the device will need to interpret the data in the registers differently, with the least significant bytes being stored in the first register and the second least significant bytes being stored in the second register.

It is important for devices to correctly interpret the data in registers regardless of the storage format being used, as this ensures that the correct values are read and written when using Modbus functions.

An example

In the table above, the "Storage Format" column indicates whether the registers are stored in Big-Endian or Little-Endian format. "Register 1" and "Register 2" represent two 16-bit registers that are used to store the 32-bit value 0x12345678. The "32-bit Value" column shows the correct and incorrect interpretation of the 32-bit integer in hexadecimal format. The "Decoded Hex Value" column shows the decimal equivalent of the 32-bit value.

In the Big-Endian format, the most significant bytes (0x1234) are stored in the first register, and the second most significant bytes (0x5678) are stored in the second register. This is the correct way to interpret the value of the registers, resulting in the correct 32-bit value of 0x12345678.

In the Little-Endian format, the least significant bytes (0x5678) are stored in the first register, and the second least significant bytes (0x1234) are stored in the second register. This is also the correct way to interpret the value of the registers, resulting in the same correct 32-bit value of 0x12345678.