MODBUS RTU
Modbus RTU is a serial transmission version of Modbus, typically used over RS-232 or RS-485 physical interfaces.
Modbus RTU frame structure
The structure of a Modbus RTU frame can be seen below. Note that Modbus RTU uses a binary format rather than ASCII and does not include explicit frame delimiters like <CR><LF>.
Modbus RTU frame structure | ||
|---|---|---|
Field | Size | Description |
Start | 3.5-character silence | Minimum silent period used as frame delimiter to indicate the beginning of a new Modbus RTU message (*). |
Address | 1 byte | Identifies the target slave device. Range: [1,247], with 0 reserved for broadcast. |
Function code | 1 byte | Defines the register type. See function code table below for details. |
Data | N bytes | Contains the requested data (register addresses and values). |
CRC | 2 bytes | A cyclic redundancy check (CRC) to verify the integrity of the transmitted data and detect communication errors. |
End | 3.5-character silence | Minimum silent period used as frame delimiter to indicate the end of a Modbus RTU message (*). |
Modbus register addressing
Modbus typically organizes data into four register types:
Modbus register addressing | ||||
|---|---|---|---|---|
Register type | Address range | Size | Read/write | Purpose |
Coils | 00001 - 09999 | 1-bit | Read/Write | Controls binary outputs (e.g., turning devices ON/OFF). |
Discrete inputs | 10001 - 19999 | 1-bit | Read-only | Stores binary input signals from e.g. sensors or switches. |
Input registers | 30001 - 39999 | 16-bit | Read-only | Stores input data (e.g., sensor readings or status) that the master can only read. |
Holding registers | 40001 - 49999 | 16-bit | Read/Write | Stores output or configuration data that the master can read and modify. |
Modbus function codes
The function codes define the type of action or request made by the master device.
Modbus function codes | |
|---|---|
Function code | Description |
01 | Read coils |
02 | Read discrete inputs |
03 | Read holding registers |
04 | Read input registers |
05 | Write single coil |
06 | Write single holding register |
15 | Write multiple coils |
16 | Write multiple holding registers |
Example
In Modbus communication, you can define the order in which data appears within register addresses. For example, if input registers are selected (see Modbus RTU configuration under serial configuration for instructions) and you select roll, pitch, and yaw as shown in the table below, roll will be stored in registers 30001 and 30002, pitch in 30003 and 30004, and so on. Since each value is stored as a 32-bit IEEE 754 single-precision floating-point number, it occupies two 16-bit Modbus registers.
Register address | Parameter | Data type | Unit | Register value (U16) decimal | Numerical value |
|---|---|---|---|---|---|
30001 | roll (MSB) | Single precision float (IEEE 754) | [deg] | -16751 | -0.28 |
30002 | roll (LSB) | -10128 | |||
30003 | pitch (MSB) | Single precision float (IEEE 754) | [deg] | 16648 | 8.55 |
30004 | pitch (LSB) | -17940 | |||
30005 | yaw (MSB) | Single precision float (IEEE 754) | [deg] | 17331 | 359.91 |
30006 | yaw (LSB) | -3246 |
Let's examine how the roll value (2.34 degrees) is retrieved from Modbus registers.
Register address | Parameter | Data type | Unit | Register value (U16) binary | Register value (U16) decimal | Numerical value |
|---|---|---|---|---|---|---|
30001 | roll (MSB) | Single precision float (IEEE 754) | [deg] | 01000000 00010101 | 16405 |
-0.28 |
30002 | roll (LSB) | 11000010 10001111 | 49807 |
These two 16-bit registers form a 32-bit binary word in big-endian format. When combined in big-endian order, the full 32-bit IEEE 754 floating-point representation is: 10111110 10010001 11011000 01110000.
IEEE 754 single-precision floating-point numbers are structured as follows:
Bit position | Component | Extracted bits |
|---|---|---|
Bit 1 | Sign (S) | 1 (negative) |
Bits 2-9 | Exponent (E) | 01111101 (decimal: 125) |
Bits 10-32 | Mantissa (M) | 00100011101100001110000 |
The roll value is computed using the IEEE 754 formula:
Roll value = (-1)s x 1.M x 2(E-127) = (-1)1 x 1.001000111011000011100002 x 2(125−127) = −1.139708 x 2-2 = −0.284854.
Due to rounding errors, the final roll angle is -0.28 degrees.
(*) The duration of the silent period depends on the baud rate. The available baud rates for RS-232 and RS-485 can be seen in serial configuration.