GB/T 19582.1-2008Modbus industrial automation network specification -- Part 1: Modbus application protocol (English PDF)

5.1 Protocol description

The Modbus protocol defines a simple protocol data unit (PDU) independent of the underlying communication layer. Modbus protocol mappings on a specific bus or network can introduce additional fields into the Application Data Unit (ADU), as shown in Figure 3.

Figure 3 -- General Modbus frame

Modbus application data units are created by the client initiating Modbus transaction processing. The function code indicates to the server which operation to perform. The

Modbus application protocol establishes the request format for client initiation.

The function code field of the Modbus data unit is encoded in one byte. The valid code range is decimal 1 ~ 255 (128 ~ 255 are reserved for exception responses). When a message is sent from the client to the server device, the function code field informs the server which operation to perform. A function code "0" is invalid.

Sub-function codes are added to some function codes to define multiple operations.

The message data field sent from the client to the server device includes additional information that the server uses to perform the operation defined by the function code.

This field also includes discrete values and register addresses, the number of items to be processed, the actual number of data bytes in the field.

In some requests, the data field may be absent (0-length), in which case the server does not need any additional information. The function code only indicates the operation.

If a correctly received Modbus ADU contains no errors related to the requested Modbus function, the server's response data field to the client includes the requested data. If an error related to the requested Modbus function occurs, this field includes an exception code, which the server application can use to determine the next action to take.

For example, the client might be able to read the on/off states of a set of discrete outputs

TCP Modbus ADU = 253 bytes + MBAP (7 bytes) = 260 bytes.

The Modbus protocol defines three types of PDUs. These are.

- Modbus request PDU, mb_req_pdu;

- Modbus response PDU, mb_rsp_pdu;

- Modbus exception response PDU, mb_excep_rsp_pdu.

Define mb_req_pdu as.

mb_req_pdu={function_code, request_data}, where.

function_code=[1 byte] Modbus function code.

request_data=[n bytes], this field is related to the function code and typically includes information such as variable references, variable counts, data offsets, sub-function codes.

Define mb_rsp_pdu as.

mb_rsp_pdu={function_code, response_data}, where.

function_code=[1 byte] Modbus function code.

response_data = [n bytes], this field is related to the function code and typically includes information such as variable references, variable counts, data offsets, sub-function codes.

Define mb_excep_rsp_pdu as.

mb_excep_rsp_pdu = {exception-function_code, exception_code}, where.

exception-function_code = [1 byte] Modbus function code + 0x80.

exception_code = [1 byte] Modbus exception code, defined in the table “Modbus exception codes” (see Chapter 8).

5.2 Data encoding

Modbus uses the most significant byte to store addresses and data items at low addresses.

This means that when sending multiple bytes, the most significant byte is sent first. For example.

Register size Value

16 bits 0x1234 The first byte sent is 0x12 followed by 0x34

Note. For more detailed information, see reference [1].

......