What is the Modbus protocol?
MODBUS is an application layer messaging protocol, positioned at layer 7 of the OSI model. Provides client/server communication between connected devices on different types of buses or networks.
The de facto industry serial standard since 1979, MODBUS continues to enable millions of automation devices to communicate.
MODBUS is a request/response protocol and offers services specified by function codes. MODBUS function codes are elements of Request/Response PDUs.
STRUCTURE OF MODBUS RTU TELEGRAMS
In order to make settings in the device, the respective Modbus registers must be addressed via a telegram. The basic structure of a Modbus telegram is shown below:
| Start | Indirizzo | Funzione | Data | CRC | Stop |
| 3,5 byte | 1 byte | 1 byte | N x 8 bits | 2 bytes | 3,5 bytes |
The data area has a different structure depending on whether the telegram is a request or a reply and on the function code used.
In RTU mode, messages begin with a silent interval of at least 3.5 characters. Depending on the transmission speed set in the network, this interval is most easily implemented as a multiple of the duration of one character.
The first field subsequently transmitted is the device address in the range 01…F7h (247d), (248-255 are reserved for Modbus). Allowed characters for all fields: hexadecimal 0-9, A-F. Networked devices constantly monitor the network bus, even during ‘silence’ intervals.When the first field (address field) is received, the device decodes it to determine whether the message is directed to it. After the last character transmitted, an identical interval of at least 3.5 characters indicates the end of the message, a new message can begin after this interval.
The complete telegram must be transmitted as a continuous data stream. In the case of a silent interval of more than 1.5 characters before the end of the telegram, the receiving device deletes the message and assumes that the next byte is the address field of a new message.
Similarly, if a new message begins before the end of the 3.5 character silence interval, the receiving device will consider this new message as a continuation of the previous message. This will trigger an error, as the value in the final CRC field will not be valid for combined messages.
