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7.1.1 Remote Terminal Unit (RTU) mode
Modbus protocol messages in Remote Terminal Unit (RTU) mode are transmitted in binary form from
the transmitter to the recipient as serial RS-485 communication, for example.
Client
(Master)
Transmitter
Recipient
Server
(Slave)
Binary code, e.g.: 90 -> 01011010 (8 bits = 1 byte)
Recipient
Transmitter
Note:
The maximum length of a message must not exceed 256 bytes.
7.1.2 Protocol configuration
A Modbus protocol message has defined start and end points. The receiving device recognises the
beginning of the message and reads out the server address. This allows a server to recognise whether
it has been addressed by the client and whether the message has been sent completely. If it has
arrived complete, the server uses error check bytes and parity bits to check the completeness of the
message. If the error check or parity fails, the message is discarded.
In RTU mode, the message starts with a silent interval of at least 3.5 characters. The first byte contains
the device address. Client and server devices continuously monitor the network, even during a silent
interval.
After the last transmitted byte, a silent interval of at least 3.5 characters indicates the end of the
message. After this silent interval, a new message can begin.
Protocol configuration
Start
Application data unit
End
Waiting
time
min. 3.5
characters
Address
Protocol data unit
CRC Error
Check
Waiting
time
min. 3.5
characters
Function code
Dates
1 byte
1 byte
n byte
(0...252 byte) 2 byte (16 bits)
1 to 247* devices;
0 as broadcast
message
1…17
Checksum
* please note: up to 32 devices are possible without a Modbus repeater!
The entire message must be transmitted in one continuous flow. If a silence interval of more than
1.5 characters occurs before the completion of the message, the receiving device will discard the
incomplete message. Subsequently, the device assumes that the next byte contains the device
address of a new message.
Similarly, if a new message starts earlier than 3.5 characters after the previous message. In this case, the
receiving device considers the message as a continuation of the previous message. Consequently, an
error occurs because the value of the combined message in the final CRC field (check field) is invalid!
