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2. MODBUS Protocol Description
2.1 MODBUS Address Rules
The MODBUS Serial Line protocol is a Primary/Replica protocol. Only one Primary at a time is connected to the bus. One or
multiple (up to 247) Replica nodes also are connected to the same serial bus.
The MODBUS Primary node has no specific address. Only the Replica nodes must have an address. The Replica nodes'
addresses are in the range of 1...247 decimal (0 is reserved as the broadcast address). This address must be unique on a
MODBUS serial bus.
2.2 MODBUS Protocol Frame
MODBUS protocol frame is made up of additional address, function code, data and error check.
Table 2.2.1 General MODBUS Frame
Additional Address
Function Code
Data
Error Check
Two different serial transmission modes are defined: RTU (Remote Terminal Unit) mode and ASCII (American Standard Code for
Information Interchange) mode.
2.3 RTU Transmission Mode
2.3.1 Byte Format
The delivery sequence of byte or word is sent in the following order (left to right): LSB (Least Significant Bit) ---> MSB (Most
Significant Bit).
The format for each byte (11 bits) in RTU mode is:
• Bits/Byte: 1 start bit
• 8 data bits, LSB sent first
• 1 bit for parity completion
• 2 stop bits
Table 2.3.1 Bit Sequence in RTU Mode (Specific Case of No Parity)
Start Bit
Data Bit
Stop Bit
Stop Bit
1
1
2
3
4
5
6
7
8
1
1
4
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