Physical overview

Layer ISO-OSI Model
7 Application MODBUS Application Protocol
6 Presentation not defined
5 Session not defined
4 Transport not defined
3 Network not defined
2 Data link MODBUS Serial Line Protocol
1 Physical EIA/TIA-485   (or EIA/TIA-232)

MODBUS Data Link Layer 

  • Each Slave must have a unique address (1…247)
  • Address 0 is reserved for broadcast messages from Master -> Slave (all Slaves)

Modbus RTU-Frame

Byte 0: Slave Address
Byte 1: Funktion Code
Byte 2-254: Data
Byte 2-254: CRC Low Byte
Byte 3-255: CRC High Byte

  • The maximum size of one Modbus-RTU frame is 256 byte.
  • Two frames are seperated by a silent intervall of at least 3,5 characters.


Transmission of one bit:
8–bit binary
1 start bit
8 data bits, least significant bit sent first
1 bit for parity completion
1 stop bit

Serial settings

Baudrate: 9600 (Standard) others optional
Parity: Even (None should also be implementes – others optional)
Stopbit: 1 (if none parity 2)