V. Modbus

Felix Lin

Computer Network

Review

OSI Model Modbus

Application Layer

Presentation Layer

Session Layer

Transport Layer

Network Layer

Data Link Layer

Physical Layer

Modbus

NA

NA/Ports

NA/TCP

NA/IP

RS-232/MAC

Serial/Ethernet

Mapping

Overview

• Published by Modicon in 1979• Modbus is a Protocol, NOT RS232 or

RS485 neither• Use for communication between

individual equipment• RTU/ASCII• Usually over RS232, RS485 or TCP/IP.• Take TCP 502 port to use

Protocol Stack

Use Case

General Frame

• ADU – Application Data Unit– Maxima size is 256Byte (inherited from RS485)

• PDU – Protocol Data Unit– Maxima size is 253Byte

(ADU – 1Byte Server Address – 2Byte CRC)

• Use big-endian representation

How It Works?

Data Model

Primary tables Object type TypeDiscretes Input Single bit RO

Coils Single bit RW

Input Registers 16- bit word RO

Holding Registers 16- bit word RW

Type of Function Codes

• Public– Data Access (Read/Write)• Bit Access• Word Access(16bits)

– Diagnostic– Other

• User Defined • Reserved for Exception

(128~255)

Function Codes(Data Access)

Code Description Type

Bit Access

0x01 Read Coils Read

0x02 Read Discrete Inputs Read

0x05 Write Single Coil Write

0x15 Write Multiple Coils Write

Word Access(16bits

)

0x03 Read Holding Registers Read

0x04 Read Input Register Read

0x06 Write Single Register Write

0x10 Write Multiple Registers Write

Exception CodesCode Name

0x01 ILLEGAL FUNCTION0x02 ILLEGAL DATA ADDRESS0x03 ILLEGAL DATA VALUE0x04 SERVER DEVICE FAILURE0x05 ACKNOWLEDGE0x06 SERVER DEVICE BUSY0x08 MEMORY PARITY ERROR0x0A GATEWAY PATH UNAVAILABLE0x0B GATEWAY TARGET DEVICE FAILED TO RESPOND

Transaction State Diagram

Read Coils

Read Discrete Inputs

Read Holding Registers

Read Input Registers

Write Single Coil

Write Single Register

Write Multiple registers

Reference

• http://www.modbus.org/