What is MODBUS?
- MODBUS is a serial communication protocol.
- Published by Modicon (now Schneider Electric).
Reasons for the use of MODBUS in the industrial Environment?
- Developed with industrial application in mind.
- Openly published and royalty free.
- Easy to deploy and maintain.
- Moves raw bits or words without placing many restriction on vendors.
- It enables communication among many devices connected the same network
- MODBUS RTU.
- MODBUS ASCII.
- MODBUS TCP/IP.
- MODBUS OVER UDP.
- MODBUS PEMEX
- ENRON MODBUS
Modbus RTU VS TCP
- Modbus RTU is an open,serial(RS-232 or RS-485 ) protocol derived from the master slave architecture.
- It is a widely accepted protocol due to its ease of use and reliability.
- MODBUS RTU is widely used within Building Management system(BMS) and industrial automation system.
- MODBUS RTU messages are a simple 16-bit CRC(cyclic -redundant Checksum).
- The simplicity of these messages is to ensure reliability .
MODBUS is considered an application layer messaging protocol, providing master slave communication between devices connected together buses or networks.
The function codes of MODBUS are elements of MODBUS Request/reply PDUs(protocol data unit) Data Object Properties
- MODBUS RTU packets are only intended to send data; they do not have the capability to send parameters such as point name,resolution, units, etc.
- If the ability to send such parameters is needed, one should investigate a BACnet, EtherNeVIP, or other modern protocols.
Modbus RTU Address Requirment
- Standard RTU node addresses are 1-254, with being reserved for broadcast messages and write only.
- ‘O’ address is rarely used due to the fact there is no confirmation that the message was properly received at this slave node.
- If physical layer is RS-232 as only one node can be implemented anyway
- RS-485 limits the number of nodes to 32.
Modbus TCP VS RTU
- MODBUS TCP/IP is a variant of the MODBUS family of simple, vendor – neutral communication protocols intended for supervision and control of automation equipment.
- It covers the use of MODBUS messaging in an ‘Intranet’ or ‘internet’ environment using the TCP/IP protocols.
- The MODBUS TCP/IP is being published as a automation standard.
- In MODBUS, data transaction are traditionally stateless, making them highly resistant.
- TCP/IP allows a very large number of concurrent connections
MODBUS uses a ‘big-endian’ representation for addresses and data items. This means that when a larger numerical quantity larger than a single byte is transmitted, the most significant byte is sent first.