Background, Chronology of Data, Applications & Limitations Modbus Protocol

Modbus is a serial communications protocol published by Modicon in 1979 to be applied to the programmable logic controller (PLC). Then the protocol has become the de facto standard communications protocol in industry, and now Modbus is a two-way communication protocol most commonly used as a media liaison with the industry or other electronic media with the computer. The main reason is the extensive use of Modbus as the communications protocol is:
  

• Modbus protocol was published as an open and royalty-free 
• Modbus is relatively easy to combine with industrial networks 
• Modbus data transfer "raw bits" or "words" without limiting the type of vendor brand or type of industrial equipment manufacturers that use

Modbus allows for communication between the two-lane devices connected to the same network, for example, a system that measures temperature and humidity and communicates the results to a computer (HMI). Modbus is often used to connect to the remote computer sopervisory terminal unit (RTU) supervisory control and data acquisition system (SCADA).

Manufacturer or supplier of large and small, system integrators, end-user, open source developers, lecturers and other interested parties can become members of Modbus. Several prominent members are SoftDEL Systems, Precision Digital Corporation, Motor Protection Electronics, FieldServer Technologies and many more.

Version of the Protocol

Communication using Modbus protocol can be through the medium of the serial port (RS-232, RS-485, FO), can also be via Ethernet (LAN) and other tissues that support the Internet protocols.

Most Modbus devices communicate over a serial EIA-485 physical layer, usually the manufacturer of compact type sensor and tranducer using this because it is much more simple and low-cost. There are many variants of the Modbus protocol

• Modbus RTU - used in serial communication and use the binary representation of data values ​​that are compressed as the communication protocol. RTU format follows the request command / data transfer with a cyclic redundancy check checksum error checking mechanism (error-checking) to ensure data reliability. Modbus RTU is the most common implementation of all existing versions of Modbus. A Modbus RTU message should be transmitted continuously with no pause between characters. Each Modbus message framing or separated by periods of idle time (without communication or communication port is closed or OFF). Communication via Modbus RTU is often used in small-scale monitoring system where sensors and HMI computers located not very far.
• Modbus ASCII - Modbus protocol is used on the type of serial communication and the use of ASCII characters in a single communication protocol. ASCII data format uses a longitudinal redundancy check checksum check for errors in data transfer. Data on Modbus ASCII messages framed by a colon or a colon (':') in advance and followed by a newline (CR / LF). Communication between electronic devices to a computer via Modbus ASCII protocol is commonly used in telephone network or GSM Modem is not available if an adequate infrastructure network such as LAN or Fiber-Optic network there.
• Modbus TCP / IP or Modbus TCP - Modbus protocol variant can only be used for communication over TCP / IP or commonly known as a LAN. Modbus TCP / IP does not require a checksum calculation on the last layer to handle errors such as data transfer on serial communication. Modbus TCP / IP is faster in transferring data than the Modbus RTU Modbus ASCII especially. On the application of SCADA or Automation system is complex IED devices which are used in large numbers and diverse, or where the level of the dense traffic of data transfer, I would recommend using Modbus TCP / IP to achieve real-time rates are higher. Of course the IED with the Port TCP / IP itself is relatively more expensive than the RS-485 port.
• Modbus over TCP / IP or Modbus over TCP - This is a different variant of the Modbus Modbus TCP checksum which is used or the calculation of data transfer errors (error-checking) are included in the payload such as Modbus RTU.
• Modbus Plus (Modbus + or MB +) - Modbus Plus (Modbus + or MB +) also exists and is a version of the extension of all Modbus versions, but only exclusive for SCHNEIDER ELECTRIC alone. Modbus requires special co-processors to handle the rapid rotation of the token as HDLC. Modbus type uses twisted pair cable at a speed of 1 Mbit / s and includes transformer isolation at each node. Special interface is required as a liaison Modbus Plus to a computer, typically using a ISA card (SA85), PCI or PCMCIA bus is specially made for the MB +.

Modeling of data and function requests data on the four variants of Modbus over other than MB + is essentially identical, only the encapsulation or fromat frame / frames are different.

Communication and Devices

Any device that wants to communicate via Modbus protocol should be given a unique address or address should not be the same as other devices. In serial communications, and network node only MB + is assigned as the Master who can initiate a command, unlike the case with Ethernet, any device can send Modbus command, although usually only one master device to do so.

Modbus command contains the Modbus address of the device that you want to go or who want to be asked to communicate .. Only the intended device will act on the orders, although other devices may also receive a message / command (the exception is a special broadcastable command is sent to node 0 acting but not recognized). All Modbus commands contain checking the information, to ensure that orders are coming not damaged or error. The basic Modbus commands can instruct an RTU to change the value of a control register or reading a port Input / Output, and simultaneously instruct the device to send back one or more values ​​contained in the register that is accessed or changed.

There are many modems and gateways are supported by Modbus, Modbus is a protocol because it is a very simple and often copied by manufacturers of electronic devices and networks. Some of them, there are specially made ​​devices designed for this protocol. Different implementations use wires, wireless communication and even SMS or GPRS. Classic problem of the monitoring system designers with a wireless network / wireless, SMS and GPRS is that they make the system must be able to include high latency and troubleshooting time.