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Modbus is a serial communications protocol originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Simple and robust, it has since become a ''de facto'' standard communication protocol, and it is now a commonly available means of connecting industrial electronic devices. The main reasons for the use of Modbus in the industrial environment are: * developed with industrial applications in mind * openly published and royalty-free * easy to deploy and maintain * moves raw bits or words without placing many restrictions on vendors Modbus enables communication among many devices connected to the same network, for example a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Many of the data types are named from its use in driving relays: a single-bit physical output is called a ''coil'', and a single-bit physical input is called a ''discrete input'' or a ''contact''. The development and update of Modbus protocols has been managed by the Modbus Organization since April 2004, when Schneider Electric transferred rights to that organization. The Modbus Organization is an association of users and suppliers of Modbus compliant devices that seeks to drive the adoption and evolution of Modbus. == Protocol versions == Versions of the Modbus protocol exist for serial port and for Ethernet and other protocols that support the Internet protocol suite. There are many variants of Modbus protocols: * ''Modbus RTU'' — This is used in serial communication & makes use of a compact, binary representation of the data for protocol communication. The RTU format follows the commands/data with a cyclic redundancy check checksum as an error check mechanism to ensure the reliability of data. Modbus RTU is the most common implementation available for Modbus. A Modbus RTU message must be transmitted continuously without inter-character hesitations. Modbus messages are framed (separated) by idle (silent) periods. * ''Modbus ASCII'' — This is used in serial communication & makes use of ASCII characters for protocol communication. The ASCII format uses a longitudinal redundancy check checksum. Modbus ASCII messages are framed by leading colon (':') and trailing newline (CR/LF). * ''Modbus TCP/IP or Modbus TCP'' — This is a Modbus variant used for communications over TCP/IP networks, connecting over port 502.〔Modbus Messaging on TCP/IP Implementation Guide V1.0b, s3.1.3〕 It does not require a checksum calculation as lower layers already provide checksum protection. * ''Modbus over TCP/IP or Modbus over TCP or Modbus RTU/IP'' — This is a Modbus variant that differs from Modbus TCP in that a checksum is included in the payload as with Modbus RTU.〔(Remote Modbus Network Monitoring )〕 * ''Modbus over UDP'' — Some have experimented with using Modbus over UDP on IP networks, which removes the overheads required for TCP 〔(Java implementation )〕 * ''Modbus Plus (Modbus+, MB+ or MBP)'' Modbus over Fieldbus (Modbus+ or MB+), also exists, but remains proprietary to Schneider Electric. requires a dedicated co-processor to handle fast HDLC-like token rotation. It uses twisted pair at 1 Mbit/s and includes transformer isolation at each node, which makes it transition/edge triggered instead of voltage/level triggered. Special interfaces are required to connect Modbus Plus to a computer, typically a card made for the ISA (SA85), PCI or PCMCIA bus. * ''Modbus PEMEX''- This variant is an extension of standard Modbus with support for historical and flow data. It was designed for process control and never gained widespread adoption 〔http://www.rtaautomation.com/modbus/〕 * ''Enron Modbus''- This variant is an extension of standard Modbus with support for 32 bit Integer and Floating Point variables, and historical and flow data. Data types are mapped using standard addresses.〔http://www.simplymodbus.ca/Enron.htm〕 The historical data is used to meet an American Petroleum Institute (API) industry standard for how data should be stored〔http://www.calscan.net/pdf/ModBus%20Driver%20Development%20Guide%201v11.pdf〕 Data model and function calls are identical for the first 4 variants of protocols; only the encapsulation is different. However the variants are not interoperable as the frame formats are different. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Modbus」の詳細全文を読む スポンサード リンク
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