Modbus is the name of a communication protocol originally develop for programmable logic controllers (PLCs) in 1979 by the company Gould-Modicon. It enables data exchange between a master (controller) and several slaves (field devices). The protocol is characteriz by its openness and flexible application options. In the industrial environment, the protocol has develop into a de facto standard. It is used, for example, for communication between control and measurement systems. Data exchange is possible via serial connections such as EIA-232 or EIA-485 as well as via Ethernet and TCP. The support operating modes are RTU, ASCII and TCP. Since 2007, Modbus TCP has been includ in the ICE 61158 industrial standards series.
Architecture, functionality and operating modes
The communication protocol works on the basis of the master/slave principle. A master can control the accessible slave devices or query measurement data from them via the fieldbus implemented via Iran Phone Number List serial connections or Ethernet. Each bus participant can be reached via its unique address. Broadcast messages can be distributed over the bus using the address 0. The master usually initiates communication by querying a client. Various object types are defined for read and write access. The following three transmission variants are provided as operating modes.
The RTU (Remote Terminal Unit Mode)
Operating mode involves an exchange of information using binary codes that cannot be read by humans via serial interfaces. The RTU communication type offers a high data throughput. RTU messages begin with a pause of at least 3.5 characters, follow by the 8-bit recipient address and the function code, which is also eight bits long. This is follow by the actual data. The message is check using a Cyclic Redundancy Check (CRC). Each message ends with a pause of at least 3.5 characters.
In contrast to the RTU operating mode, the ASCII operating Cell Phone Number Database mode uses human-readable ASCII characters to transmit information. The data throughput is lower than with RTU. Each message begins with a colon. This is follow by the recipient address, the function code and the actual message data, which varies in length. Longitudinal Redundancy Check (LRC) is use for error checking. The end of each message is mark with the CRLF (Line Feed) character.
The TCP operating mode is based on the client
Server model and maps the field bus on the Ethernet medium Bulk Database with TCP/IP packets. Like RTU, the TCP operating mode uses binary codes. The TCP port use is 502. Each message begins with the two-byte transaction number. This is follow by the protocol character, the number of subsequent bytes, the address, the function field and the message data of varying lengths. In 2018, a secure variant of the TCP operating mode base on Transport Layer Security (TLS) with encryption and role-base access control was publish. The TCP port use in this variant is port 802.
