Modicon programmable controllers can communicate with each other and with other devices over a variety of networks. Supported networks include the Modicon Modbus and Modbus Plus industrial networks, and standard networks such as MAP and Ethernet. Networks are accessed by built–in ports in the controllers or by network adapters, option modules, and gateways that are available from Modicon. For original equipment manufacturers, Modicon ModConnect ‘partner’ programs are available for closely integrating networks like Modbus Plus into proprietary product designs. The common language used by all Modicon controllers is the Modbus protocol. This protocol defines a message structure that controllers will recognize and use, regardless of the type of networks over which they communicate. It describes the process a controller uses to request access to another device, how it will respond to requests from the other devices, and how errors will be detected and reported. It establishes a common format for the layout and contents of message fields. The Modbus protocol provides the internal standard that the Modicon controllers use for parsing messages. During communications on a Modbus network, the protocol determines how each controller will know its device address, recognize a message addressed to it, determine the kind of action to be taken, and extract any data or other information contained in the message. If a reply is required, the controller will construct the reply message and send it using Modbus protocol. On other networks, messages containing Modbus protocol are imbedded into the frame or packet structure that is used on the network. For example, Modicon network controllers for Modbus Plus or MAP, with associated application software libraries and drivers, provide conversion between the imbedded Modbus message protocol and the specific framing protocols those networks use to communicate between their node devices. This conversion also extends to resolving node addresses, routing paths, and error–checking methods specific to each kind of network. For example, Modbus device addresses contained in the Modbus protocol will be converted into node addresses prior to transmission of the messages. Error–checking fields will also be applied to message packets, consistent with each network’s protocol. At the final point of delivery, however – for example, a controller – the contents of the imbedded message, written using Modbus protocol, define the action to be taken.
MODICON, Inc. (1996) Modicon Modbus Protocol Reference Guide