8a – Modbus overview

Publisher Summary This chapter provides an insight into Modbus protocol. Modbus is an application layer messaging protocol that provides client/server communication between devices connected to different types of buses of networks. The Modbus protocol implements a client/server architecture and operates essentially in a request/ response mode, irrespective of the media access control used at layer 2. This client/server model is based on four types of messages: Modbus requests, which are the messages sent on the network by the clients to initiate transactions; Modbus confirmations, which are the response messages received on the client side; Modbus indications, which are the request messages received on the server side; and Modbus responses, which are the response messages sent by the servers. These messaging services of the client/server model are used to exchange real-time information between two device applications, between device applications and devices, or between devices and HMI/SCADA applications. A typical Modbus message frame format includes four fields: address field, function field, data field, and error-check field. The Modbus Messaging Protocol (layer 7) needs additional support at the lower layers in order to get the message across. A popular method is the use of a master/slave (half-duplex) layer 2 protocol, transmitting the data in serial format over RS-232, RS-485 or Bell 202 type modem links. Other methods include MODBUS+ (half-duplex over RS-485), or MAP.