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Modbus or MODBUS is a client/server data communications protocol in the application layer. [1] It was originally designed for use with programmable logic controllers (PLCs), [2] but has become a de facto standard communication protocol for communication between industrial electronic devices in a wide range of buses and networks.
A remote terminal unit (RTU) is a microprocessor-controlled electronic device that interfaces objects in the physical world to a distributed control system or SCADA (supervisory control and data acquisition) system by transmitting telemetry data to a master system, and by using messages from the master supervisory system to control connected objects. [1]
RS-485 is used as the physical layer underlying many standard and proprietary automation protocols used to implement industrial control systems, including the most common versions of Modbus and Profibus. DH 485 is a proprietary communications protocol used by Allen-Bradley in their line of industrial control
Modbus RTU or ASCII or TCP; oBIX - Open Building Information Exchange is a standard for RESTful Web Services-based interfaces to building control systems developed by OASIS. UPB - 2-way Peer to Peer Protocol; VSCP - Very Simple Control Protocol is a free protocol with main focus on building- or home-automation; xAP – Open protocol
Many are designed to send information only when the master station polls the RTU. Typical legacy SCADA protocols include Modbus RTU, RP-570, Profibus and Conitel. These communication protocols, with the exception of Modbus (Modbus has been made open by Schneider Electric), are all SCADA-vendor specific but are widely adopted and used.
The DNP3 protocol supports time synchronization with an RTU. The DNP Protocol has time stamped variants of all point data objects so that even with infrequent RTU polling, it is still possible to receive enough data to reconstruct a sequence of events of what happened in between the polls.