The ability of PAC’s or PLC’s to communicate process or event information up to SCADA and BMS systems, or to the administrative levels within organisations is of paramount importance for today’s control systems. The protocols used need to be open and backed by international standards organisations to ensure reliable operation across all hardware platforms.
Listed below are several powerful network protocols meeting the above requirements, with each being specifically designed to suit a particular industry, or application type. AIEglobal recently used all three in an 8 MW Trigeneration project. OPCuA was used for real time PLC to PLC and SCADA communications, BACnet was used for SCADA to BMS communications, and DNP3 was used to communicate from a PLC to a Load Shedding Controller.
OPC_uA (OPC Foundation)
A secure platform independent protocol for data communication from automation devices up to the corporate level. Data may transferred using a local intranet or over the internet. OPC_uA supports client discoverable devices where read/write data is predefined in the device and subscribed to by the client. The device server takes care of transmitting the data to the client based on “change of state” and minimum update requirements.
BACnet (ASHRAE, OPC & ANSI)
The BACnet protocol was specially developed for building automation, with emphasis on control of HVAC plants, fire control panels, intrusion detection and access control systems. It is an object based protocol that provides browse services for object discovery and connection.
DNP3 (Distributed Network Protocol- DNP.org) IEEE 1815-2010
The driving force behind the development of the DNP3 protocol where the Electricity and Water utilities that required a vendor independent protocol which could collect event data from remote stations for processing by centralized SCADA systems. The big difference with DNP3 is that the field devices buffer events in between reads by the central SCADA system. Buffering time stamped events ensures that real time events are captured even if the SCADA update rate is slow.
