A Multiagent Architecture Based in aFoundation Fieldbus Network Function Blocks
Vinicius Ponte, Dennis Brandão, Adrião Duarte Dória Neto, Jorge Dantas de Melo · InTech eBooks · 2010
Foundation Foundation Fieldbus ProtocolThe term FOUNDATION Fieldbus (FF) indicates the protocol specified by the Fieldbus Foundation and standardized by IEC 1 standards number 61158 (IEC, 2000) and 61784 at profile CPF 2 -1/1 (IEC, 2003).It is a digital, serial, bidirectional, and distributed protocol, which interconnects field devices such as sensors, actuators and controllers.Basically, this protocol can be classified as a LAN (Local Area Network) for instruments used in process and industrial automation, with the ability to distribute the control application through a network.This protocol is based on the ISO/OSI (International Organization for Standardization/Open System Interconnection) seven layer reference model (ISO, 1994).Although being based on the ISO/OSI model, the FF does not use the network layer, the transport layer, the section layer, nor the presentation layer, because it is restricted to local applications.The entire network structure of the FF concentrates on the physical layer, the data link layer (DLL) and the application layer.Besides these three implemented layers, the protocol defines an additional layer named User Application Layer.The FF Physical Layer, named H1, uses a shielded twisted pair cable as communication medium.The H1 specifies a 31.25 kBit/s bit rate with Manchester codification over a bus powered channel.The network topology configuration is flexible: it is typically configured with a trunk and several spurs, attending certain physical and electrical limitations regarding maximum spur lengths and number of transmitters.The DLL carries the transmission control of all messages on the fieldbus and its protocol grants to the FF network temporal determinism for critic process control data.The communication is based on a master-slave model with a central communication scheduler (master), named Link Active Scheduler or LAS.This node performs the medium access control (MAC).Two types of DLL layer are standardized: Basic and Link Master.A Basic DLL transmitter does not have LAS capabilities, it operates passively as a communication slave.A Link Master DLL transmitter, on the other hand, can execute LAS functions and thus, if the active LAS node fails, become the LAS node.The FF Data Link Layer supports two transmission policies: one addressed to scheduled cyclic data, and another to sporadic (unscheduled) background data.These two communication policies share the physical bus, but they are sequentially segmented in cyclic time slots or periods.In the scheduled communication period, most process variables generated by periodic processes are transmitted cyclically according to a static global schedule table loaded on the LAS node.This cyclic transmission mode has higher priority over acyclic transmission modes.A periodic process can be defined as a process initiated at predetermined points in time, also called a time-triggered process.The period for the network cyclic process is typically from tenths to hundreds of milliseconds, and it is mandatory to consider that the generated data must be delivered before the next data is available.This type of periodic data is usually related to measurement and control variables (Cavalieri et al., 1993).