Technical Aspects of Foundation Fieldbus

Foundation fieldbus (FF) is an all-digital, serial, two-way communication system for industrial applications.

Foundation fieldbus specifications include two different configurations: H1 and HSE.

H1 Foundation Fieldbus (FF)

H1 FF communication system is mainly applied to distributed continuous process control; running at 31.25 kbps. It connects digital field equipment such as sensors, actuators and inputs/outputs allowing the integration of an existing 4-20 mA signal transmission. In summary the H1 FF has the following properties:

• Two-wire (ungrounded) network cable.
• DC power is conveyed over the same two wires as digital data.
• 100 ohm (nominal) characteristic impedance.
• 31.25 kbps data rate.
• Manchester encoding
• Differential voltage signaling i.e. 0.75 volts peak to peak transmit minimum; 0.15 volts peak-to-peak receive threshold minimum.

Given that DC power is conveyed over the same two wires as the digital data, it means each device only requires connecting to two wires in order to function on an H1 network segment.

Below the Fieldbus application layer, H1 FF directly presents that data link layer (DDL) that is managing access to the communication channel. A physical layer (PhL) that deals with the problems of interfacing with the physical medium. A network and system management layer is also present.

The figure below shows how the H1 FF architecture compares against the ISO OSI (Open System Interconnection) reference model.

HSE Foundation Fieldbus (FF)

HSE {High-Speed Ethernet} FF is mainly designed for discrete manufacturing applications; it provides integration of controllers such as distributed control systems (DCS) and programmable logic controllers (PLCs), H1 subsystems, foreign protocols (e.g. Modbus and Profibus DP); conventional input/output {I/O} (e.g. digital I/O, 4-20 mA), data servers, and workstations.

HSE FF enhances the H1 applications of Foundation fieldbus by providing a high-speed backbone, redundancy and bringing capabilities for multiple protocols.

The key feature of the HSE FF is the use of Internet architecture for high-speed discrete control and in most cases, for interconnecting several H1 segments in order to achieve a plant-wide fieldbus network.

HSE FF defines an application layer and associated management functions, designed to operate over a standard Transmission Control Protocol (TCP)/Unit Datagram Protocol (UDP)/Internet Protocol (IP) stack over twisted pair of wires or fiber optic switched Ethernet.

The figure below illustrates how HSE FF compares against the ISO OSI reference model.

Figure (1c) illustrates how Foundation fieldbus is applied in industrial communication networks.

Related articles:

• What is Fieldbus in Industrial Communication Networks?
• Basic Features of Foundation Fieldbus (FF)
• How RS-232, RS-422 and RS-485 Networks are applied in Instrumentation
• Features of HART Communication Protocol