Introduction

The FOUNDATION fieldbus can be flexibly used in process automation applications. The specification supports bus-powered field devices as well as allows application in hazardous areas. The Fieldbus FOUNDATION’s slogan ‘... dedicated to a single international fieldbus’ expresses the organization’s
claim to establishing an international, interoperable fieldbus standard. Fieldbus technology replaces the expensive, conventional 4 to 20 mA wiring in the field and enables bidirectional data transmission. The entire communication between the devices and the automation system as well as the process
control station takes place over the bus system, and all operating and device data are exclusively transmitted over the fieldbus (see also Lit./4/). The communication between control station, operating terminals and field devices simplifies the start-up and parameterization of all components. The communication functions allow diagnostic data, which are provided by up-to-date field devices, to be evaluated. The essential objectives in fieldbus technology are to reduce installation costs, save time and costs due to simplified planning as well as improve the operating reliability of the system due to additional performance features. Fieldbus systems are usually implemented in new plants or existing plants that must be extended. To convert an existing plant to fieldbus technology, the conventional wiring can either be modified into a bus line, or it must be replaced with a shielded bus cable, if required.

Note: To ensure trouble free operation, the communication system must be designed and configured by experts. For this purpose, a variety of assistance as well as comprehensive documentation can be obtained. This Technical Information does not claim to replace this type of support, but aims at explaining the basic principle of operation of the FOUNDATION fieldbus as well as its special characteristics to users, who have an interest in this technology. It must also be noted that the FF specification is not yet completed at this stage – November 1999 – so that the facts presented here can be subject to future changes.

Historical development

In 1992 an international group, the ISP – ‘Interoperable Systems Project’, was founded with the intention to create an internationally uniform fieldbus standard for use in hazardous environments. At the same time, the manufacturers and users of the French FIP (Flux Information Processus; previously:
Factory Instrumentation Protocol) established the international user organization WorldFIP. Together with the FIP North America, they were a strong counterweight to the ISP consortium. In 1994, for technical, economic and political reasons, the ISP and the WorldFIP merged to form the Fieldbus FOUNDATION. The aim of the Fieldbus FOUNDATION was and is to create a single, international fieldbus standard for hazardous environments which will find widespread use as IEC standardized fieldbus. The same goal is pursued by the PROFIBUS user organization with its PROFIBUS PA fieldbus. While the PROFIBUS PA has its roots and its largest user community in Europe, the FOUNDATION fieldbus manufacturers and users are concentrated in America and Asia. The Fieldbus FOUNDATION utilized some elements from the FIP for the specification of their FOUNDATION fieldbus (FF) as well as – similar to PROFIBUS PA – details from the ISP specification. This is why the physical bus design of both fieldbus systems is the same. Also, the device interface for application, which is based on function blocks, exhibits many common features. This is due to the fact that both systems have similar ambitions. However, when taking a closer look and comparing the system functions, it can be seen that there are also great differences.

User organization
The Fieldbus FOUNDATION is an independent not-for-profit organization which aims at developing and maintaining an internationally uniform and successful fieldbus for automation tasks, the FOUNDATION fieldbus. Members include users and manufacturers of field devices and automation systems. The Fieldbus FOUNDATION incorporates various workshops which are responsible, among others, for technical support, marketing and support of the members.

Approval of devices
The Fieldbus is an open bus standard which enables devices of different manufacturers to be integrated in one system and, if required, interchanged (interoperability). This is only feasible when all the devices exactly meet the specification. Devices approved by the Fieldbus FOUNDATION are a guarantee for the user and the manufacturer that they comply with the specification.

The FOUNDATION fieldbus provides a broad spectrum of services and functions compared to other fieldbus systems:

• Intrinsic safety for use in hazardous environments

• Bus-powered field devices

• Line or tree topology

• Multi-master capable communication

• Deterministic (predictable) dynamic behavior

• Distributed data transfer (DDT)

• Standardized block model for uniform device interfaces (interoperability, interchangeability’)

• Flexible extension options based on device descriptions

The characteristic feature of distributed data transfer enables single field devices to execute automation tasks so that they are no longer ‘just’ sensors or actuators, but contain additional functions. For the description of a device’s function(s) and for the definition of a uniform access to the data, the FOUNDATION fieldbus contains predefined function blocks (see ‘User application’ on page 29). The function blocks implemented in a device provide information about the tasks the device can perform. Typical functions provided by sensors include the following:

• 'Analog Input’ or

• ‘Discrete Input’ (digital input).

Control valves usually contain the following function blocks:

• ‘Analog Output’ or

• ‘Discrete Output’ (digital output).

The following blocks exist for process control tasks:

• ‘Proportional/Derivative’ (PD controller) or

• ‘Proportional/Integral/Derivative’ (PID controller).

If a device contains such a function block, it can control a process variable independently.

The shift of automation tasks – from the automation level down to the field –results in the flexible, distributed processing of control tasks. This reduces the load on the central process control station which can even be replaced entirely in small-scale installations. Therefore, an entire control loop can be implemented as the smallest unit, consisting only of one sensor and one control valve with integrated process controller which communicate over the FOUNDATION fieldbus (see Fig Complete control loop based on the FOUNDATION Fieldbus).
The enhanced functionality of the devices leads to higher requirements to be met by the device hardware and comparably complex software implementation and device interfaces.

Layered communications model
The FOUNDATION specification is based on the layered communications model and consists of three major functional elements (Fig. Structure and description of the FF communication layers):

• Physical Layer

• Communication “Stack”

• User Application

The User Application is made up of function blocks and the device description. It is directly based on the Communication Stack. Depending on which blocks are implemented in a device, users can access a variety of services. System management utilizes the services and functions of the User Application and the application layer to execute its tasks (Figs. b and c). It ensures the proper cooperation between the individual bus components as well as synchronizes the measurement and control tasks of all field devices with regard to time. The FOUNDATION fieldbus layered communications model is based on the ISO/OSI reference model. As is the case for most fieldbus systems, and in accordance with an IEC specification, layers three to six are not used. The
comparison in Fig. 3 shows that the Communication Stack covers the tasks of layers two and seven and that layer seven consists of the Fieldbus Access Sublayer (FAS) and the Fieldbus Message Specification (FMS)

Physical layer
The specification of the FOUNDATION Fieldbus is not yet completed at this stage. However, it is certain that the topology of a FF system complies with the IEC Fieldbus model in many aspects.
The IEC fieldbus solves pending communication tasks by using two bus systems, the slow, intrinsically safe H1 bus and the fast, higher-level H2 bus with 1 to 2.5 MBit/s (see IEC fieldbus model. The physical design of the H1 bus of the FOUNDATION fieldbus complies exactly with the specifications of the IEC fieldbus model. The specification of the H2 bus is not yet completed and the publication of the preliminary specification (PS) has been announced. However, it is certain that the High Speed Ethernet (HSE) will be used.

Structure of the FOUNDATION fieldbus

H1 bus
The following summary gives a brief overview of the basic values and features of the H1 bus. For more details, refer to the various ‘Application Guides’ of the Fieldbus FOUNDATION (e.g., AG 140, AG 163)
The H1 bus specification is based on the IEC 61158-2 (see Lit./2/):

• Manchester coding is used for data transfer. The data transfer rate is 31.25 kBit/s.

• Proper communication requires that the field devices have enough voltage. Each device should have minimum 9 volts. To make sure that this requirement is met, software tools are available which calculate the resulting currents and terminal voltages based on the network topology, the line resistance and the supply voltage.

• The H1 bus allows the field devices to be powered over the bus. The power supply unit is connected to the bus line in the same way (parallel) as a field device. Field devices powered by supply sources other than the bus, must be additionally connected to their own supply sources.

• With the H1 bus it must be ensured that the maximum power consumption
  of current consuming devices is lower than the electric power supplied by
  the power supply unit.

Mixed topology for an H1 network

Length of spurs

• Network topologies used are usually line topology or, when equipped with junction boxes, also star, tree or a combination of topologies (Fig. above). The devices are best connected via short spurs using tee connectors to enable connection/disconnection of the devices without interrupting communication.

• The maximum length of a spur is limited to 120 meters and depends on the number of spurs used as well as the number of devices per spur (Fig. below).

• Without repeaters, the maximum length of an H1 segment can be as long as 1900 meters. By using up to four repeaters, a maximum of 5*1900 m = 9500 m can be jumpered. The short spurs from the field device to the bus are included in this total length calculation.

Fieldbus cable types and maximum bus lengths

• The number of bus users per bus segment is limited to 32 in intrinsically safe areas. In explosion-hazardous areas, this number is reduced to only a few devices due to power supply limitations (see EEx-i instrumentation below).

• Various types of cables are useable for fieldbus (Fig. 7). Type A is recommended as preferred fieldbus cable, and only this type is specified for the maximum bus length of 1900 m.

• Principally, there need to be two terminators per bus segment, one at or near each end of a transmission line.

• It is not imperative that bus cables be shielded, however, it is recommended to prevent possible interferences and for best performance of the system.

EEx-i instrumentation
The H1 bus can be designed intrinsically safe (Ex-i) to suit applications in hazardous areas. This requires that proper barriers be installed between the safe and the explosion hazardous area (Fig. 8). In addition, only one device, the power supply unit, must supply the fieldbus with power. All other devices must always, i.e. also when transmitting and receiving data, function as current
sinks. Since the capacity of electrical lines is limited in intrinsically safe areas depending
on the explosion group – IIB or IIC – (see Fig. 9), the number of devices that can be connected to one segment depends on the effective power consumption of the used devices. Since the FOUNDATION fieldbus specification is not based on the FISCO model,  the plant operator himself must ensure that intrinsicsafety requirements are met when planning and installing the communications network. For instance, the capacitance and inductance of all line segments and devices must be calculated to ensure that the permissible limit values are observed (Fig. 10).

High Speed Ethernet (HSE)
The HSE is based on standard Ethernet technology. The required components are therefore widely used and are available at low costs. The HSE runs at 100 Mbit/s and cannot only be equipped with electrical lines, but with optical fiber cables as well.

The Ethernet operates by using random (not deterministic) CSMA bus access. This method can only be applied to a limited number of automation applications because it requires real-time capability. The extremely high transmission rate enables the bus to respond sufficiently fast when the bus load is low
and devices are only few. With respect to process engineering requirements, real-time requirements are met in any case. If the bus load must be reduced due to the many connected devices, or if several
HSE partial networks are to be combined to create a larger network, Ethernet Switches must be used (see Fig. 4). A switch reads the target address of the data packets that must be forwarded and then passes the packets on to the associated partial network. This way, the bus load and the resulting bus
access time can be controlled to best adapt it to the respective requirements.

Bridge to H1-HSE coupling
A communications network that consists of an H1 bus and an HSE network results in a topology as illustrated in Fig. 4. To connect the comparatively slow H1 segments to the HSE network, coupling components, so-called Bridges, are required. Similar to HSE, the specification of this bus component
has not been completed up to now. A Bridge is used to connect the individual H1 buses to the fast High Speed Ethernet. The various data transfer rates and data telegrams must be adapted and converted, considering the direction of transmission. This way, powerful and widely branched networks can be installed in larger plants.

Communication stack
The field devices used with the FOUNDATION fieldbus are capable of assuming process control functions. This option is based on distributed communication which ensures that

• each controlling field device can exchange data with other devices (e.g. reading measuring values, forwarding correction values),

• all field devices are served in time (‘in time’ meaning that the processing of the different control loops is not negatively influenced),

• two or more devices never access the bus simultaneously.

To meet these requirements, the H1 bus of the FOUNDATION fieldbus uses a central communication control system.

Link Active Scheduler – LAS
The Link Active Scheduler (LAS) controls and schedules the communication on the bus. It controls the bus activities using different commands which it broadcasts to the devices. Since the LAS
also continuously polls unassigned device addresses, it is possible to connect devices during operation and to integrate them in the bus communication. Devices that are capable of becoming the LAS, are called ‘Link Master’. ‘Basic devices’ do not have the capability to become LAS. In a redundant system containing multiple Link Masters, one of the Link Masters will become the LAS if the active LAS fails (fail-operational design).

Communication control
The communication services of the FF specification utilize scheduled and unscheduled data transmission. Time-critical tasks, such as the control of process variables, are exclusively performed by scheduled services, whereas parameterization and diagnostic functions are carried out using unscheduled communication services.

• Scheduled data transmission

To solve communication tasks in time and without access conflicts, all time-critical tasks are based on a strict transmission schedule. This schedule is created by the system operator during the configuration of the FF system. The LAS periodically broadcasts a synchronization signal (TD: Time Distribution) on the fieldbus so that all devices have exactly the same data link time. In scheduled transmission, the point of time and the sequence are exactly defined.

This is why it is called a deterministic system. Fig. 11 presents the schedule for a system with two sensors and two control
valves. The schedule determines when the devices process their function blocks (AI, A0, PID) and when it is time to transmit data. Each activity to be executed has been scheduled for a certain time. This time is defined by an offset value which reflects the delay referred to the start of the schedule.
Based on this schedule, a transmission list is generated which defines when a specific field device is prompted to send its data. Upon receipt of the message, the respective device (‘publisher’) broadcasts the data in the buffer to all devices on the fieldbus which are configured to receive the data (‘subscriber’). This type of data transmission is therefore called the ‘publisher- subscriber’ method.
The LAS cyclically transmits the data according to the list for all data buffers in all devices. Each cyclical data transmission is explicitly activated by the LAS (Fig. 12):

• If a device (e.g. device 1: Sensor) is prompted to publish its measured data, the LAS issues the Compel Data (CD) command to the device.

• Upon receipt of the CD, the device publishes the data in the buffer.

• The ‘subscribers’ of this message (e.g. device 3: Control valve) can read
  and evaluate this data accordingly.

Each field device receives a separate schedule. This enables system management to know exactly what task is to be executed when and when data must be received or sent. Example: For the above mentioned schedule, the following time sequence of actions results as shown in Fig. 13.

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