Foundation Fieldbus H1 Technology

Foundation Fieldbus H1 Technology 1
  • The Foundation Fieldbus is a modern industrial automation technology that is well-known for its ability to effectively collect and send large volumes of data in complex process control environments. 
  • In contrast with standard systems, Foundation Fieldbus smoothly maintains a variety of instrument and process data types in addition to process variables and control signals. 
  • At the core of this capability lies a well-structured communication model, comprising three integral components: the physical layer, the data link and application layer, and the user layer.
What are the layers of FOUNDATION fieldbus? 1

One of the FOUNDATION fieldbus protocol versions is Foundation Fieldbus H1. Field devices can communicate with each other and with the control system via the bi-directional Foundation H1 (31.25 kbit/s) communications protocol.

  • The primary function of the physical layer is to handle the translation of messages into physical signals and vice versa, ensuring effective communication between devices.
  • It establishes a standardized electrical interface for all Foundation fieldbus devices, promoting seamless integration.
  • Foundation fieldbus H1 segments operate within the power range of 9-31 volts DC, drawing approximately 15-20 mAmps of current per device.
  • The communication speed is set at 31.25Kbaud, meeting the necessary requirements for reliable data transfer.
  • Compliance with approved standards such as IEC61158-2 and ANSI/ISA 50.02, part 2, ensures consistency and compatibility.
  • The physical layer’s ability to run on existing field wiring over extended distances enhances its practicality, and support for two-wire devices adds to its versatility.
  • Additionally, the option for intrinsic safety provides an extra layer of security.
What are the layers of FOUNDATION fieldbus? 2
What is Foundation Fieldbus signal?
What is Foundation Fieldbus signal? 1
  • This layer integrates various techniques to control data transfer via the fieldbus.
  • Its primary responsibility is to provide a consistent way of organizing function-block execution, communication scheduling, and data packaging.
  • It makes effective process control and device interoperability possible by ensuring consistency in data handling.
  • By providing an organized method of data transmission, the data link and application layers enhance the communication model’s reliability and consistency.
  • The user layer, which is positioned at the top of the communication stack, acts as an interface between other layers and applications.
  • Resource blocks, transducer blocks, and functional blocks are located on the user layer and are responsible for defining and implementing device capabilities, which include control and diagnostics.
  • An essential component of the user layer is device description, which allows the host system to understand and communicate with these blocks without requiring specialized programming.
  • This layer improves the Foundation fieldbus system’s overall usability and adaptability by providing as some kind of bridge between the applications and the underlying communication infrastructure.
Scheduled communications
  • In Foundation Fieldbus, scheduled communications involve devices and functional blocks executing and exchanging process control information on a repeating cycle. 
  • The timing is determined by a master schedule in the Link Active Scheduler (LAS), residing in the host system or one of the devices on the segment. 
  • This deterministic approach ensures regular and precise execution, reducing process variability and improving plant performance.
Unscheduled communication 2
  • Foundation Fieldbus supports unscheduled (acyclic) communication for non-time-critical information such as configuration, alarms, events, trend data, operator display, diagnostic, and status information. 
  • These transmissions have flexible timing and use a token-passing method, allowing each device on the segment the opportunity to transmit messages during the allotted time.
  • Foundation Fieldbus incorporates various data redundancy checks to avoid message-bit errors. 
  • Each device is designated to check for problems and label the data it sends with a status indicating whether the quality of the data is good, bad, or uncertain. 
  • This feature ensures reliability, and the status information is available to the host system.
  • Every device on a Foundation Fieldbus segment shares the same time, thanks to the application clock. 
  • This system management function periodically broadcasts the time (local or universal coordinated time) to all devices. 
  • Alarms and events are time-stamped at the device where they occur, providing superior time resolution and accuracy for activities like sequence of events recording and analysis.
  • The Link Active Scheduler (LAS) function is crucial for maintaining the central, deterministic schedule for communication between devices on a segment. 
  • It improves overall communication reliability by compelling each device to transmit cyclic data when scheduled. 
  • Multiple backup LAS ensures graceful degradation in case of failure, enhancing system reliability.
  • In Foundation Fieldbus, device address assignment is critical for identifying signals associated with each device on the multidrop bus. 
  • The protocol avoids error-prone methods like dip switches, and online addressing, configured automatically, helps prevent addressing errors. Default addresses can be overridden when necessary.
  • Foundation Fieldbus simplifies device and parameter identification through a tag-based service. 
  • Instead of relying on hardware or register addresses, devices or variables are found by tags (e.g., “FT-101”). 
  • A find tag query is sent on the bus, and devices respond with complete path information, avoiding duplicate tag assignments. This feature streamlines the identification process for improved efficiency.
  • Fieldbus blocks are compact software modules, each with designated inputs and/or outputs tailored for specific functions or information types. 
  • In Foundation Fieldbus technology, three main block types are utilized: Resource blocks, Transducer blocks, and Function blocks.
Foundation Fieldbus Blocks
  • The resource block encompasses comprehensive information about the overall device, including details such as manufacturer, device type, and serial number. 
  • It serves as a crucial identifier during project execution, facilitating device tagging and commissioning. 
  • In ongoing operations, maintenance technicians utilize the resource block to access device configuration, status information, and perform diagnostics. 
  • This block’s significance lies in enabling the detection of potential device issues before they impact the process.
  • With a focus on the “wetted parts” of an apparatus, the transducer block enables local input/output operations to control actuators or displays and read sensors. 
  • It serves as an essential connection between the data-centric area of process control and the actual sensors and actuators. 
  • Transducer blocks are used for calibration, unit setting, and other tasks related to assuring accurate input or output during project execution.
  • These blocks are used by maintenance technicians for diagnostics, troubleshooting, calibration, and other continuous operations tasks that maintain the functionality and health of the device.
  • Multiple transducer blocks may exist within a single device, each handling specific aspects such as sensors, actuators, local displays, or diagnostics.
function block of Fieldbus Foundation
  • Function blocks provide the fieldbus environment’s control-system behavior. 
  • Process control is achieved through the connection of discrete and analog input and output blocks over a fieldbus with a range of control algorithms, such splitter, PID, and characterizer. 
  • Even operating a control loop completely in field devices without involving the host system is feasible and often preferable. 
  • A basic device might only contain one function block for input or output. Devices that are more complex may include many input and output blocks in addition to blocks for control and monitoring.
  • Control engineers employ function blocks to carry out the control strategy throughout project execution. 
  • The function blocks give the operators the functions and process control information they require to run the plant when it is in operation. 

Field device function blocks can be used to demonstrate the following example control loop:

FF Control Loop
  • Incorporates an AI function block.
  • Converts analog signals from the flow sensor into digital data.
  • Provides real-time information on the flow rate.
  • Contains PID and AO function blocks.
  • PID function block calculates control signal based on desired setpoint and measured flow rate.
  • AO function block converts control signal into physical action to adjust valve opening.
  • AI function block from flow transmitter provides flow rate data.
  • PID function block in control valve compares actual flow rate with setpoint and generates control signal.
  • AO function block adjusts valve opening based on control signal to regulate flow rate.
  • Establishes a closed-loop system for continuous monitoring and adjustment of flow rate to meet process requirements.
H1 Link Active Scheduler
  • Facilitates PID loop communication scheduling and unscheduling.
  • Manages the communication process between PID loops.
  • Enables coordination and timing of PID loop activities.
  • Ensures efficient utilization of resources for PID loop operations.
  • Supports dynamic adjustment of scheduling based on system requirements.
  • Optimizes PID loop performance and responsiveness.
  • Enables prioritization and allocation of resources for critical PID loop tasks.
  • Enhances overall system stability and reliability through effective scheduling mechanisms.
  • Provides flexibility for adapting to changing process conditions or priorities.
  • Enhances control system efficiency by managing PID loop communication effectively.

Sundareswaran Iyalunaidu

With over 24 years of dedicated experience, I am a seasoned professional specializing in the commissioning, maintenance, and installation of Electrical, Instrumentation and Control systems. My expertise extends across a spectrum of industries, including Power stations, Oil and Gas, Aluminium, Utilities, Steel and Continuous process industries. Tweet me @sundareshinfohe

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