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IEC TR 61804-6: Function Blocks for Process Control – Engineering Design Guide
Key Insight
IEC TR 61804-6 defines the application guide for function blocks (FB) in process control devices, standardizing how measurement, actuation, and processing functions are represented across different fieldbus protocols and device description technologies.
IEC TR 61804-6 defines the application guide for function blocks (FB) in process control devices, standardizing how measurement, actuation, and processing functions are represented across different fieldbus protocols and device description technologies.
1. Scope and Architectural Foundation
IEC TR 61804-6, published in 2012 as a technical report, is part of the IEC 61804 series that specifies function blocks (FB) for process control. While the core parts of the series (IEC 61804-2 through -5) define the FB concept, device model, and EDDL (Electronic Device Description Language) integration, Part 6 serves as an application guide. It addresses how function blocks map to real process control devices — transmitters, actuators, and controllers — within distributed control systems (DCS) and fieldbus environments.
The standard’s architectural model defines three block classes: Analog Input (AI), Analog Output (AO), and PID Controller blocks as the foundation, extended by digital/discrete blocks, arithmetic blocks, and signal characterization blocks. Each block encapsulates its algorithm, parameters, and status information in a standardized way, enabling device interoperability regardless of the underlying communication protocol (PROFIBUS PA, FOUNDATION Fieldbus, HART, or WirelessHART).
Engineering Insight: The true value of IEC 61804 function blocks becomes apparent during system integration. When a pressure transmitter from Manufacturer A is replaced with one from Manufacturer B, the AI block ensures that the signal scaling, filtering, and alarm parameters remain consistent at the DCS level — no control logic modification required. This plug-and-play interoperability can reduce commissioning time by 30–50% on large projects.
2. Function Block Parameterization and EDDL
2.1 Block Structure and Parameters
Each IEC 61804 function block contains three parameter categories: input parameters (received from the process or upstream blocks), output parameters (delivered to actuators or downstream blocks), and contained parameters (configuration, diagnostic, and status data). The standard defines mandatory and optional parameters for each block type, ensuring a minimum level of functionality while allowing manufacturers to add proprietary features.
| Block Type | Mandatory Parameters | Typical Scan Rate | Primary Application |
|---|---|---|---|
| Analog Input (AI) | PV, OUT, SIMULATE, PV_FTIME, ALARM_SUM | 50–200 ms | Pressure, temperature, level transmitters |
| Analog Output (AO) | SP, OUT, READBACK, SIMULATE, MODE_BLK | 50–200 ms | Valve positioners, variable-speed drives |
| PID Controller | SP, PV, OUT, GAIN, RESET, RATE, TRK_VAL | 100–500 ms | Feedback control loops |
| Signal Characterization (SC) | IN, OUT, CURVE_X, CURVE_Y | 100–500 ms | Linearization, square-root extraction |
| Arithmetic (AR) | IN_1..IN_4, OUT, COMP_HI, COMP_LO | 100–500 ms | Ratio control, flow compensation |
2.2 EDDL Integration
EDDL (IEC 61804-3) is the device description technology that enables configuration tools to interpret and display function block parameters. IEC TR 61804-6 provides detailed guidance on EDDL application — including how to define block parameter menus, write validation rules, and create help text for operators. The report emphasizes the importance of proper status handling: every analog output must propagate quality and sub-status information (good, uncertain, bad, test) so that the control system can detect device faults, communication losses, and process anomalies.
Design Warning: A common EDDL implementation error is failing to define the MODE_BLK parameter correctly. In IEC 61804, each block has a target mode, actual mode, permitted mode, and normal mode. If the permitted mode list does not include Auto and O/S (out of service), the block cannot transition between commissioning and operational states. This is one of the most frequent causes of “block not responding” errors during DCS commissioning.
3. Practical Implementation Guidance
IEC TR 61804-6 offers several critical implementation guidelines for system integrators and device manufacturers:
Block Execution Order: In a control loop, blocks must execute in a defined sequence — AI block first (read process), then PID (compute), and AO block last (write to actuator). The standard recommends allocating the AI and PID blocks to the field device and the AO block to the actuator, minimizing communication overhead on the fieldbus segment. For high-speed loops (e.g., compressor surge control), all three blocks should reside in the same physical device or be connected via a dedicated high-speed backbone.
Alarm and Event Handling: Each AI and PID block includes an ALARM_SUM parameter that aggregates process alarms (HI, HI_HI, LO, LO_LO, rate-of-change). The standard specifies alarm priority levels from 0 (no action) to 15 (most critical), allowing the DCS to filter nuisance alarms and prioritize operator response. IEC TR 61804-6 recommends setting the alarm deadband to at least 1% of the sensor span to prevent alarm chatter.
Simulation and Bypass: For commissioning and maintenance, each block supports a SIMULATE parameter that overrides the process input with a manually entered value. The standard mandates that the simulation status is clearly indicated in the output quality field so that downstream blocks and the DCS know the value is not live. This feature is essential for loop testing without process intervention — for example, simulating a high-level alarm in a tank to verify the shutdown logic.
4. Frequently Asked Questions
Q1: What is the relationship between IEC 61804 and IEC 61499?
IEC 61804 focuses on function blocks for process control devices (transmitters, actuators) in continuous process industries. IEC 61499 is a more general function block standard for distributed industrial automation systems, supporting event-driven execution and suitable for discrete manufacturing. While both use the FB concept, IEC 61804 blocks are simpler and optimized for cyclic execution typical of DCS environments.
Q2: Can IEC 61804 function blocks be used in safety-instrumented systems (SIS)?
The standard’s function blocks are not certified for use in safety-critical applications (SIL 2 or higher) without additional measures. For SIS applications, use blocks certified to IEC 61508 or IEC 61511, which include enhanced diagnostics, diversity, and fail-safe behavior. The IEC 61804 blocks can, however, be used for non-safety monitoring functions within the same device, provided the safety and non-safety functions are sufficiently separated.
Q3: How does wirelessHART support IEC 61804 function blocks?
WirelessHART (IEC 62591) implements the IEC 61804 AI, AO, and PID blocks with minor adaptations for time-synchronized mesh networking. The key difference is that block execution is not strictly cyclic; updates occur at the network’s configured publish rate (typically 1–60 seconds). The standard’s status handling becomes critical here — the wireless block output includes Hold or Uncertain quality flags if a communication path is temporarily disrupted, preventing spurious process upsets.
Q4: What is the maximum number of function blocks a field device can support?
The standard does not specify a maximum, but practical limitations arise from the device’s processor speed, memory, and the fieldbus segment’s communication bandwidth. A typical pressure transmitter supports 2–4 AI blocks (for primary and auxiliary variables). A multi-variable flow transmitter may support 8–12 blocks including AI, flow calculation, and totalizer functions. For devices with more than 16 blocks, segment scheduling and update rates must be carefully evaluated.
