The HART (Highway Addressable Remote Transducer) protocol is a widely adopted communication standard in process automation, enabling digital data transmission alongside traditional 4-20 mA analog signals. One of its key features for improving efficiency is Burst Mode, which allows field devices to periodically broadcast process data and diagnostics without constant polling from the host system.

This mode reduces communication overhead, lowers power consumption in devices, and provides faster updates—making it valuable for real-time monitoring in industrial environments. This guide explains Burst Mode in detail, including its operation, configuration, trigger mechanisms, and differences between wired and wireless implementations.

Overview of the HART Protocol

HART combines analog and digital communication, allowing seamless upgrades from legacy systems. Its physical layers include:

• FSK (Frequency Shift Keying): Most common, superimposed on 4-20 mA loops.
• RS-485, PSK (Phase Shift Keying), and IR (Infrared): For specific applications.
• IEEE 802.15.4: Foundation for WirelessHART, using TDMA for scheduled communication.
• IEEE 802.3 (Ethernet): For HART-IP.

The protocol uses a master-slave request-response model by default. Burst Mode shifts this to allow slaves to initiate broadcasts, improving efficiency for fixed-format data like process variables (PV) and status.

What is Burst Mode in HART?

In Burst Mode, a field device periodically broadcasts data and diagnostic information without requiring the master to poll it continuously. This is ideal for transmitting key process variables and status updates.

Key characteristics:

• On wired networks (e.g., FSK), only one device can burst at a time to avoid collisions.
• In WirelessHART, all devices operate in a burst-like mode, scheduled via TDMA slots—multiple devices can transmit without interference.
• Burst Mode devices are configured by the master and broadcast responses to specific HART commands.

Benefits include faster update rates (up to 3-4 per second in wired, scheduled in wireless), reduced master workload, and lower power use for battery-powered devices.

Configuring Burst Mode

Devices supporting Burst Mode must implement at least three configurable burst messages. Each can have unique commands, variables, periods, and triggers.

Primary configuration commands:

Additional commands for I/O systems/WirelessHART adapters: 101 (read subdevice burst messages) and 102 (map subdevices).

Supported burst commands include 1, 2, 3, 9, 38, and 48 (plus 33 if implemented).

Burst Period Configuration (Command 103)

Defines minimum (fast) and maximum (slow) update rates.

Maximum period: Up to 3600 seconds. Allowed values include discrete steps (e.g., 0.02, 0.05, 0.1, 0.5, 1, 2, 4, 8, 16, 32 seconds) or any value from 60-3600 seconds.

The device uses the minimum rate when triggers are met; otherwise, it falls back to the maximum.

Burst Trigger Modes (Command 104)

Triggers control when the device switches to the faster update rate. Options:

• Continuous: Always bursts at minimum rate.
• Window: Bursts at minimum rate if the source variable deviates beyond a symmetric window around the last sent value.
• Rising: Triggers when value rises above a set level.
• Falling: Triggers when value falls below a set level.
• On-Change: Triggers on any change in the burst message content (common for discrete devices).

For non-continuous modes, the device reverts to the maximum rate when conditions are not met.

Device Variables and Command Selection (Commands 107 & 108)

• Command 107: Maps up to 8 device variables (slots 0-7; code 250 = unused). Command 33 supports only first 4.
• Command 108: Selects the HART command response to burst (e.g., Command 1 for PV).

If trigger classification mismatches the command, the device may auto-switch to continuous and issue a warning.

Enabling Burst Mode (Command 109)

Enables/disables bursting on specific link layers:

• 0: Off
• 1: Token-Passing (wired)
• 2: TDMA (WirelessHART)
• 3: Both
• 4: HART-IP

WirelessHART handles scheduling automatically; bandwidth issues may delay or adjust rates.

Burst Mode State Machine

Burst Mode devices follow a state machine to manage transmissions safely on shared links:

• Burst Acknowledge (BACK): Unsolicited responses.
• Timers: BT (Burst Timer), RT1/RT2 (backoff/listen times), STO (slave timeout).

In idle state:

• Enable command → Prepare immediate BACK.
• Detect traffic → Back off with RT1.
• Receive addressed request → Respond normally.
• BT expires → Listen (RT2) then transmit BACK.

Timers vary by physical layer (e.g., FSK: RT1 = 33 character times).

Wired vs. WirelessHART Burst Mode

• Wired HART: Optional; only one burster per network; collision avoidance via timers.
• WirelessHART: Mandatory and scheduled; all devices “burst” in assigned TDMA slots; supports event-driven bursts.

Conclusion

HART Burst Mode enhances communication efficiency by enabling proactive data broadcasting, reducing latency, and supporting advanced diagnostics. Proper configuration of periods, triggers, and commands is essential for optimal performance. For the latest specifications, refer to FieldComm Group resources or device manuals.

This detailed understanding helps engineers implement reliable, high-performance HART systems in process industries.