HART protocol burst mode: communication characteristics and performance optimization
To enable regular transmission of data and diagnostic information from field devices to master control units, the data link layer of the HART protocol defines an operational mode. In this mode, field devices can periodically broadcast data over the communication link. When a field device performs synchronous cyclic broadcasting of data and diagnostic information without requiring continuous polling from the master control unit, it operates in burst mode.
In wired 4-20ma hart communication links, regardless of the number of field devices, only one device may operate in burst mode at any time. Wireless HART devices all operate in burst mode, but within the same time slot and channel on the time division multiple access (TDMA) communication layer, only one device can transmit simultaneously.
The English description for Burst-mode devices is "Burst-Mode Device". This is a digital response device that periodically provides process data and diagnostic information. It functions as an independent broadcast device, with its operating mode configured by the master station equipment.
1、Configuration content for Burst operation mode
The Burst operation mode supports periodic publication of process data using "burst" messages. In this mode, field devices are instructed to continuously transmit process data and status information without requiring any additional actions from the master station. The master station configures this operation mode for field devices through the following commands.
- Command 103: Write burst cycle;
- Command 104: Write burst trigger mode;
- Command 105: Read burst mode configuration;
- Command 107: Write burst device variables;
- Command 108: Write burst mode command ID;
- Command 109: Burst mode controlAdditionally, two commands are specifically designed for I/O systems and WirelessHART adapters:
- Command 101: Reads the sub-device for burst messages;
- Command 102: Maps the sub-device to burst messages.
In addition, devices implementing burst mode must be configured with at least three burst messages. Each burst message must support different configurations. Each burst message can represent distinct commands, trigger conditions, update cycles, device variable sets, etc.
2、Burst configuration period
The master station configures the device's burst cycles using Command 103, setting both the minimum and maximum burst periods. The upper limit of the minimum burst period varies across different physical layers, as shown in the table below.
Minimum Update Rates Allowed by Physical Layer(in Seconds)
The maximum burst cycle duration cannot exceed 3600 seconds. Select from the options below that are not shorter than the minimum burst cycle duration.
Update Periods Allowed(in Seconds)
The device must transmit Burst information at the shortest burst period whenever the trigger conditions specified in Command 104 are met. When these configured trigger conditions are not satisfied, the field device must transmit Burst information at the longest burst period.
3、Configuration of Burst Trigger Mode
The main station configures the device's Burst triggering mode using Command 104, with five predefined triggering modes available.
- Continuous Trigger Mode: Non-stop burst messages are released in the shortest possible burst intervals.
- Window Trigger Mode (Window): When the source value deviates and exceeds the specified window threshold, messages are sent at the shortest burst cycle.
- Rising Trigger Mode: When the source value exceeds the specified trigger threshold, messages are sent at the shortest burst cycle.
- Falling Trigger Mode: When the source value drops below the specified trigger threshold, messages are sent at the shortest burst cycle.
- Change Trigger Mode (On-Change): Send messages with the shortest burst cycle whenever any value in the message changes.
For I/O systems, when burst messages originate from a slave device, if the engineering unit within the slave device changes, the burst message must be transmitted using the shortest burst period specified in Command 103.
3.1 Window Trigger Mode (Window)
The window is a symmetric interval centered around the last transmitted value. When the source value deviation remains within the specified window, the next Burst message is sent at the maximum update cycle. If the deviation exceeds the designated window, the next Burst message is transmitted at the shortest burst cycle. The diagram below illustrates the window mode.
3.2 Rising Trigger Mode（Rising）
When the source value remains below the specified Trigger Level, the next Burst message is sent at the maximum update cycle. Once the Trigger Level is exceeded, the update frequency changes. As long as the source value stays above the Trigger Level, Burst messages are published at the update cycle rate. If the value falls below the Trigger Level, the update frequency drops to the maximum update cycle.
3.3 Falling Trigger Mode（Falling）
The burst message is triggered when the source value falls below the specified trigger threshold. The burst message will be published at the rate specified by the update cycle; otherwise, the update time will decrease to the maximum update cycle.
3.4 Change Trigger Mode (On-Change)
The burst message is triggered when any value in the burst message changes, and it will be published at the rate specified by the update cycle. Otherwise, the update time will be reduced to the maximum update cycle.