Basics of wirelessHART network protocol

The WirelessHART™ standard provides a robust wireless protocol for the full range of process measurement, control, and asset management applications. It adds wireless capabilities to the HART Protocol while maintaining compatibility with existing HART devices, commands, and tools.

A wirelessHART device uses nominal 2.4GHz transmission frequency using IEEE 802.15.4 standard radios because meeting these criteria allows WirelessHART devices to be unlicensed according to FCC (Federal Communications Commission) standards.

Wireless HART network: 

WirelessHART is a subset of the HART industrial instrument communication standard as of version 7, communicating process data over 2.4 GHz radio waves. WirelessHART is a wireless mesh network communications protocol for process automation applications.

 

Each individual instrument in the HART wireless connection is connected through a mesh network. Each individual instrument is connected to a common input and adjustment instruments. If an instrument is far from the gateway or the route is blocked, it can not connect to the gateway. Although you can communicate with the gateway through other instruments. Therefore, each device in the mesh network can serve as a router for messages from other devices.

The purpose of a mesh network is to provide redundant data pathways in case of device failure or changes in the environment interrupting radio communication between devices.

A network administrator responsible for configuring the network, scheduling communications between devices, managing message routes, and monitoring the state of the network. Network Manager can be integrated into the gateway, the host application or the process automation controller.

This extends the reach of the network and provides redundant communication paths to increase reliability. Network Manager determines redundant routes based on latency, efficiency and reliability. To ensure that redundant routes remain open and unobstructed.

WirelessHART protocol:

 

Physical Layer: 

  • 2.4 GHz to 2.5 GHz (“ISM” – Industrial, Scientific, Medical) signal band
  • O-QPSK modulation (offset quadrature phase-shift keying)
  • 250 kbps data rate
  • Direct-sequence spread-spectrum (DSSS) with frequency-hopping between 15 channels within that band for security and interference reduction
  • TDMA (Time-Division Multiple Access) bus arbitration, with 10-millisecond timeslots allocated for device transmission
  • Variable transmit power, with 10 dBm (10 milliwatts) being default

Data Link Layer:

  • Network ID number uniquely identifies each WirelessHART network, allowing multiple networks to overlap the same physical area
  • Channel “blacklisting” – automatically avoids hopping to noisy channels

Network Layer:

  • “Mesh” networking
  • Signal repeating – devices may act as “repeaters” for other devices too far away from the master unit
  • A Network Manager device determines communication routes between field devices, as well as timing schedules
  • Four levels of data message priority (listed from highest to lowest): Command, Process data, Normal ( all messages other than command), Alarm.

Application layer:

  • 128-bit encryption of data
  • Backward-compatibility with a wired-HART command structure and DDL (Device Description Language)
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