The HART Protocol is a master-slave communication protocol which means that during normal operation, each slave (a field device) communication is initiated by a request (or command) from the master (host) communication device. The master or host is generally a distributed control, PLC, or PC-based asset management system for example. The slave device is typically a field measurement device such as pressure, level, temperature, flow or other transmitters.
In order to make certain any HART-enabled device from any supplier can communicate properly and respond to a command with the correct information, the set and types of commands are defined in the HART Specifications and implemented in all HART registered devices.
Users need not worry about these commands because they are included in the functions of the host. The specific capabilities of a device (device specific commands) are available to the host when the host is given the instructions included in the Device Description (DD) of a specific device.
An important point is that defined device status indications are included with each communication response to the host. The host then interprets these status indicators and may provide basic device diagnostic information.
The HART Command Set provides uniform and consistent communication for all field devices. Host applications may implement any of the necessary commands for a particular application. The command set includes three classes:
All devices using the HART Protocol must recognize and support the universal commands. Universal commands provide access to information useful in normal operations (e.g., read primary variable and units).
A Partial List of HART universal Commands:
- Read manufacturer and device type
- Read primary variable (PV) and units
- Read current output and percent of range
- Read up to four pre-defined dynamic variables
- Read or write eight-character tag, 16-character descriptor, date
- Read or write 32-character message
- Read device range values, units, and damping time constant
- Read or write final assembly number
- Write polling address
Common Practice commands provide functions implemented by many, but not necessarily all, HART Communication devices.
A Partial List of HART Common Practice Commands:
- Read selection of up to four dynamic variables
- Write damping time constant
- Write device range values
- Calibrate (set zero, set span)
- Set fixed output current
- Perform self-test
- Perform master reset
- Trim PV zero
- Write PV unit
- Trim DAC zero and gain
- Write transfer function (square root/linear)
- Write sensor serial number
- Read or write dynamic variable assignments
Device Specific commands represent functions that are unique to each field device. These commands access setup and calibration information, as well as information about the construction of the device. Information on Device Specific commands is available from device manufacturers.
A Partial List of HART Device Specific Commands:
- Read or write low-flow cut-off
- Start, stop, or clear totalizer
- Read or write density calibration factor
- Choose PV (mass, flow, or density)
- Read or write materials or construction information
- Trim sensor calibration
- PID enable
- Write PID set point
- Valve characterization
- Valve set point
- Travel limits
- User units
- Local display information