HART communication
occurs between two HART-enabled devices, typically a field device
and a control or monitoring system. Communication occurs using standard
instrumentation grade wire and using standard wiring and termination
practices.
HART provides two simultaneous communication channels: the 4-20mA
analog signal and a digital signal. The 4-20mA signal communicates
the primary measured value (in the case of a field instrument) using
the 4-20mA current loop - the fastest and most reliable industry
standard. Additional device information is communicated using a
digital signal that is superimposed on the analog signal. The digital
signal contains information from the device including device status,
diagnostics, additional measured or calculated values, etc. Together,
the two communication channels provide a complete field communication
solution that is easy to use and configure, is low-cost and is very
robust.
HART Data—Overview
- Digital data: 35-40 valuable data items standard in every HART
device
- Device identification: device tag, supplier, device type and
revision, device serial number
- Calibration data: upper and lower range values, upper and lower
sensor limits, PV damping, last calibration date
- Process variables: primary variable plus secondary measurements
and multivariable parameters
- Status/diagnostic alerts: device malfunction, configuration
change, power fail restart, loop current fixed or saturated, primary
or secondary variable out of limits, communication error, plus
more
HART Commands
HART device communicate via commands. The command
set includes three classes: universal, common practice, and device
specific.
| Universal |
All devices using the HART
Protocol must recognize and support the universal commands.
Universal commands provide access to information useful in
normal operations. For example: read primary variable and
units, read manufacturer and device type, read current output
and percent of range and read sensor serial number and limits.
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| Common Practice |
Common practice commands provide functions
implemented by many, but not necessarily all, HART communication
devices. The HART specifications recommend devices to support
these commands when applicable. Examples of common practice
commands are: read a selection of up to four dynamic variables,
write damping time constant, write transmitter range, set
fixed output current and perform self-test. |
| Device Specific |
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 or in the
Field Device Specification document. Examples of device specific
commands: read or write sensor type, start, stop or clear
totalizer, read or write alarm relay set point, etc. |
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