What is HART?
HART (Highway Addressable Remote Transducer) is an important communications protocols used in plant automation. HART is owned by an independent, non-profit HART communication foundation.
This protocol was developed by Rosemount and is regarded as an open standard. Its main advantage is that it enables the end-user to keep the existing 4-20 mA instrumentation cabling and simultaneously to use the same wires to carry digital information superimposed on the analogue signal.
The HART protocol has two formats for digital transmission of data:
- Poll /response mode
- Burst or broadcast mode
In the poll/response mode the master polls each of the smart devices on the highway and requests the relevant information. In burst mode the field device continuously transmits process data without the need for the host to send request messages.
Commands:
This allows the host device to obtain and interpret field device data. There are three classes of commands:
- Universal commands.
- Common practice commands.
- Device Specific commands.
Universal commands:
Read Manufacturer and device type. Read PV and units. Read current output and percent of range. Read up to 4 pre defined dynamic variables. Read or write 8-character tag, 16-character descriptor, date. Read or write 32 character message. Read device range, units and damping time constant. Read or write final assembly number. Write polling address.
Common Practice Commands:
Read selection of up to 4 dynamic variables. Write damping time constant. Write device range. Calibrate (Set zero, set span). Set fixed output current. Perform self-test. Trim PV zero. Write PV units. Trim DAC zero and gain. Write transfer function (square root/linear). Write sensor serial number. Read or write dynamic variable assignments.
Instrument Specific Commands
Read or write low flow cut-off value. Start, stop or clear totaliser. Read or write density calibration factor. Choose PV (mass flow or density). Read or write materials or construction information. Trim sensor calibration.