What is HART?

The HART signal

HART ("Highway Addressable Remote Transducer") is a popular digital communication protocol designed for industrial process measurement applications. The special feature is that it uses a low-level modulation superimposed on the standard 4-to-20 mA current loop which is now widely used for such measurements. Because the HART signal is small, and composed of sine waves, its average value is zero and does not significantly affect the accuracy of the analogue current signal, which can therefore still be used. This provides compatibility with existing systems, while allowing simultaneous digital communication for device configuration, status checking, diagnostics and so forth.

The HART message structure

The structure of a HART message is shown below:

The preamble, of between 5 and 20 bytes of hex FF (all 1's), helps the receiver to synchronise to the character stream.

The start character may have one of several values, indicating the type of message: master to slave, slave to master, or burst message from slave; also the address format: short frame or long frame.

The address field includes both the master address (a single bit: 1 for a primary master, 0 for a secondary master) and the slave address. In the short frame format, the slave address is 4 bits containing the "polling address" (0 to 15). In the long frame format, it is 38 bits containing a "unique identifier" for that particular device. (One bit is also used to indicate if a slave is in burst mode.)

The command byte contains the HART command for this message. Universal commands are in the range 0 to 30; common practice commands are in the range 32 to 126; device-specific commands are in the range 128 to 253.

The byte count byte contains the number of bytes to follow in the status and data bytes. The receiver uses this to know when the message is complete. (There is no special "end of message" character.)

The status field (also known as the "response code") is two bytes, only present in the response message from a slave. It contains information about communication errors in the outgoing message, the status of the received command, and the status of the device itself.

The data field may or may not be present, depending on the particular command. A maximum length of 25 bytes is recommended, to keep the overall message duration reasonable. (But some devices have device-specific commands using longer data fields.) See also the HART data field.

Finally, the checksum byte contains an "exclusive-or" or "longitudinal parity" of all previous bytes (from the start character onwards). Together with the parity bit attached to each byte, this is used to detect communication errors.

Uses of HART

Using HART digital communication, up to four measurements can be transmitted in a single message. Multivariable instruments have been developed to take advantage of this. In addition, when using only digital communication, several instruments can be connected in parallel "multidrop" on a single pair of wires (with their analogue currents each set to a minimum value, usually 4 mA). Each device has its own address, so a host can communicate with each one in turn.

HART was developed by Rosemount Inc in the mid-1980's, but has been made completely open, and all rights now belong to the independent HART Communication Foundation, which supports the protocol and oversees further development.

There are now upward of 100 member companies in the HCF, most of whom have HART products on the market. Virtually all measurement types are available, also valve positioners. A number of DCSs and PLCs provide HART-compatible inputs, so they can check device status, and read multiple measurements from a single field instrument.

You can find out more about available products from the Virtual HART Book. For more details of the technology, try the HCF web site at www.hartcomm.org, or ask your favourite instrument supplier. There is also a technical overview booklet, available from me or from the HCF.

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