HIGHWAY ADDRESSABLE REMOTE TRANSDUCER

HIGHWAY ADDRESSABLE REMOTE TRANSDUCER

The Highway Addressable Remote Transducer protocol (HART) is an early version of the fieldbus, a digital, industrial automation protocol. The largest advantage is the fact that it is based on a conventional 4 to 20 mA standard which serves the transmission of analog signals. The existing wiring of the old system can still be used, and parallel operation of both is possible.
Because of the basis of the 4 to 20 mA standard, the Highway Addressable Remote Transducer protocol is one of the most widely distributed industry protocols. Data are transmitted according to the Bell 202 standard using frequency shift keying technology. The Highway Addressable Remote Transducer protocol offers very good user friendliness for users using the conventional 4 to 20 mA protocol who want to use a “smarter” system with a wider scope of services.

The protocol was developed by Rosemount Inc. in the middle of the 1980s as a proprietary communication protocol which was to be used in the company’s own intelligent fieldbus devices. This soon developed into the Highway Addressable Remote Transducer protocol. In 1986, it was changed to an open protocol. Since then, the abilities of the protocol have been expanded by successive changes to the specifications.

Different modes

There are two modes of operation for Highway Addressable Remote Transducer protocol devices: an analog/digital mode and a multi-drop mode.

In point-to-point operation (analog/digital), the digital signals are applied to the 4 to 20 mA system. As such, the 4 to 20 mA current and the digital signal, are valid output values of the device. The polling address of the device is set to “0”. Only one device can be used per signal cable pair. With this, a signal defined by the user can be sent as a 4 to 20 mA signal. Other signals are applied digitally to the 4/20 mA signal with the Highway Addressable Remote Transducer protocol. For example, a pressure indication can be sent as a 4 to 20 mA signal, but a range of pressures or temperatures can also be sent over the same lines in a digital form.

Only the digital signals are used in multi-drop operation (digital). With this, the analog loop current is fixed to 4 mA. In multi-drop mode, more than one device can be operated per signal cable pair. Revisions 3 to 5 of the Highway Addressable Remote Transducer protocol permit polling addresses in the range from 1 to 15. HART 6 (and above) permits an address up to 63. Each device must have a unique own address.

Packet structure

The Highway Addressable Remote Transducer protocol is composed of the following individual components:

Name Length (Bytes) Purpose
Preamble 5-20 Synchronization and definition of the transmitter
Start byte 1 Specifies the master number
Address 1-5 Defines slave, master and starts burst mode
Command 1 Numerical value for the command to be executed
No. of  data bytes 1 Specifies the size of the data field
Status Master (0)
Slave (2)
Execution and safety repetitions
Data 0-253 Data to be transmitted
Checksum 1 Checksum of all bytes from the start byte to the data bytes

Preamble

Currently, all new devices based on the Highway Addressable Remote Transducer protocol are implemented with a 5 byte preamble, as any larger value reduces the transmission speed. However, here the master devices are responsible for backward compatibility. The master communication to a new device starts with the max, preamble of 20 bytes and it is then reduced until the preamble size is suitable for the current device.

Start byte

This byte contains the master number and indicates the start of the communication packet.

Address

Specifies the destination address of the device existing in the Highway Addressable Remote Transducer protocol. The original addressing scheme used only 4 bits, which limited the number of devices to 16, including the master. The newer system users 38 bits in order to specify the device address. This address is requested by the device either with command 0 or command 11.

Command

This is a 1 byte numerical value in order to indicate the command to be executed. Command 0 and command 11 are used as described in order to contact a device.

Number of data bytes

Indicates the number of data bytes following the status information.

Status

The status field is missing for the master and has two bytes for the slave. This field is used by the slave to inform the master whether the task has been completed or not and about the current device status.

Data

The data are sent in this field. The data in this field are independent of the command to be executed.

Check sum

The check sum is formed of an XOR command. Starting with the start byte and ending with the last byte of the data field.