HART Communication Protocol (Highway Addressable Remote Transducer) is a hybrid analog digital industry automation protocol. The most important advantage is that it can communicate through a long 4-20 mA analogue loop instrumentation loop, sharing a pair of cables used by analog host systems only. HART is widely used in process and instrumentation systems ranging from small automation applications to highly sophisticated industrial applications.
According to Sanjay [1] due to the large installed base of 4-20 mA systems worldwide, the HART Protocol is one of the most popular industry protocols today. The HART protocol has created a good transition protocol for users who want to use the old 4-20 mA signal, but want to implement the "smart" protocol.
The protocol was developed by Rosemount Inc., built from Bell 202's early communication standards in the mid-1980s as an exclusive digital communication protocol for their smart field instruments. It soon evolved into HART and in 1986 made the protocol open. Since then, protocol capabilities have been enhanced by successive revisions to specifications
The protocol was developed by Rosemount Inc., built from Bell 202's early communication standards in the mid-1980s as an exclusive digital communication protocol for their smart field instruments. It soon evolved into HART and in 1986 made the protocol open. Since then, protocol capabilities have been enhanced by successive revisions to specifications.
Video Highway Addressable Remote Transducer Protocol
Mode
There are two main operational modes of the HART instrument: point to point (analog/digital) mode, and multi-drop mode.
Point to point
In point-to-point mode a digital signal is spread over a 4-20 mA loop current. Both 4-20 mA and digital signals are valid signaling protocols between controllers and measuring devices or final control elements.
The instrument polling address is set to "0". Only one instrument can be mounted on each pair of instrument signal cables. One signal, generally determined by the user, is set to a 4-20 mA signal. Other signals are digitally transmitted over a 4-20 mA signal. For example, the pressure can be sent as 4-20 mA, representing various pressures, and the temperature can be sent digitally over the same cable. In point-to-point mode, the digital part of the HART protocol can be viewed as a kind of digital loop interface at this time.
Multi-drop
In the Multi-drop mode, the analog loop current is set at 4 mA and it is possible to have more than one instrument on the signal loop.
Revision of HART 3 through 5 addresses allowed polls from instruments in the range 1-15. HART revision 6 and then allowed addresses up to 63. Each instrument must have a unique address. It can be recovered at any time
Maps Highway Addressable Remote Transducer Protocol
Package structure
The HART request package has the following structure:
Opening
Currently all newer devices implement five-byte openings, because anything larger reduces communication speed. However, the master is responsible for backing support. The master communication to the new device starts with the maximum opening length (20 bytes) and then reduced after the preamble size for the currently specified device.
Start delimiter
This byte contains the Master number and specifies the communication packet started.
Address
Specifies the destination address as applied in one of the HART schemes. The original addressing scheme uses only four bits to determine the address of the device, which limits the number of devices to 16 including the master.
The newer scheme uses 38 bits to determine the device address. This address is requested from the device using Command 0, or Command 11.
Command
This is a one byte numeric value representing which command to execute. Command 0 and Command 11 are used to request the device number.
Number of bytes of data
Specifies the number of bytes of communication data to follow.
Status
Status field does not exist for master and two bytes for slave. This field is used by a slave to tell his master whether it has completed the task and what his current health status is.
Data
The data contained in this field depends on the command to be executed.
Checksum
The checksum consists of XOR of all bytes starting from the initial byte and ending with the last byte of the data field, including the byte.
Manufacturer code
Every manufacturer participating in the HART convention is given an identification number. This number is communicated as part of the basic device identification command used when first connected to the device.
References
External links
- FieldComm Group
- Open Source.NET Project
- Hart Simulator Wireless
Source of the article : Wikipedia
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