Highway Addressable Remote Transducer Protocol
Highway Addressable Remote Transducer (HART ) is a standardized, widely used communication system to build industrial fieldbuses. It enables digital communications for multiple participants (field devices ) via a common data bus. HART is based on especially on the equally widespread 4/20-mA-Standard ( for transmission of analog sensor signals). Existing lines of the old system can be used directly and both systems are operated in parallel. The HART was developed in the 1980s by the company for its Rosemount field devices. In 1989, the HART standard by the HART Communication Foundation ( HCF) has been launched. Since 2007 is part of the HART fieldbus standard IEC 61158. Seat of the HART Communication Foundation in Europe Basel ( Switzerland ).
Benefits of HART
- Bus or network structure: About a 4/20-mA-Kabel several HART field devices can communicate bidirectionally and acyclic.
Data transfer is performed after the Bell 202 standard via Frequency Shift Keying ( FSK). The low-frequency analog signal is a high frequency vibration ( / - 0.5 mA) superimposed. A digital " 1" at the frequency of 1.2 kHz (1200 Hz) and a "0" is displayed with the frequency 2.2 kHz (2200 Hz).
HART specifies several protocol levels in the OSI model and allows the transfer of process and diagnostics data and control signals between field devices and higher-level control system. Standardized parameter sets can be used for cross-vendor operation of all HART devices. Most major manufacturers of sensors ( field devices ) offer devices with - some optional - HART communication. Typical applications include transducers for the measurement of mechanical and electrical variables.
The communication between the devices can according to the master / slave principle, take place, that is the slave - the field device - only sends information when the master - a request has been made - the management and control center. A further possibility of communication is the burst mode. In this case, the slave sends after initiation of communication unsolicited messages to the master.
In September 2007, the new WirelessHART standard has been set and published by the HCF (HART Communication Foundation ). The radio transmission is based on the wireless communication standard IEEE 802.15.4 ( ISM band ) and TDMA used as the transfer method. The communication provides an encryption based on the Advanced Encryption Standard (AES 128 ) to allow the transfer of data and the parameterization of field devices can not be changed without authorization.
WirelessHART is a part of the new HART 7 specification and was standardized as IEC 62591:2010. The completion of the HART test specification is expected by mid 2008. Thus, the first WirelessHART -compliant instruments are likely to still be available in 2008.
The network organizes itself ( Meshed Network). If a field device can be added to an existing network, only one type password must be entered in the topology map will be self-organizing. Transmission distances in outdoor areas up to 3 km from participant to participant. In spatially successive participants, therefore, significantly larger distances can be covered, since the intermediate participants were, act as a repeater.
Built-in redundancy: If a participant fails as transmission, the transmission is established automatically via another subscriber. Are transmitting the same information as in the wired HART. The measured value transmission cycles depend on the system configuration and are typically between 15 seconds and several hours.
Typical areas of application of WirelessHART:
- Large distances to be bridged, such as the monitoring tank
- Measuring devices that are mounted on rotating or moving parts
- With obstacles in the signal path as train tracks or rivers
- Parameterization and operation of HART components to controllers that are not yet capable of HART
- Subsequent or temporary installation of field devices
- Open and standardized by the support of all in the HCF (HART Communication Foundation ) registered HART devices.