Posts
HIGHWAY ADDRESSABLE REMOTE TRANSDUCER
HART® is an open protocol that was originally developed in the late 1980's by Fisher Rosemount to communicate with their range of Smart field devices. Over the years it has become a de facto standard for communicating with SMART devices in the Process industry. Over 100 manufacturers utilise the HART® protocol in over 560 different products, from simple temperature transmitters to gas detectors.
The HART® protocol is a powerful communication technology used to realise the full potential of digital field devices whilst preserving the traditional 4-20mA signal. The HART® protocol extends the system capabilities for two way digital communication with smart instruments.
HART® offers the best solution for smart field device communications and has the widest base of support of any field device protocol worldwide. More instruments are available with the HART® protocol than any other digital communications technology. Almost any process application can be addressed by one of the products offered by HART® instrument suppliers. Unlike other digital communication methods the HART protocol gives a unique communication solution that it is backward compatible with currently installed instrumentation. This ensures that investments in existing cabling and current control strategies remain secure into the future.
The HART® digital signal is superimposed onto the standard 4-20mA signal. It uses Bell 202 standard Frequency Shift Keying (FSK) signal to communicate at 1200 baud. The digital signal is made up of two frequencies, 1200Hz and 2200Hz, representing bits 1 and 0 respectively. Sine waves of these two frequencies are superimposed onto the analogue signal cables to give simultaneous analogue and digital communications. As the average value of the FSK signal is always zero there is no effect on the 4-20mA analogue signal. A minimum loop impedance of 230ohms is required for communication.
HART® is a master-slave protocol - this means that a field device only replies when it is spoken to. Up to two masters can connect to each HART® loop. The primary master is usually the DCS (Distributed Control System), the PLC (Programmable Logic Controller) or a PC. The secondary master can be a hand held configurator or another PC running an instrument maintenance software package. Slave devices include transmitters, actuators and controllers that respond to commands from the primary or secondary master.
The digital communication signal has a response time of approx. 2-3 updates per second without interrupting the analogue signal.
HART® Commands
The HART® protocol provides uniform and consistent communication for all field devices via the HART® command set. This includes three types of Command:
Universal
All devices using the HART® protocol must recognise and support these commands. They provide access to information useful in normal operations.
Common Practice
These provide functions implemented by many but not all HART® communication devices.
Device specific
These represent functions that are unique to each field device. They access set up and calibration information as well as information on the construction of the device.
The HART® Communication Protocol is an open standard owned by more than 100 member companies in the HART® Communication Foundation HCF. The HCF is an independent, non-profit organisation, which provides worldwide support for application of the technology and
ensures that the technology is openly available for the benefit of the industry.
The HART® protocol is a powerful communication technology used to realise the full potential of digital field devices whilst preserving the traditional 4-20mA signal. The HART® protocol extends the system capabilities for two way digital communication with smart instruments.
HART® offers the best solution for smart field device communications and has the widest base of support of any field device protocol worldwide. More instruments are available with the HART® protocol than any other digital communications technology. Almost any process application can be addressed by one of the products offered by HART® instrument suppliers. Unlike other digital communication methods the HART protocol gives a unique communication solution that it is backward compatible with currently installed instrumentation. This ensures that investments in existing cabling and current control strategies remain secure into the future.
The HART® digital signal is superimposed onto the standard 4-20mA signal. It uses Bell 202 standard Frequency Shift Keying (FSK) signal to communicate at 1200 baud. The digital signal is made up of two frequencies, 1200Hz and 2200Hz, representing bits 1 and 0 respectively. Sine waves of these two frequencies are superimposed onto the analogue signal cables to give simultaneous analogue and digital communications. As the average value of the FSK signal is always zero there is no effect on the 4-20mA analogue signal. A minimum loop impedance of 230ohms is required for communication.
HART® is a master-slave protocol - this means that a field device only replies when it is spoken to. Up to two masters can connect to each HART® loop. The primary master is usually the DCS (Distributed Control System), the PLC (Programmable Logic Controller) or a PC. The secondary master can be a hand held configurator or another PC running an instrument maintenance software package. Slave devices include transmitters, actuators and controllers that respond to commands from the primary or secondary master.
The digital communication signal has a response time of approx. 2-3 updates per second without interrupting the analogue signal.
HART® Commands
The HART® protocol provides uniform and consistent communication for all field devices via the HART® command set. This includes three types of Command:
Universal
All devices using the HART® protocol must recognise and support these commands. They provide access to information useful in normal operations.
Common Practice
These provide functions implemented by many but not all HART® communication devices.
Device specific
These represent functions that are unique to each field device. They access set up and calibration information as well as information on the construction of the device.
The HART® Communication Protocol is an open standard owned by more than 100 member companies in the HART® Communication Foundation HCF. The HCF is an independent, non-profit organisation, which provides worldwide support for application of the technology and
ensures that the technology is openly available for the benefit of the industry.