Fieldbus and HART are two main communication technologies used in process intrumentation. HART and FOUNDATION fieldbus enable many of the same benefits such as remote configuration and diagnostics, each technology has some unique characteristics. Consider the comparison between HART and Fieldbus in different aspects:
Technologies:
HART:
HART Field Communications Protocol is superimposed on the 4-20mA signal and provides two way communications with smart field instruments without compromising the integrity of the measured data. HART communicates at 1200 bps and provides a host with two or more digital updates per second from a field device.
Fieldbus:
Foundation Fieldbus is a true Multidrop System and has numerous advantages including:
- Wiring savings
- Hardware savings-fewer devices (instruments barriers and I/O)
- Documentation savings-Simpler layout and drawings
- Reduced Engineering costs
- Footprint savings
- Multi-variable field devices
- Interoperability and freedom of choice
- Reduced Commissioning and startup costs
- Reduced downtime
- Integrity improved
- DCS future capacity savings
- Control in the field
- Device Plug and Play
- On line configuration
- Time Stamping and Synchronisation
- Obsolescence Robustness
- Status Information
- Tag Search Capability
- Interface to High Speed Ethernet
- Trend storage in the field device
- Predictive Maintenance
- DCS future capacity savings
- Automated data collection for asset management
Transmitter Diagnostics:
HART technology has the ability to provide diagnostics for the device only, this includes any predictive options. Thus HART has no knowledge of other devices. FF however communicates with other devices, opening a whole new potential in regards to advanced diagnostics such as plant health systems. For both HART and FF you will hear all about the “wonderful” transmitter diagnostics which can tell you every little thing about the status of the transmitter.
Multidrop Capability:
HART:
HART provides a single pair of wire to each field device and powers the device over the 4 to 20 ma signal current.HART has only a limited capability in digital communication, which can support only 15 devices.However it is practically only about 3 because of the slow SERIES loop.
Fieldbus:
FF is a true multidrop technology with a theoretical limit 32 devices which is practically 12-16. Furthermore FISCO, FNICO and High Power Trunk concepts have radically changed the number of instruments that can be installed in Ex environments.
Downloads:
HART:
HART technology does not allow downloads.HART devices have to use ASIC chips for processors to conserve power. The program for these processors is contained in a mask that is used to make the chips as a result upgrading the program in a HART device requires changing out the entire device.
Fieldbus:
Foundation Fieldbus allows field devices to have software upgrades downloaded over the bus. FF devices can afford to have general-purpose processors and a downloadable stored program.
Power Limitation:
HART:
35 milliwatts – 4mA available for the HART signal- Cannot “mirror” Fieldbus however may provide a 80% solution
Fieldbus:
FF minimum power requirement of 8mA –No spec limit- Ultimate FF segment power budget – FF devices order of magnitude more power a Even for IS