Comparison between Conventional (4-20ma) connection, Foundation-Fieldbus, and HART?

What is a foundation field bus?

A field bus can be described as the connection between industrial devices. The major purpose of a field bus is to replace the point-to-point link between the field devices and control system, instead of that Fieldbus would provide a digital link. In a field bus, the transfer of information would take place in small size packets in a serial way. There are many advantages for serial transmission over parallel and point-to-point transmission like there would be only a few transmission lines. So basically, Fieldbus is an all-digital, serial two-way, multi-drop communication system.

Foundation fieldbus

What are the major components of a foundation field bus?

  • Cables
  • Terminal blocks
  • Connectors
  • Couplers
  • Terminators
  • Power supplies
  • IS barrier
  • Measurement and control device
  • Host devices
  • Bridges

What is FF H1?

This is a field bus that is used to do the communication, between the field device and the control system. This is a bidirectional communication protocol and it would provide the communication and power over the standard twisted power wiring.

What is foundation HSE?

The high-speed ethernet would connect devices in an industrial network and mostly it would connect the input-output subsystem.

How does foundation Fieldbus work?

The foundation Fieldbus uses the OSI layers to do its operation. The three used OSI layers are application, data link, physical, and in addition it would use a fourth layer which is the user layer.

Application layer

This layer is broken into Fieldbus message specification and Fieldbus access sublayer

Physical layer

This would provide the high-speed data communication network, both the communication signal and also the current for each device would be transmitted by this section.

Datalink layer

This layer would be useful to transmit the data between devices when a device receives data then it would publish the data on the bus so that all the devices can use it.

User layer

It would allow interoperability between the devices.

What is the major difference between conventional connection and foundation Fieldbus connection?

Conventional VS Foundation fieldbus connection

Conventional system vs Foundation Fieldbus system

In this system, the analog and digital instruments would be wired individually to a centralized controller. The control functions would be processed by the centralized controller and the passive device would accept the command and provide the feedback. In a conventional system, two wires would be used to connect a field instrument. The wires would carry the electrical power to the device. The device would signal its measured value to the DCS controller by varying the current it uses between the 4-20mA. The controller would collect the data from the devices and it would do the required calculations and then it would send the command.

In the case of the Fieldbus, it would also use two wires to carry the power to the devices. But in this case, a number of devices would share the same Fieldbus wire. So due to this, the devices can send data to each other without the help of the DCS controller. The Fieldbus network is a digital, multidrop, bidirectional serial bus communication, so it would connect the isolated field devices such as controllers, transducers, actuators, and sensors. So due to this each device are capable to do certain functions such as diagnostic, control, and maintenance. So with the help of the Fieldbus connection, the operators would be able to communicate with the other field devices too. We could achieve a distributed control network with the help of the Fieldbus system and the data transmission would take place digitally. Each field device would be capable to carry out its own control and thus it would inform if there is any problem with the device.

Conventional VS Fieldbus
 Conventional wiring Foundation Fieldbus
TerminationsMore terminations would be requiredFewer terminations
TopologyOne to oneMultidrop  
Transmission method4-20mA DC analog signalDigital signal
Transmission directionOne wayBidirectional
Signal typeSingle signalMultiplex signal
Multivariable transmissionThere is no multivariable transmissionSupports multivariable transmission
AccuracyChances of error are high due to the A/D and D/A conversionHigh accuracy due to digital transmission
InteroperabilityData exchange between the devices of different manufacturers is not possibleDigital data would be exchanged between the devices of different manufacturers
Conventional VS Fieldbus

What are the advantages of Foundation Fieldbus over the conventional connection?

  • Field wiring costs can be reduced
  • The same bus can be used for the analog and discrete device
  • Excellent support for analog I/O
  • Incorporates discrete devices into the same segments
  • Provides control in the field capabilities
  • Provides redundancy option for power and communication devices
  • Available for intrinsically safe installation
  • Various field devices would work with a variety of host systems, regardless of the manufacturer
  • It would provide greater controllability and process information
  • Less space would be used when compared to the conventional type
  • Devices can communicate with each other
  • In the Fieldbus network devices can do the control, diagnostic, and maintenance functions
  • There is no need for online diagnosis
  • There is no need to record the device information manually
  • We can replace a number of devices that uses the 4-20ma analog standard with one Fieldbus device
  • It would eliminate the inaccuracies of the A/D and D/A conversions of the analog representative signals, such as the 4-20mA
  • No need for the calibration of the representative signals to improve the accuracy
  • There is no need to set up the lower and upper range limit at the device level
  • Improved systems performance
  • Reduced maintenance cost

What is HART?

Highway addressable remote transducer is a communication protocol, which is created for industrial process measurement and control. Mostly it is called a hybrid protocol because it combines the digital and analog protocol. The digital signal won’t affect the analog reading.


How does Hart work?

The Hart communication protocol would be superimposed to the 4-20ma signal and thus two-way communications are possible. Mostly a single pair cable would be used for each device and the 4-20ma signal is mostly used for the process variable. We can connect many devices in the same current loop, providing digital data from each device. HART can provide the master/slave communication but there could only be two masters.

What are the major features of HART?

  • It is compatible with the standard 4-20ma wiring
  • Digital data can be transmitted simultaneously
  • Less risk due to the highly accurate and robust protocol
  • Ease of implementation for the maximum upfront, cost-effectiveness
  • Broad product selection with compatible devices and software applications from most process automation providers

What is the major difference between HART and Foundation Fieldbus?

Multivariable transmission In digital mode onlyMultivariable transmission is possible
Communication performance 100 bits/sec31,250 bits/sec good communication speed
Transmitter diagnosticsNo knowledge of other devicesDevice + other devices
Diagnostic feature It doesn’t have the processing powerIt can monitor the process, machinery monitoring
Two-way communication There is no data transmission between devicesFieldbus devices can communicate with each other
Advanced device featuresNoYes
Commissioning speedVery lessGood commissioning speed
PowerPower limitations 35milliwatt, 4ma available for Hart signal  FF has a minimum power requirement of 8ma with no specification limit
AccuracyAverageGood accuracy
Process handlingNot capable to handle the complex processCan handle the complex process


post-graduate in Electronics & communication.

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