Is there any difference in signal transmitted in HART and Fieldbus or not?

Please help me. I'm totally Confused. I found that both are digital signals superimposed on an analog signal then what is the difference? Please help.

 

HART was defined to use a modulation technique called FSK or Frequency Shift Keying in order to superimpose data on top of a DC 4-20ma signal. The HART FSK is modulated at 1200 Hz, and both signals can be carried at the same time on the same wire. When the signals are received at the HART/Analog Input card in the DCS, a high-pass filter allows the 1200 Hz HART signal to be detected separately from the 4-20ma DC signal. The 4-20ma DC signal passes through a 250 ohm precision resistor to produce a 1-5 volt signal that is then converted by an Analog to Digital converter in the input card, producing the process variable signal. HART is considered to be one of the members of the family of digital protocols call "fieldbus" and is type 20 of the IEC standard for fieldbuses along with Foundation Fieldbus, PROFIBUS, ControlNet, and others. Only the digital portion of HART is covered by this standard, which also specifies a 9600 Hz version of HART, also FSK modulated, but no merged with a 4-20ma DC carrier. The HART data carried by either form of HART is identical, and represents that data in digital form. One datum value that can be carried on HART is the Process Variable that is a digital representation of the same data carried by the current between 4 and 20ma DC. The value of HART, however is to carry the other data produced by smart field instruments such as the temperature of the transmitter, both high and low pressures of a differential pressure transmitter, and other secondary variables.

Foundation Fieldbus and PROFIBUS-PA are two very widely used fieldbuses, neither of which is superimposed on a DC signal. Both use Manchester Encoding to form essentially an AC baseband signal with a trapezoidal wave form to reject noise. The protocol for each is quite different, but they are designed to carry the same Process Variable value in digital format, along with other data that is available in the smart field transmitter.

Dick Caro
Chair, ISA S50

 

Thank you Caro for your information. Now I'm little confident about the concept of the HART, but still I 'm confused about FieldBus (Foundation). Would you please tell how the signal is transmitted in Fieldbus Protocol.

I Thank you once again for your valuable information.
Vaseem Mirza

 

Foundation Fieldbus is designed to connect "smart" field instruments with each other, and when required to a host DCS, PLC, or HMI. The basic instrument communicates in Foundation Fieldbus H1 protocol as previously described. The instrument is smart enough to sent "alert" messages when something is out of limits, but most communications uses the publish/subscribe format. Any device such as another instrument on the same Foundation Fieldbus H1 segment, a DCS, PLC, or HMI may subscribe to the value produced by the instrument that may be a PV (measurement), alarm status, secondary variable, etc. at a specific synchronous interval. When that interval occurs, the desired value is then published on the network for all subscribers to listen.

Foundation Fieldbus instruments are capable of processing the raw measurement signal by linearizing, filtering, and converting to engineering units values. Function block may be configured to execute in these field instruments to subscribe to external values in the same segment, and perform many standard computations including PID closed loop control. Control outputs are sent via the Foundation Fieldbus H1 segment to the final control element, an AO function block, usually contained in a control valve positioner.

The communications protocol for Foundation Fieldbus H1 is specified in IEC 61158-2 Type 1. This is a conventional data communication protocol designed for short messages up to 255 bytes, and secured by a 24-bit cyclic redundancy check. All messages are acknowledged. Critical loop timing is controlled by a Link Active Scheduler (LAS,) a function that must exist for each control loop, and may be resident in any Foundation Fieldbus H1 device. The LAS performs link arbitration and assures that measurements are delivered just-in-time to the PID control function block.

There is also a higher level network called Foundation Fieldbus HSE based on UDP/IP messaging carried over an Ethernet base signal, but only a few suppliers offer HSE protocol networks.

 

http://iamechatronics.com/notes/general-engineering/294-h1-foundation-fieldbus-physical-layer

 

Thank you once gain Caro sir, and thanks to David for the information. Now my almost- almost all doubts are clarified. Thank you once again.

Regards
Vaseem Mirza

 

Get the basics on FOUNDATION fieldbus from this brochure:
http://www.fieldbus.org/images/stories/technology/aboutthetechology/overview/fieldbus_brochure.pdf

In depth from this book:
www.isa.org/fieldbuses

Cheers,
Jonas