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Anonymous Poster

Calibration Loop Resistance (250 Ohms)

11/27/2010 1:36 AM

Hi all! Can any body give clear explanation why of using resistor (250 Ohms) in series with the supply voltage when performing Bench Calibration for HART communicator to work but when the field instrument is mounted at the site and power source is coming from DCS, no resistor at all but the HART communicator can be able to communicate with the smart devices?
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user-deleted-1105
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#1

Re: Calibration Loop Resistance (250 Ohms)

11/27/2010 6:23 AM

Just maybe if you told us what DCS stands for we could offer an explanation.
I've yet to meet a power supply which doesn't have some internal resistance (even if it is very very small)
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Anonymous Poster
#2
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Re: Calibration Loop Resistance (250 Ohms)

11/27/2010 8:56 AM

When performing Bench Calibration of such flow, Pressure or Temp transmitter we usually have separate power supply. We employed 250 ohms resistor so as the HART communicator to be able to communicate the x'mtr. My query is when the instrument is installed at the site and its power source is coming from DCS then the HART communicator can communicate the instrument without the 250 Ohms resistor in series the power supply line?
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user-deleted-1105
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#3
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Re: Calibration Loop Resistance (250 Ohms)

11/27/2010 9:58 AM
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GW
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#7
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Re: Calibration Loop Resistance (250 Ohms)

11/28/2010 11:56 AM

DCS = Distributed Control System

PLC = Programmable Logic Controler

The terms were coined about 20 years ago. Reliance started calling their Automax system DCS or Distributed Control System when it achieved the ability to have multiprocessor, multitasking, networked programable logic controlers. Honeywell and others also coined the DCS term. In general DCS is used for the BIG control systems in industrial plants that run most of the logic and control loops and operator interfaces (MMI = Man Machine Interface, or the politically correct HMI = Human Machine Interface (a terminal or touchscreen etc.)).

If we create enough techno babble people think we have something great for the equivalent of a Bank ATM (Automatic Teller Machine).
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Iris
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#4

Re: Calibration Loop Resistance (250 Ohms)

11/27/2010 10:58 AM

For 4-20mA DC loops, the means of signal transmission is current, the means of 'reading' the signal is always voltage, whether the signal is analog or HART.

The HART signal is a 1200 baud FSK signal superimposed on the loop's DC 4-20mA signal.

Some minimum loop impedance is needed so that the HART signal is useable and can be 'seen' by a HART master.

The analog inputs on the DCS have a precision shunt resistor installed at each analog input. It is commonly 250 ohms.

Typically the IR drop needed for HART is developed by the loop receiver's (indicator, recorder, controller, PLC or DCS analog input) loop resistance or even the wire resistance in the loop.

But on the bench, it is not uncommon to power up a transmitter with just a DC power supply and then need to connect a HART master (handheld or HART modem) for setup. In the absence of a receiver device (and its analog input resistance) wired into the loop on the bench, there is insufficient loop resistance for a HART master device to 'see' the FSK HART data. The internal resistance of a DC power supply is insufficient and its filter caps act as a low pass filter. Inserting a loop resistor develops the HART signal so the HART master can see it.

Somewhere on the HART Foundation web site the minimum loop resistance needed is cited and I seem to recall it's actually about 230 ohms. 250 ohms is most commonly recommended because 250 ohm resistors work for that purpose and 250 ohm resistors are typically available in the instrument shops where these types of bench tests are done because the majority of manufacturers (not all, but most) use 250 ohm resistors for analog input resistors.

The HART handhelds usually have a pair of banana jacks on the handheld for the purpose of plugging in a banana plug adapter with a 249 or 250 ohm resistor across its inputs to provide the loop resistance needed.

So the answers are:
a) the resistance needed by a HART master to read the HART FSK signal is already wired into the loop at the DCS analog input, but is absent on the bench when just a power supply is in the loop.

b) Yes, the HART master can communicate in a DCS loop without an additional 250 ohm resistor (the one needed for bench cal) because most DCS's have 250 ohms in each analog loop by design. The loop resistance needed is already there, supplied by the DCS's AI module.
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Anonymous Poster
#5
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Re: Calibration Loop Resistance (250 Ohms)

11/27/2010 6:24 PM

Thank you so much Iris for the enlightenment. You discussed the topic very detailed and you addressed my query. By that I gave you good rate.
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Shyam
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#6
In reply to #4

Re: Calibration Loop Resistance (250 Ohms)

11/27/2010 11:21 PM

250 ohms drop resistance is most suited one. 250 Ohms 4mA makes 1V drop across resistor, 250 ohms 20mA makes 5V drop across resistor. It is a very common signal range for many ADCs and LOGIC ICs.

HART signal is AC over DC, and requires amplifier in transmitter and receiver. Zero impedance will kill the signal. Hence, good reason to have load resistance from 100 ohms to 250 ohms range.

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Carl_E
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#8

Re: Calibration Loop Resistance (250 Ohms)

03/03/2011 1:29 PM

An excellent video demonstrating the need for a minimum loop resistance for HART to function is on youtube here:

http://www.youtube.com/watch?v=XJ4A-0kGGkI&feature=related

The HART Foundation specifies a minimum of 230 ohms in a loop in its document

HART Communication Application Guide
HCF_LIT-039 Rev. 1.0 Preliminary
Date of Publication: March 25, 2010

On page 9, the document states

"A minimum loop impedance of 230 Ω is required for communication."

Document is publicly available here:
http://www.hartcomm.org/hcf/documents/appguide.pdf
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