HelpHi people...

Can somebody tell me what is the deference betwen this two signals?
Regards

Armando ABRAHAM

 

With a 4-20mA signal you can transmit one analog signal That is not any difference - the main control signal are transmitted as 4-20mA in both cases. On top of that you can send other signals, modulated in approximately the same way as a modem sends information over a phone line is it possible to send commands and other digitalized information over the same wire as the 4-20mA signal uses. This is what you do with HART.


/Johan Bengtsson

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They are basically the same signal but the 4~20mA has HART protocol signal superimposed to the analog 4~20mA.
This way, the signal can not carry whatever variable you are measuring or transmitting with 4~20mA but also others , like intrument
information, temperature , pressure, status, tag, depending on what the Hart Instrument can make available for the user...

Good Luck!
Gil

 

On the Hart signals, there are the *superimposed* digital signals which give/take information/program for the device. (Like Asi, in which there are 24 V DC + digital signals).

 

Dear Friend

In my humble opion the 4~20 ma is continous single whereas hart or brain protocal has limted signal i.e only use to be in a way of giving set point.

 

The HART protocol encodes digital information on top of the current loop. If your device doesnt' understand HART, then it just looks like plain old 4-20.

 

FYI:

The HART standard imposes a digital signal at 1200 bps on a standard 4-20mA system using frequency shift keying (FSK - Bell 202 Standard) - with two sine waves (1.2 kHz for "1" and 2.2 kHz for "0") superimposed on the DC analogue (4-20mA) signal. The average value of an FSK signal is always zero, so the 4-20 mA signal is not affected. You can get two digital updates per second - I am not sure on the exact performance statistics.

If you want to use HART with multi-drop, then the 4-20 mA component is ignored and you just communicate "digitally" with HART (the trunk is only two wires, so with all the devices chattering away the 4-20 mA signal is useless). Alternatively, you can use single drop and get both HART and 4-20 mA data.

In the "digital" mode, you normally read four (or so) data points and can write to data points too. The bus is master/slave with one master such as a DCS, PLC or PC. Sometimes, the master is just used for configuration via HART and then the devices are single drop and just 4-20 mA with the HART being ignored.

Try "http://www.hartcomm.org/":http://www.hartcomm.org/ if you want to know more.

R

 

The HART protocol uses a small symetrical high frequency ON/OFF current superimposed on the normal 4-20mA signal. Most devices have no
problems with this superimposed information, however if you are using analog high speed controllers (e.g. to control speed of big
compressors) this could cause serious problems, especially if the derivative function of the PID controller is used because this will cause
the controller to react immediately on every jump (as in digital ON/OFF signal) in the measured value.

 

Ranjan is correct. HART uses Bell 2002 signall that does not affect the 4-20ma Signal. The other part of this is that the signalling occurs at a high frequency that is normally filtered out by traditional process control system. In effect, HART supports two communications channels separated in frequency over a single pair of wires: the 4-20ma (0-100Hz) and modualted (ie. analog) half-duplex communication (1000Hz - 2500Hz).

HART actually supports 2 masters (not one) using an implied token passing algorithm. This allows access to a wide range of process data. In addition, diagnostic and configuration properties are also available. Since the HART Application Layer is well-defined there are dozens of standardized data items that must be supported by any field device claiming HART Compatibility.

If you need more information I will be glad to assist.

best regards

Wally Pratt ([email protected])
Chief Engineer
HART Communication Foundation
http://www.hartcomm.org

 

This is the best explained correct answer of the question. Keep it up for providing explanatory info!!

> The HART standard imposes a digital signal at 1200 bps on a standard 4-20mA system using frequency shift keying (FSK
> - Bell 202 Standard) - with two sine waves (1.2 kHz for "1" and 2.2 kHz for "0") superimposed on the DC analogue
> (4-20mA) signal. The average value of an FSK signal is always zero, so the 4-20 mA signal is not affected. You can
> get two digital updates per second - I am not sure on the exact performance statistics.

> If you want to use HART with multi-drop, then the 4-20 mA component is ignored and you just communicate
> "digitally" with HART (the trunk is only two wires, so with all the devices chattering away the 4-20 mA signal is
> useless). Alternatively, you can use single drop and get both HART and 4-20 mA data.

> In the "digital" mode, you normally read four (or so) data points and can write to data points too. The bus is
> master/slave with one master such as a DCS, PLC or PC. Sometimes, the master is just used for configuration via HART
> and then the devices are single drop and just 4-20 mA with the HART being ignored.

> Try http://www.hartcomm.org/ if you want to know more.

 

If you want to learn more about HART and a current example of PLC implementation, we recently published a Control.com article explaining the fundamentals. It doesn't cover every scenario and use case, but if you want to gain a little better understanding, I'd encourage you to check it out.

Getting Starting with the HART Protocol for Sensing Instruments