In instrumentation, what characteristics of cable to be considered to achieve reliable transmission of hart signal from instrument to controller?why?

Fiber optic.

MICC cables & Optical Caable, beacause the important concept applied in many instrumentation signal systems is that of "live zero," a standard way of scaling a signal so that an indication of0 percent can be discriminated from the status of a "dead" system. Take the pneumatic signal system as an example: if the signal pressure range for transmitter and indicator was designed to be0 to12 PSI, with0 PSI representing0 percent of process measurement and12 PSI representing100 percent, a received signal of0 percent could be a legitimate reading of0 percent measurement or it could mean that the system was malfunctioning (air compressor stopped, tubing broken, transmitter malfunctioning, etc.). With the0 percent point represented by0 PSI, there would be no easy way to distinguish one from the other.

Dear Sohail:

You are talking about the calibration of an instrument device/mechanism/transmitter on standard analog signal of0  to20mA where one will assigne corresponding process parameters (e.g. pressure, temperature) for each assigne discreet value within0 to20mA.

The question was more on the reliability of the cable when transmitting analog, hart, modbus, profibus or any field bus type signal. If you go back on  basic of transmission line theory; one parameter to consider is the resistance of copper  cable, obviously because as distance increases, the resistance increases and so current decreases (ohm's law) and thus, the size of cable must be adjusted to meet operational current of the instrument.

Since, multi-pairs trunk cable constitute as RLC circuit and that when we are transmitting a signal near voice frequency (0 to3KHz), then it important to minimize the capacitive and inductive effect of the cable and so RC time constant will tend to be minimal too. Sometimes when the cable is too long, it act as an antenna that often pick up noise (from pump, motor). In order to minimize the effect of noise, it importance to have twisted and shielded type of cable. Just go back to electric theory and one will figure out the correct answer and thus lead you to good decision making when doing engineering.

In summary,  the resistance, inductance and capacitance of the cable must be considered to produce reliable transmission. I hope this make sense.

Henry Olazo (ECE/MSCS)

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+966=561-858-361

The Resistance of the cable, Because resistace can effect the current flow and thus the control. 

Apart from that, the noise insulation is also to be considered

i really didn't try configuring with hart devices but the rumor has it you should consider the cable length and it should be shielded twisted pair cable , grounding the cable at one point only near the device you programing

you should avoid noise and interference