Answer / g pravin

at 4-20ma with +24V power supply, no spark will be
generated.& 1 VDC at 4 mA of loop current and 5 VDC at 20
mA of loop current as per standard of analog i/o cards.
if we use 0 to 20ma instead of 4 to 20 ma then we can not
find actual position zero reference output, for example if
we have a motorized valve, it's full close at that time we
get 0ma (0%) reference and at fully open position we get
20ma (100%) reference, in this condition if power supply of
is off then we also get 0ma reference, so here in this kind
of
Condition we can't get actual result like that gate is
actually close or any other problems as per explanation of
above

Answer / venbha123

The 4 mA current has been selected for zero, so that we can distinguish the process zero condition from the supply failure condition. it is called the LIVE ZERO. If the process is zero then output will be 4 ma. If the instrument power is not available then it will give 0 ma. We can ensure the online  availability of the instrument for measurement.

The noise pick up in the signal cables may be in the order of say 3.5 mA and so 4 mA has been fixed as zero. As a convention the span to zero ratio is fixed as 5, so that the span output signal comes to 20 mA. (3 to 15 psi, 1 to 5 Volts,0.2 to 1.0 bar etc.)

For remote signal transmission the cureent signal is preferred to voltage because of the drop in the signal cable. The supply voltage is 24 V will make up for the loss in case of current signal transmission.  

Answer / shijith c

only at 4-20ma output, we are getting linearity. Also, at 4-20ma with +24V power supply, no spark will be generated. for these reasons only 4-20 ma is selected.

in DCS or PLC we can not put all ai or di,there is calculation through which we can accomodate certain no of digital and analog signal.I need to know that calculation

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