Question { Atesco, 46690 }

What is expansion of SMART Instruments? And What is it?

Answer

Actually there is no special expansion for SMART.
Because,

An expansion is not shorted to give SMART. But the
professionals trying to create from it.

(For example, People can create some expansionS for
INSTRUMENTATION as well.)

Simply they named it as SMART. thats all.

Question { ONGC, 14377 }

Why we use 4 - 20 ma?

Answer

To operate a Digital signal we need 1-5Vdc. For Analog
Instruments we are using 250 ohms as load resistance. When
we converting this signal into current By Ohm's
Law V=IR
I=V/R
Put, V=1V Then I=1/250=4mA
Put, V=2V Then I=2/250=8mA
Put, V=3V Then I=3/250=12mA
Put, V=4V Then I=4/250=16mA
Put, V=5V Then I=5/250=20mA

*Also, since this is linear region
*Not possible to cause spark
*Since it is a current signal we can transmit it into long
distance.
*Suppose if the instrument is dead we can identify it.