Answer Posted / george mathews

Analog signal is the signal that we get in the form 4 - 20
mA. form the instrument on the field. It is categorized as
AI & AO depending upon their movement either from instrument
to DCS or from DCS to Instrument. IT it is from DCS to Field
instrument it is called AO , and if it is from Field
Instrument to DCS it is called AI.

Like wish Digital value are in 1 and 0 form. And we
categorized DI and DO the same way we do in case of AI & AO.
From DCS to Field Instrument it is DO and form Field
instrument to DCS it is called DI.

The main deference between analogue and digital signal is
that in a AI & AO signal the command defince what is the
present status of any instrument ( 4mA = 0%,8ma = 25%, 12ma
= 50% , 16ma = 75% , 20ma = 100% open or close.)

But in DI & DO we have only 1 & 0 , so either with this
command we can either open and close any device 100%. (ie
100% open or 100% close.)

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