hart, smart, differences etc.

Please could anyone explain me in detail about the differences in 1) smart, hart (what are all these) protocols, differences,when are
the used? for what purpose 2), why 4-20 ma signals used.what is 4-20 ma? 3) Please can any one forward me the details at this address snipped-for-privacy@yahoo.com
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here. All logos and trade names are the property of their respective owners.