Translation:Shulchan Aruch/Orach Chaim/57
57: LAW OF BAR'KHU AND ITS RESPONSE. COMPRISING 2 SECTIONS.
1
The prayer leader says: Bar'khu et Adonai ha-mevorach, and they answer after him: Barukh Adonai ha-mevorach l'olam va'ed, then the chazzan returns and says: Barukh Adonai ha-mevorach l'olam va'ed.
And the have practiced that the prayer leader lengthens "Bar'khu", and the congregation say "Yitbarakh va-yishtabach etc." while he is extending "bar'khu" (Tur).
2
A place where they're accustomed to call out at their fellows between Kaddish and Bar'khu to Yotzer Or, or to speak regarding the needs of the public, they err. (And see above, end of Siman 54).