Editing Digital current loop interface (section)

==History==
Long before [[RS-232]] was made a recommended standard in 1960, current loops were used to send digital data in [[Asynchronous start-stop|serial]] form for [[teleprinter]]s.  More than two teleprinters could be connected on a single circuit allowing a simple form of networking.<ref name=PARR98 >E. A. Parr, ''Industrial Control Handbook'', Industrial Press Inc., 1998, {{ISBN|0831130857}}, page 648</ref> Older teleprinters used a 60 [[Ampere|mA]] current loop.  Later machines, such as the [[Teletype Model 33]], operated on a lower 20&nbsp;mA current level and most early [[minicomputer]]s featured a 20&nbsp;mA current loop interface, with an RS-232 port generally available as a more expensive option.  The original [[IBM PC]] [[serial port]] card had provisions for a 20&nbsp;mA current loop.<ref name=appnote>http://www.bb-elec.com/Learning-Center/All-White-Papers/Current-Loop/Current-Loop-Application-Note/curentlp.pdf {{Webarchive|url=https://web.archive.org/web/20170420195811/http://www.bb-elec.com/Learning-Center/All-White-Papers/Current-Loop/Current-Loop-Application-Note/curentlp.pdf |date=2017-04-20 }}  Current Loop Application  Note, retrieved July 11, 2017</ref>