▁by ▁the ▁T IA ▁as ▁T IA - 4 8 5 , ▁but ▁engine ers ▁and ▁applications ▁gu ides ▁continue ▁to ▁use ▁the ▁R S - 4 8 5 ▁design ation . ▁ ▁The ▁initial ▁edition ▁of ▁E IA ▁R S - 4 8 5 ▁was ▁dated ▁April ▁ 1 9 8 3 . ▁ ▁R S - 4 8 5 ▁only ▁spec ifies ▁elect rical ▁characteristics ▁of ▁the ▁generator ▁and ▁the ▁receiver : ▁the ▁physical ▁layer . ▁It ▁does ▁not ▁specify ▁or ▁recommend ▁any ▁communic ations ▁protocol ; ▁Other ▁standards ▁define ▁the ▁protocol s ▁for ▁communication ▁over ▁an ▁R S - 4 8 5 ▁link . ▁The ▁for ew ord ▁to ▁the ▁standard ▁references ▁The ▁Tele communic ations ▁Systems ▁Bul letin ▁T SB - 8 9 ▁which ▁contains ▁application ▁gu idel ines , ▁including ▁data ▁signal ing ▁rate ▁vs . ▁cable ▁length , ▁stub ▁length , ▁and ▁configurations . ▁ ▁Section ▁ 4 ▁defines ▁the ▁elect rical ▁characteristics ▁of ▁the ▁generator ▁( trans mit ter ▁or ▁driver ), ▁receiver , ▁trans ce iver , ▁and ▁system . ▁ ▁These ▁characteristics ▁include : ▁definition ▁of ▁a ▁unit ▁load , ▁voltage ▁ranges , ▁open ▁circuit ▁volt ages , ▁th resh olds , ▁and ▁trans ient ▁toler ance . ▁It ▁also ▁defines ▁three ▁generator ▁interface ▁points ▁( signal ▁lines ); ▁A , ▁B ▁and ▁C . ▁ ▁The ▁data ▁is ▁transm itted ▁on ▁A ▁and ▁B . ▁ ▁C ▁is ▁a ▁ground ▁reference . ▁This ▁section ▁also ▁defines ▁the ▁logic ▁states ▁ 1 ▁( off