▁signal ▁levels , ▁the ▁speed , ▁format , ▁and ▁protocol ▁of ▁the ▁data ▁transmission ▁are ▁not ▁specified ▁by ▁R S - 4 8 5 . ▁Inter oper ability ▁of ▁even ▁similar ▁devices ▁from ▁different ▁manufact ur ers ▁is ▁not ▁assured ▁by ▁comp liance ▁with ▁the ▁signal ▁levels ▁alone . ▁ ▁Sign als ▁▁ ▁The ▁R S - 4 8 5 ▁differential ▁line ▁consists ▁of ▁two ▁signals : ▁ ▁A , ▁which ▁is ▁low ▁for ▁logic ▁ 1 ▁and ▁high ▁for ▁logic ▁ 0 ▁and , ▁ ▁B , ▁which ▁is ▁high ▁for ▁logic ▁ 1 ▁and ▁low ▁for ▁logic ▁ 0 . ▁ ▁Because ▁a ▁mark ▁( logic ▁ 1 ) ▁condition ▁is ▁tradition ally ▁represented ▁( e . g . ▁in ▁R S - 2 3 2 ) ▁with ▁a ▁negative ▁voltage ▁and ▁space ▁( logic ▁ 0 ) ▁represented ▁with ▁a ▁positive ▁one , ▁A ▁may ▁be ▁considered ▁the ▁non - in ver ting ▁signal ▁and ▁B ▁as ▁in ver ting . ▁The ▁R S - 4 8 5 ▁standard ▁states ▁( par ap hr ased ): ▁ ▁For ▁an ▁off , ▁mark ▁or ▁logic ▁ 1 ▁state , ▁the ▁driver ' s ▁A ▁terminal ▁is ▁negative ▁relative ▁to ▁the ▁B ▁terminal . ▁ ▁For ▁an ▁on , ▁space ▁or ▁logic ▁ 0 ▁state , ▁the ▁driver ' s ▁A ▁terminal ▁is ▁positive ▁relative ▁to ▁the ▁B ▁terminal . ▁ ▁The ▁truth ▁tables ▁of ▁most ▁popular ▁devices , ▁starting ▁with ▁the ▁SN 7 5 1 7 6 , ▁show ▁the ▁output