▁D + ▁as ▁alternative ▁for ▁B ▁( high ▁for ▁mark ▁i . e . ▁idle ) ▁ ▁T X − / R X − ▁or ▁D − ▁as ▁alternative ▁for ▁A ▁( low ▁for ▁mark ▁i . e . ▁idle ) ▁ ▁In ▁addition ▁to ▁the ▁A ▁and ▁B ▁connections , ▁an ▁optional , ▁third ▁connection ▁may ▁be ▁present ▁( the ▁T IA ▁standard ▁requires ▁the ▁presence ▁of ▁a ▁common ▁return ▁path ▁between ▁all ▁circuit ▁grounds ▁along ▁the ▁bal anced ▁line ▁for ▁proper ▁operation ) ▁called ▁SC , ▁G ▁or ▁reference , ▁the ▁common ▁signal ▁reference ▁ground ▁used ▁to ▁by ▁the ▁receiver ▁measure ▁the ▁A ▁and ▁B ▁volt ages . ▁This ▁connection ▁may ▁be ▁used ▁to ▁limit ▁the ▁common - mode ▁signal ▁that ▁can ▁be ▁im pressed ▁on ▁the ▁receiver ▁inputs . ▁The ▁allow able ▁common - mode ▁voltage ▁is ▁in ▁the ▁range ▁− 7 V ▁to ▁+ 1 2 V , ▁i . e . ▁ ± 7 V ▁on ▁top ▁of ▁the ▁ 0 - 5 V ▁signal ▁range . ▁Fail ure ▁to ▁stay ▁within ▁this ▁range ▁will ▁result ▁in , ▁at ▁best , ▁signal ▁cor ruption , ▁and , ▁at ▁worst , ▁damage ▁to ▁connected ▁devices . ▁▁ ▁Care ▁must ▁be ▁taken ▁that ▁an ▁SC ▁connection , ▁especially ▁over ▁long ▁cable ▁runs , ▁does ▁not ▁result ▁in ▁an ▁attempt ▁to ▁connect ▁dispar ate ▁grounds ▁together ▁– ▁it ▁is ▁wise ▁to ▁add ▁some ▁current ▁limit ing ▁to ▁the ▁SC ▁connection . ▁Gr ounds ▁between ▁buildings ▁may ▁vary ▁by ▁a ▁small ▁voltage