▁rule ▁of ▁thumb , ▁the ▁speed ▁in ▁bit / s ▁multip lied ▁by ▁the ▁length ▁in ▁metres ▁should ▁not ▁exceed ▁ 1 0 8 . ▁Thus ▁a ▁ ▁cable ▁should ▁not ▁signal ▁faster ▁than ▁. ▁ ▁In ▁contrast ▁to ▁R S - 4 2 2 , ▁which ▁has ▁a ▁driver ▁circuit ▁which ▁cannot ▁be ▁switched ▁off , ▁R S - 4 8 5 ▁drivers ▁use ▁three - state ▁logic ▁allowing ▁individual ▁transmit ters ▁to ▁be ▁de activ ated . ▁This ▁allows ▁R S - 4 8 5 ▁to ▁implement ▁linear ▁bus ▁top ologies ▁using ▁only ▁two ▁w ires . ▁The ▁equipment ▁located ▁along ▁a ▁set ▁of ▁R S - 4 8 5 ▁w ires ▁are ▁inter change ably ▁called ▁nodes , ▁stations ▁or ▁devices . ▁The ▁recommended ▁arrangement ▁of ▁the ▁w ires ▁is ▁as ▁a ▁connected ▁series ▁of ▁point - to - point ▁( mult id ro pped ) ▁nodes , ▁i . e . ▁a ▁line ▁or ▁bus , ▁not ▁a ▁star , ▁ring , ▁or ▁multiply ▁connected ▁network . ▁Star ▁and ▁ring ▁top ologies ▁are ▁not ▁recommended ▁because ▁of ▁signal ▁ref lections ▁or ▁excess ively ▁low ▁or ▁high ▁term ination ▁imped ance . ▁If ▁a ▁star ▁configuration ▁is ▁un av oid able , ▁special ▁R S - 4 8 5 ▁repe aters ▁are ▁available ▁which ▁bid irection ally ▁listen ▁for ▁data ▁on ▁each ▁span ▁and ▁then ▁re trans mit ▁the ▁data ▁onto ▁all ▁other ▁sp ans . ▁ ▁Ide ally , ▁the ▁two ▁ends ▁of ▁the ▁cable ▁will ▁have ▁a ▁term