. ▁The ▁+ 5 ▁V ▁pin ▁guarante es ▁at ▁least ▁ 5 0 0 ▁m A , ▁and ▁requires ▁devices ▁to ▁use ▁only ▁ 1 0 0 ▁m A ▁each . ▁A DB ▁also ▁includes ▁the ▁PS W ▁pin ▁which ▁is ▁attached ▁directly ▁to ▁the ▁power ▁supply ▁of ▁the ▁host ▁computer . ▁This ▁is ▁included ▁to ▁allow ▁a ▁key ▁on ▁the ▁keyboard ▁to ▁start ▁up ▁the ▁machine ▁without ▁need ing ▁the ▁Apple ▁Des ktop ▁Bus ▁software ▁to ▁interpret ▁the ▁signal . ▁In ▁more ▁modern ▁designs , ▁an ▁auxili ary ▁micro controller ▁is ▁always ▁kept ▁running , ▁so ▁it ▁is ▁econom ical ▁to ▁use ▁a ▁power - up ▁command ▁over ▁the ▁standard ▁USB ▁channel . ▁ ▁Most ▁serial ▁digital ▁interfaces ▁use ▁a ▁separate ▁clock ▁pin ▁to ▁signal ▁the ▁arrival ▁of ▁individual ▁bits ▁of ▁data . ▁However , ▁W oz nia k ▁decided ▁that ▁a ▁separate ▁wire ▁for ▁a ▁clock ▁signal ▁was ▁not ▁necessary ; ▁and ▁as ▁Apple ▁Des ktop ▁Bus ▁was ▁designed ▁to ▁be ▁low - cost , ▁it ▁made ▁econom ical ▁sense ▁to ▁leave ▁it ▁out . ▁Like ▁mode ms , ▁the ▁system ▁lo cks ▁onto ▁the ▁signal ▁rise ▁and ▁fall ▁times ▁to ▁re create ▁a ▁clock ▁signal . ▁ ▁The ▁dec oding ▁trans ce iver ▁AS IC ▁as ▁well ▁as ▁associated ▁pat ents ▁were ▁controlled ▁by ▁Apple ; ▁this ▁required ▁vend ors ▁to ▁work ▁more ▁closely ▁with ▁Apple . ▁In ▁the ▁Mac int osh ▁SE , ▁the ▁Apple ▁Des ktop ▁Bus ▁is ▁implemented ▁in ▁an ▁Apple - brand ed ▁Micro ch ip