ates ▁requests ▁for ▁a ▁particular ▁track ▁and ▁sector ▁from ▁the ▁host ▁system ▁into ▁a ▁sequence ▁of ▁head ▁position ing ▁commands , ▁including ▁setting ▁the ▁direction ▁of ▁head ▁movement , ▁in ▁or ▁out , ▁and ▁sending ▁individual ▁" ST EP " ▁commands ▁to ▁move . ▁ ▁Four ▁of ▁the ▁control ▁cable ▁p ins , ▁" HD ▁SL CT ▁ 0 " ▁through ▁" HD ▁SL CT ▁ 3 ", ▁allow ▁the ▁selection ▁among ▁up ▁to ▁ 1 6 ▁heads , ▁although ▁only ▁four ▁are ▁available ▁on ▁the ▁two - pl atter ▁ST 5 0 6 . ▁( The ▁original ▁ST 5 0 6 / ST 4 1 2 ▁interface ▁defined ▁only ▁two ▁HD ▁SL CT ▁lines , ▁providing ▁supporting ▁for ▁only ▁four ▁heads , ▁but ▁a ▁third ▁HD ▁SL CT ▁line ▁was ▁shortly ▁added ▁to ▁the ▁design ; ▁a ▁fourth ▁was ▁added ▁a ▁not ▁much ▁later ▁by ▁re def ining ▁the ▁reduced ▁Write ▁Current ▁signal , ▁needed ▁only ▁by ▁very ▁early ▁drives , ▁as ▁HD ▁SL CT ▁ 3 .) ▁Once ▁the ▁heads ▁are ▁properly ▁position ed ▁and ▁the ▁appropriate ▁head ▁is ▁selected , ▁data ▁is ▁read ▁or ▁written ▁serial ly ▁through ▁a ▁set ▁of ▁p ins ▁in ▁the ▁data ▁cable . ▁The ▁limited ▁band width ▁of ▁the ▁data ▁cable ▁was ▁not ▁an ▁issue ▁at ▁the ▁time ▁and ▁is ▁not ▁the ▁factor ▁that ▁limited ▁the ▁performance ▁of ▁the ▁system . ▁However , ▁the ▁un sh ield ed ▁cable ▁can ▁sometimes ▁be ▁sus cept ible ▁to ▁high ▁levels ▁of ▁noise . ▁ ▁The ▁ST 4