1 9 8 1 . ▁The ▁ST 2 2 5 ▁was ▁introduced ▁shortly ▁there after ▁with ▁ 2 0 ▁meg aby tes ▁and ▁half ▁the ▁height . ▁All ▁three ▁used ▁M FM ▁encoding , ▁a ▁widely ▁used ▁coding ▁scheme . ▁A ▁subsequent ▁extension ▁of ▁the ▁ST 4 1 2 ▁interface , ▁the ▁ST 4 1 2 HP ▁interface , ▁used ▁R LL ▁encoding ▁for ▁a ▁ 5 0 % ▁increase ▁in ▁capacity ▁and ▁bit ▁rate . ▁ ▁The ▁ST 5 0 6 ▁drive ▁connected ▁to ▁a ▁computer ▁system ▁through ▁a ▁disk ▁controller . ▁The ▁ST 5 0 6 ▁interface ▁between ▁the ▁controller ▁and ▁drive ▁was ▁derived ▁from ▁the ▁Sh ug art ▁Associ ates ▁SA 1 0 0 0 ▁interface , ▁which ▁was ▁in ▁turn ▁based ▁upon ▁the ▁flo ppy ▁disk ▁drive ▁interface , ▁thereby ▁making ▁disk ▁controller ▁design ▁relatively ▁easy . ▁ ▁The ▁ST 4 1 2 ▁interface ▁was ▁adopted ▁by ▁numerous ▁H DD ▁manufact ur ers ▁such ▁that ▁the ▁interface ▁became ▁a ▁de ▁fact o ▁industry ▁standard ▁for ▁disk ▁drives ▁well ▁into ▁the ▁ 1 9 9 0 s . ▁ ▁The ▁limitations ▁of ▁the ▁ST 4 1 2 ▁interface ▁are ▁ 5 ▁million ▁trans itions ▁per ▁second ▁maximum ▁on ▁data ▁lines , ▁ 1 6 ▁heads , ▁ 4 ▁drive ▁units ▁and ▁a ▁▁ ▁cable ▁length . ▁The ▁standard ▁channel ▁code ▁for ▁the ▁ST 4 1 2 ▁( and ▁ST 5 0 6 ) ▁is ▁M FM ▁with ▁one ▁data ▁bit ▁per ▁transition ▁for ▁a ▁data ▁rate ▁of ▁ 5  