ST-506
Seagate's 1980 hard-disk interface, which left the encoding to a controller on the host card while the drive was just heads and platters.
The name came from Seagate's 5 MB drive of the same designation. The interface was deliberately "dumb": the drive sent and received a raw analog data stream over two ribbon cables — one for control, one for data — while the controller on the motherboard decided whether it became MFM (5 Mbit/s) or RLL (7.5 Mbit/s). That made drive and controller separate purchases, with all the incompatibility that entailed.
The ST-412 variant added buffered seek and became the de-facto standard on the PC, XT and AT throughout the 1980s. It died when IDE moved the controller onto the drive and made the whole question of encoding the disk's private business.