History of IBM CKD Controllers information

Beginning with its 1964 System/360 announcement, IBM's mainframes initially accessed count key data (CKD) subsystems via a channel connected to separate Storage Control Units (SCUs) with attached Direct Access Storage Devices (DASD), typically a hard disk drive. This practice continued in IBM's larger mainframes thru IBM Z; however low end systems generally used lower cost integrated attachments where the function of the SCU was combined with that of the channel, typically called an Integrated File Adapter.

The System/360 selector channel was followed by the System/370 block multiplexor channel which could operate as a selector channel to allow attachment of legacy subsystems.

The SCU evolved into a Director and Controller, the latter typically labelled an "A-unit" (or A-Box") with the controller and at least one DASD physically in an A-unit. An Integrated Storage Control (ISC) is a Director within the cabinet of an IBM System. A Director could attach from one to four A-units. One or more conventional DASD, now labeled a "B-unit" could attach to an A-unit

The following sections list in order of announcement IBM mainframe CKD storage controls, categorized as conventional storage controls, director type storage controls and integrated controls attaching an A-unit.