IBM 2841

Beginning with its 1964 System/360 announcement IBM's mainframes initially accessed CKD (Count key data) 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 System/370; 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.

A storage control, frequently called a Storage Control Unit, attaches to a System/360 (or System/370) channel one or more logically separate devices. The control unit and the device each comporting to System/360 and/or System/370 Input/Output architecture. Since the control unit and attached devices are logically distinct either or both can be busy when an I/O is initiated and either can present ending status when it becomes free.

Conventional DASD storage controls listed in this section have different interfaces to the attached DASD, unique to a DASD or DASD family. In 1972, IBM introduced a standard DASD to control interface used with Director type storage controls.

The IBM 2820 Storage Control Unit (SCU) connects to one or two selector channels and can attach from one to four 2301 Drum Storage units.

The IBM 2841 Storage Control Unit was a Direct Access Storage Device (DASD) control unit introduced with System/360, first shipped in 1965 and offered until at least 1975.

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