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History of IBM CKD Controllers
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<p>Beginning with its 1964 System/360 announcement, IBM's mainframes initially accessed <a href="/facts/Count_key_data/Gxbr1GwB">count key data</a> (CKD) subsystems via a channel connected to separate Storage Control Units (SCUs) with attached <a href="/facts/Direct-access_storage_device/dr0tg5Hl">Direct Access Storage Devices</a> (DASD), typically a hard disk drive. This practice continued in IBM's larger mainframes thru <a href="/facts/IBM_Z/JJX9eAsD">IBM Z</a>; 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. </p><p>The System/360 <a href="/facts/IBM_System%2f360/7znvpy4K">selector channel</a> was followed by the System/370 <a href="/facts/IBM_System%2f360/7znvpy4K">block multiplexor channel</a> which could operate as a selector channel to allow attachment of legacy subsystems. </p><p>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 </p><p>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. </p>