History of IBM CKD Controllers
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 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.
- 1 Chronological summary
- 2 Conventional storage controls
- 3 Director type storage controls
- 4 System 360 Model 25 integrated attachment
- 5 2319A integrated attachment
- 6 Integrated controls attaching A-units
- 7 See also
- 8 Notes
- 9 References
|2820 SCU||A22-6895||4/7/1964||similar to 2841 with parallel read channels|
|2314 DASF||A26-3599||4/23/1965||Bundled SCU with 9 drives, similar to 2841|
|System 360 Model 25 DAC||A24-3510||1/4/1968|
|2314-A1 SCU||A26-3599||9/1969 est.||Unbundled 2314 DASF, SCU similar to 2841|
|2844 Aux SCU with 2314||A26-3599||1968 est.[c]||provides dual path, similar to 2841|
|2835-1,2 SCUs||GA26-1589||1/28/1970||similar to 3830-1 with parallel read channels|
|System 370 Model 145 IFA||A26-3599||9/23/1970|
|2314-B1 SCU||A26-3599||12/14/1970||For 2319B DASD, similar to 2841|
|3830-2 SCU||GA26-1617||Aug 1972||single director|
|Integrated Storage Control Unit||GA26-1620||8/2/1972||A repackaged 3830-2|
|S/370 125-0, -2 3330/3333 Direct Disk Attachment||10/4/1972|
|S/370 115-0, -2, 125-0, -2 3340/3344 Direct Disk Attachment||GA33-1506||3/17/1973|
|3830-3 SCU and
Integrated Storage Controller with Staging Adapter
|GA32-0036||10/9/1974||single director, used with 3850 MSS|
|S/370 138 Integrated File Adapter||GA24-3632||6/30/1976|
|4321/4331 DASD Adapter for 3340/3344||GA33-1526||1/30/1979|
|3880-1, 2, 3 SCU||GA26-1661||6/1/1980||dual director|
|3880-11, 13 SCU||GA32-0061||10/21/1981||dual directors, large cache|
|3880-4 SCU||GA26-1661||1983||For reference only, NOT a CKD director|
|4361 DASD Adapter for 3340/3344||GA33-1566||9/10/1984|
|3880-21, 23 SCU||GA32-0081||10/18/1984||dual directors, large cache|
|3380-CJ2 Direct Chanel Attached DASD||GC26-4497||9/1/1987||An SCU and two DASD in one unit; additional DASD can be attached|
|3990-1,2,3 SCUs||GA32-0099||11/14/1989||dual directors, model 3 has cache|
|3990-6 SCU||GA32-0274||5/19/1993||dual caching directors|
Conventional storage controls
A conventional DASD 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.
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.
||There is a proposal that this section be split into a new article titled IBM 2841. (Discuss) (February 2016)|
The 2841 was a microprogrammed control unit "intended for use in controlling access to a disk or strip file or a slow-speed drum storage unit." It connected to one or two standard System/360 channels, or could also be attached to an IBM 1130 or IBM 1800 Data Acquisition and Control System to add support for 2311 disks.
The IBM 2841 Storage Control Unit ws capable of attaching combinations of up to eight of following DASD with a limitation of up to a maximum of eighteen access mechanisms (some devices have more than one access mechanism):
- IBM 2302 Disk Storage, Models 3, 2 access mechanisms per device
- IBM 2302 Disk Storage, Models 4, 4 access mechanisms per device
- IBM 2311 Disk Storage, Model 1, 1 access mechanism per device.
- IBM 2303 Drum Storage[d], 1 access mechanism per device
- IBM 2321 Data Cell, 1 access mechanism per device
Typically DASD types are not intermixed, with a very common installation being a 2841 SCU with a number of 2311 DASD up to eight.
DASD devices introduced after this used different storage controls as discussed in the following sections.
In IBM 2314 family of SCUs and DASDs, the SCUs connect to one or two selector channels and can attach from one to nine drives, at most eight online plus one optional service drive which is accessible from the SCU for maintenance purposes. The model numbers reflect packaging differences but otherwise all versions of the 2314 family are functionally the same. The models are:
- 2314 Direct Access Storage Facility - Model l: A bundle of an SCU and nine drives, two four drive modules and one one drive module.
- 2314 Storage Control Unit Model A1 - This SCU was announced initially as a part of an A Series DASF and shortly thereafter unbundled. The unbundled DASD models were the one drive 2312 Disk Storage, the four drive 2313 Disk Storage and the two drive 2318 Disk Storage. One to nine drives could be attached as in the Model 1 DASF.
- 2314 Storage Control Unit Model B1 - Part of the B series, this SCU attaches a three drive 2319-B1 Disk Storage and optionally one or two additional, three drive 2319-B2 Disk Storages.
- 2844 Auxiliary Storage Control - This SCU is a second 2314 SCU attached to one set of 2314 DASD allowing simultaneous access to any two of the attached DASD.
3830 Model 1
3380 Model CJ2
The 3380 Model CJ2 provides both the storage functions of one 3380 DASD (two devices) and the storage control functions of a 3990 model I (two paths to A-units) in a single unit. It connects to two, four or eight block multiplexor channels. Up to three 3380 'B' units (models BJ4 or BK4 only) can be attached to the 3380 Model CJ2; providing the head-of-storage-facility function for up to 14 devices.
Director type storage controls
The 3830 Model 2 storage control and associated Integrated Storage Controls were the first instance of the separation of an IBM Storage Control Unit into a Director and Controller, the latter typically packated and labelled as an "A-unit" (or A-Box") with the controller and at least one DASD physically in an A-unit. The term "A-unit" (or "A-Box") was not initially used, instead units containing the controller and DASDs were given product type numbers, specifically 3333 for the A-unit containing 3330 DASD and 3333-11 for the A-unit containing two double capacity 3330-11 DASDs. The use of the suffixes "A" and "B" in the model number to designate an A-unit and B-units began with the 3340 DASD with the 3340 Model A2 and 3340 B2, respectively. This practice of using the digits of a suffix to a DASD Model number to represent the type of unit and the number of DASD in the unit continued for all subsequent CKD DASD, for example, the 3380 Model A4 has one controller and four access mechanisms in one unit, the 3380 Model AA4 has two controllers and four access mechanisms in one unit while the 3380 Model B4 has four access mechanisms.
The combination of an A-unit with one or more optional B-units is a "string." "String switch" is an optional feature on most A-units that allows the controller in the A-unit to be switched between two directors, thereby providing additional paths into a string of DASD.
3830 director family
The 3830-2 connects to one, two or four block multiplexer channels and can attach any combination of up to four A-units, i.e., any combination of 3333s, 3340-A2s, or 3350-A2/A2Fs up to a maximum of 32 physical drives (up to 6 additional drives attached to each A-unit).
The Integrated Storage Control for the System/370 Models 158 and 168 functions identically to the 3830-2.
The Integrated Storage Controller with Staging Adapter functions identically to the 3830-3.
The IBM 3880 Storage Control Models 1, 2 and 3 have two directors per cabinet. Each director can attach to one, two, four or eight block multiplexor channels. The Model 4 has one director but does not attach CKD DASD.
Through use of diskettes, each storage director can be initialized to attach exclusively 3330/3350 A-units or 3340 A-units or 3380 A-units, that is, all A-units attached to one director must be of one type. This allows field conversion between Models 1. 2 and 3. Directors can attach up to four intermixed 3330 or 3350 A-units or four 3340 A-units or two 3380 A-units 
Each 3380 Storage Control, Model 11, Model 13, Model 21 and Model 23 has two directors; each director can attach to one, two or four block multiplexor channels.
The 3880 Model 13 has two caching storage directors that access subsystem storage; a larger portion of subsystem storage is the cache which is used to store active data for quick access; a smaller portion of the storage is the directory which is used to locate the data stored in the cache. The cache storage director attaches only one or two 3380 A-units each of which can in turn attach up to three 3380 B-units for a total of 16 devices. Because each 3380 DASD has two actuators the 3880 Model 13 can have up to 32 device addresses. The Model 23 increased the cache size and somewhat improved performance but otherwise performed the same functions
Paging is well established in computer with dedicated CKD paging devices going back to the drums included in the S/360 announcement. The 1978 StorageTek 4305 was the first CKD device using semiconductor memory for paging.
The 3880 Model 11 has two storage directors, director 1 can access subsystem storage for paging and director 2 is for nonpaging. Director 1 attaches one string of up to eight 3350 DASDs. Director 2 attaches any combination of from one to four 3330 A-units or 3350 A-units, each A-unit attaching up to three additional B-units for up to 32 DASDs.
All 3990's are dual directors and each director can attach to four or eight block multiplexor channels. Other characteristics of specific models include:
- Multiple A-unit paths: Prior to the 3390 all directors had a single path to associated A-units; each director in the 3390 Models 2, 3 and 6 has dual paths to A-units.
- Attached DASD A-units: The 3990 Model 1 can attach up to four 3380 A-units. The 3390 Models 2, 3 and 6 can intermix up to four 3380 and 3390 A-units
- Cache: In the Models 3 and 6 one director has a cache and one director has nonvolatile storage
The Model 1 can be field upgraded to a Model 2 or 3. The Model 6 has improved reliability, availability and serviceability but is otherwise functionally the same as the Model 3. 
System 360 Model 25 integrated attachment
On January 4, 1968, IBM announced the System 360 Model 25 which included the "Disk Attachment Control" (DAC) attaching up to four 2311 DASD. This integrated attachment simulates both a channel and a storage control unit between the CPU and attached 2311 units. By merging the functions the DAC eliminates the need for a channel and a separate storage control. There are no differences between a 2311 connected to a 2841 SCU and one connected to the DAC.
2319A integrated attachment
On September 23 1970 IBM announced as part of its System/370 Model 145 an new "Integrated File Adapter" along with a new 2319-A1 DASD. On March 8, 1981 support of the 2319-A1 was announced with an Integrated File Adapter as part of the announcement of the System/370 Model 135.
The 2319-A1 contained three 2314 class disk drives plus a piece of the storage control from the system unit. Conventional 2314 type DASD, e.g., 2312, 2313 and/or 2318, could be attached to the 2319-A1.
Since it could not connect to director type storage control the 2319-A1 was not an "A-unit" as that term came to be used by IBM.
Integrated controls attaching A-units
Direct disk attachments
S/370 125-0 and -2
Direct attachment of the 3333/3330 DASD Subsystem or the 3340 DASF Subsystem or the 3340/3344 DAS Subsystem (3125-2 only) is provided. Depending on model and features up to 16 spindles can be attached. In a 3340 Subsystem the 3340 mdl A2 and its attached 3340 drives can be shared with another S/370, except 3115-0 or 3125-0, via the String Switch capability (#9315).
Direct attachment of the 3340 DASF Subsystem or the 3340/3344 DAS Subsystem (3115-2 only) is provided. Depending on the Model and feature up to eight drives can be attached. In a 3340 DASF Subsystem, via the string switch feature (#9315), the 3340 Model A2 can be shared with another S/370, except 3115-0 and 3125-0. 
S/370 138 Integrated File Adapter
One or two 3330 or 3340 A-units can be attached to the Integrated File Adapter of the S/370 Model 138. Up to three appropriate B-units may be attached to each A-unit. If 3344 B-units are installed, they may only installed in the first of the two possible strings with up to three 3340 and/or 3344 B-units in any combination.
4321/4331 DASD Adapter for 3340/3344
One or two DASD adapters are available (depending upon model) for attachment of one 3340 A-unit and with up to three additional 3340 or 3344 B-units providing up to eight devices. String Switch feature in the 3340 A-unit allows sharing of 3340/3344 DASD with another IBM processor or director that supports the A-units and string switching.
4361 DASD Adapter for 3340/3344
Up to four 3340 A-units can be attached to an optional DASD Adapter; to each 3340 A-unit can be attached from one to three 3340 B-units or 3344 B-units for a total of 32 DASD in four strings of eight. String Switch feature in the 3340 A-unit allows sharing of 3340/3344 DASD with another IBM processor or director that supports the A-units and string switching.
- IBM System/360 Input/Output Configurator, Third Ed. A22-6823-2
- IBM System/370 Input/Output Systems Configurator
- Manual numbers are generic; links are to the latest version as indicated by the highest dash number found on line.
- Unless otherwise noted dates are from IBM's Storage product profiles or Mainframes product profiles
- This model number appears in Defamation’s December 1968 edition
- According to Datamation, March 1966, p.31, the 2302 replaced the 7320 originally announced with the 2841
- STK 8890 CyberCache for STK 3350 compatibles
- The tables within each section are extracted from IBM System/370 Reference Summary GX20-1850-7, IBM Corp., February 1989, with System/360 SCUs information added from reference manuals.
- IBM System/360 Component Descriptions - 2820 Storage to Control and 2301 Drum Storage (PDF). Third Edition. IBM. September 1968. A22-6895-2.
- IBM System/360 Component Descriptions - 2841 Storage Control Unit and Associated DASD (PDF). Eighth Edition. IBM. December 1969. A26-5988-7.
- IBM System/360 Component Descriptions 2314 Direct Access Storage Facility and 2844 Auxiliary Storage Control (PDF). Seventh Edition. IBM. November 1971. GA26-3599-6.
- IBM System/360 Model 25 Functional Characteristics (PDF) (First ed.). IBM. January 1968. A24-3S10-0.
- "multi-disc drive". Datamation: 206. September 1969.
IBM is now offering from one to eight active disc drives for its 2314 direct access storage facility
- Reference Manual for IBM 2835 Storage Control and IBM 2305 Fixed Head Storage Module (PDF). Third Edition. IBM. August 1971. GA26-1589-2.
- Reference Manual for IBM 3830 Storage Control Model 1 and IBM 3330 Disk Storage (PDF). Sixth Edition. IBM. November 1976. GA26-1592-5.
- Reference Manual for IBM 3830 Storage Control Model 2 (PDF). Sixth Edition. IBM. April 1977. p. 11. GA26-1592-5.
- IBM 3880 Storage Control Models 1, 2, 3 and 4 Description Manual (PDF) (Tenth ed.). IBM. September 1987. GA26-1661-9.
- IBM (March 1982). IBM 3880 Storage Control Model 11 Description (PDF) (First ed.). GA32-0061-0.
- IBM (June 1982). IBM 3880 Storage Control Model 13 Description (PDF) (First ed.). GA32-0067-0.
- see IBM System/360 architecture, Unit Status
- Thoburn, F. J. (November 1970). "A Transmission Control Unit for High-speed Computer-to-computer Communication". IBM Journal of Research and Development: 614–619. doi:10.1147/rd.146.0614. CiteSeerX: 10
.1 .1 .94 .3749.
- ...IBM 3380 DASD Direct Channel Attach Model CJ2, VM, Between-Release Support Information
- IBM (March 1982). IBM 3880 Storage Control Model 11 Description (PDF) (First ed.). GA32-0061-0.
- "Now Memorex fills the gap in your system's performance." Datamation Advertisement, August 1978, p. 85-6
- "It's how IBM's newest cache makes DASD faster". Datamation: 119. November 1984.
- Cornwell, Michael (September 2009). "Innovative Solid State Storage Architecture" (PDF). http://www.snia.org/. SNIA. Retrieved December 31, 2015.
StorageTek 4305 1978 First Solid State DiskExternal link in
- IBM 3990/9390 Storage Control Introduction. Storage Subsystem Library (Ninth ed.). IBM. October 1996. GA32-0098-08.
- System/370 Model 145 Announcement Letter
- System/370 Model 135 Announcement Letter
- Universal Sales Manual Hardware Products. IBM. February 15, 1994.
- "IBM System/370 Input/Output Systems Configurator" (PDF). IBM. April 1988. Retrieved January 1, 2016.