3B series computers

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Image of a 3B15 computer

The 3B series computers are a line of minicomputers that were produced by AT&T Computer Systems' Western Electric subsidiary for use with the company's UNIX operating system and its in-house micro-programmed central processing units. The line primarily consists of the models 3B20, 3B5, 3B15, 3B2, and 3B4000. The series is notable for controlling a series of electronic switching systems (ESS) for telecommunication, for general computing purposes, and for serving as the historical software porting base for commercial UNIX.

3B high-availability processors[edit]

The original series of 3B computers include the models 3B20C, 3B20D, 3B21D, and 3B21E.

The 3B (3B20D/3B20C/3B21D/3B21E) is a 32-bit microprogrammed duplex (redundant) high availability processor unit with a real-time operating system. It is used in the telecommunications environment and was first produced in the late 1970s at the WECo factory in Lisle, Illinois. It uses the Duplex Multi Environment Real Time (DMERT) operating system which was renamed UNIX-RTR (Real Time Reliable) in 1982. The Data Manipulation Unit (DMU) provided arithmetic and logic operations on 32-bit words using AMD 2901 bipolar 4-bit processor elements.[1] The first 3B20D was called the Model 1. Each processor's control unit consisted of two frames of circuit packs. The whole duplex system required many seven foot frames of circuit packs plus at least one tape drive frame (most telephone companies wrote billing data on magnetic tapes), and many washing machine sized (and look with the open top door) disk drives. For training and lab purposes a 3B20D could be divided into two "half-duplex" systems. A 3B20S consisted of most of the same hardware as a half-duplex but used a completely different operating system.

The 3B20C was briefly available as a high-availability fault tolerant multiprocessing general purpose computer in the commercial market in 1984. The 3B20E was created to provide a cost reduced 3B20D for small offices that did not expect such high availability. It consisted of a virtual "emulated" 3B20D environment running on a stand-alone general purpose computer (the system was ported to many computers but primarily runs on the Sun Microsystems Solaris environment).

There have been many improvements made to the 3B20D UNIX-RTR system in both software and hardware throughout the 1980s, 1990s, and 2000s. These included some remarkable features such as disk independent operation (DIOP: the ability to continue essential software processing such as telecommunications after duplex failure of redundant essential disks) and Off-line Boot (the ability to split in half, boot up the previously out-of-service half, verify successful boot), and Switch Forward (switch processing to the previously out-of-service half). The processor was re-engineered and renamed in 1992 as the 3B21D. It is still in use as of 2016 as a component of many Alcatel-Lucent products such as the 2STP, 4ESS and 5ESS (both wireline and wireless).

General-purpose computers[edit]

A 3B2 model 400

The general purpose family of 3B computer systems includes the 3B2, 3B5, 3B15, 3B20S, and 3B4000.

These computers were named after the successful 3B20D. The 3B20S (simplex) ran using the UNIX operating system and was developed at Bell Labs and produced by WECo in 1982 for the general purpose internal Bell System use, and later the mini-computer market. The other 3B computers were also created for this market and eventually were running UNIX System V from AT&T.


The 3B20S was built using virtually the same hardware as the 3B20D. The machine was approximately the size of a sizeable refrigerator, requiring a minimum of 170 square feet floor space.[2] It was in use at the 1984 Summer Olympics where around twelve 3B20S served the email requirements of the Electronic Messaging System, which was built to replace the man-based messaging system of earlier olympiads. It connected around 1800 user terminals and 200 printers.[3]


3B2/300 motherboard

The 3B2 was introduced using the WE-32000 32-bit microprocessor with memory management chips that supported demand-paging. The 3B2 Model 300 was approximately 4 inches (100 mm) high and the 3B2 Model 400 was approximately 8 inches (200 mm) high. The 300 was soon supplanted by the 3B2/310, which featured the WE-32100 CPU as did all follow on models. The Model 400 allowed more peripheral slots and more memory. It also had a built-in 23 megabyte QIC tape drive managed by a floppy disk controller (nicknamed the "floppy tape"). These three models used standard MFM5 14" hard disk drives. The 3B2/600 offered an improvement in performance and capacity. It featured a SCSI controller for the 60 megabyte QIC tape and two internal full-height disk drives. The 600 was approximately twice as tall as a 400, and was oriented with the tape and floppy disk drives opposite the backplane (instead of at a right angle to it as on the 3xx, 4xx and later 500 models). Early models used an internal Emulex card to interface the SCSI controller with ESDI disks, with later models using SCSI drives directly. The 3B2/500 was the next model to appear, essentially a 3B2/600 with enough components removed to fit into a 400 case. One internal disk drive and several backplane slots were sacrificed in this conversion. Unlike the 600 which, because of its two large fans was quite loud, the 500 was tolerable in an office environment, like the 400. The 3B2/700 was an uprated version of the 600 featuring a slightly faster processor. The 3B2/1000 was an additional step in this direction.


The 3B5 was built using the older Western Electric WE-32000 32-bit microprocessor. The initial versions had discrete memory management unit hardware built using gate arrays and supported segment-based memory translation. IO was programmed using memory-mapped techniques. The machine was approximately the size of a dishwasher, though adding the reel-to-reel tape drive increased it to the size of a refrigerator.

These computers used SMD hard drives.


The 3B15 was the faster follow-on to the 3B5, but with similar large form factor.

3B1 desktop workstation[edit]

Officially named the AT&T UNIX PC,[4] AT&T introduced a desktop computer in 1985 that was often dubbed the 3B1. However, this workstation was unrelated in hardware to the 3B line, and was based on the Motorola 68010 microprocessor. It used a derivative of Unix System V Release 2 by Convergent Technology. The system was also known as the PC-7300. The system was tailored for use as a productivity tool in office environments and as an electronic communication center.[4]

See also[edit]


  1. ^ J. O. Becker, The 3B20D PROCESSOR and DMERT Operating System (The Bell System Technical Journal, January 1983, Vol. 62, No. 1, Part 1), Page 193
  2. ^ 3B20S Processor System Index and Description, Western Electric Co., July 1981.
  3. ^ Olympics electronic messaging system demonstrated in IEEE Explore, November 1983, page 113.
  4. ^ a b AT&T, Select Code 999-601-311IS, AT&T UNIX PC Owner's Manual (1986)

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