 thumb|320px|image of a 3b15 computer the 3b series computers were a line of micro-programmable minicomputers produced by at&t computer systems's western electric subsidiary. ==high-availability processors== 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.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 the first 3b20d was called the model 1. each processor's control unit consisted of 2 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 (operating system) 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 2010 as a component of many alcatel-lucent products such as the 4ess and 5ess. 
AT&T Alcatel-Lucent Minicomputers