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. 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 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.