The Interface
Dr. An Wang, Robert Howard and Prentice Robinson developed the Centronics parallel interface at Wang Laboratories. Wang had a surplus stock of 20,000 Amphenol 36-pin micro ribbon connectors that were originally used for one of their early calculators. The connector has become so closely associated with Centronics that it is now popularly known as the “Centronics connector”. The Centronics parallel interface quickly became an industry de facto standard. Manufacturers of the time tended to use various connectors on the system side, so a variety of cables were required. When IBM implemented the parallel interface on the IBM PC, they used the DB25F connector at the PC-end of the interface, creating the now familiar parallel cable with a DB25M at one end and a 36 pin micro ribbon connector at the other. HP adopted Centronics parallel on their printer models and introduced a bidirectional version known as Bitronics on the LaserJet 4 in 1992. The Bitronics and Centronics interfaces were superseded by the IEEE 1284 standard in 1994.
Centronics parallel is generally compliant with IEEE 1284 compatibility mode. The original Centronics implementation called for the busy lead to toggle with each received line of data (busy by line), whereas IEEE 1284 calls for busy to toggle with each received character (busy by character). Some host systems or print servers may use a strobe signal with a relatively low voltage output or a fast toggle. Any of these issues might cause no or intermittent printing, missing or repeated characters or garbage printing. Some printer models may have a switch or setting to set busy by character; others may require a handshake adapter.
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