parallel port

parallel port
Name	Parallel interface
Invented	1970s
Designer	Centronics, IBM
Type	Computer bus
Connectors	DB-25, Centronics connector
Data width	8 bits (typical)
Voltage levels	TTL, RS-232 (historical variants)
Successors	Universal Serial Bus, IEEE 1284, FireWire

parallel port

The parallel interface is a computer peripheral interface designed to transfer multiple bits simultaneously across multiple conductors. It was widely adopted for connecting printers, scanners, storage devices, and instrumentation to personal computers produced by companies such as IBM, Hewlett-Packard, and Compaq. The interface influenced peripheral ecosystems involving firms like Centronics, Microsoft, Intel, Apple Inc., and standards bodies including IEEE.

Overview

The interface implements a multi-wire link that conveys eight or more data lines alongside control and status signals between host controllers from vendors like IBM and peripheral implementations by Centronics and Canon. Early personal computer platforms such as the IBM PC, Commodore 64, and Apple II used the port to connect printers from manufacturers like Epson and Brother. Peripheral device classes included printers, plotters, tape drives, network adapters from firms like 3Com, and instrumentation compatible with test-equipment vendors such as Agilent Technologies.

History and Development

Origins trace to the printer interface developed by Centronics in the 1970s and adoption by IBM for microcomputer systems in the 1980s. The de facto coupling between Centronics physical connectors and IBM PC parallel port implementations established a broad market; influential companies such as Hewlett-Packard and Epson produced peripherals to that ecosystem. Formalization efforts by IEEE and subsequent cooperative work among Microsoft, Intel, and peripheral vendors produced standards to extend functionality and bidirectional signaling. Market shifts driven by corporations like Apple Inc. and standardization efforts by ISO and IEC influenced later revisions and successor technologies.

Technical Specifications and Signaling

Electrical signaling used transistor–transistor logic levels standardized by industry actors including Intel and Motorola. The typical data path comprises eight parallel data conductors accompanied by control lines and status returns; timing and handshaking schemes evolved through proposals from IEEE committees and manufacturer specifications from Centronics and IBM. Implementation considerations included signal integrity across cable assemblies manufactured by suppliers such as 3M and Belden', termination strategies informed by National Semiconductor application notes, and EMI compliance assessed against standards produced by Underwriters Laboratories and IEC.

Connector Types and Pinouts

Physical interconnects featured the Centronics connector on peripheral ends and DB-25 or DB-25 male connectors on host ends used by systems from IBM and Compaq. Other regional and vendor variants used edge connectors or custom housings implemented by Apple Inc. and certain workstation vendors. Pinout assignments for data, control, and status lines were specified in vendor documentation from Centronics and later harmonized in standards published with contributions from IEEE and ISO.

Modes of Operation and Standards

Operation modes progressed from unidirectional, printer-oriented communication to multiple bidirectional and high-speed modes. Notable formalizations included specifications influenced by IEEE working groups that led to standardized bidirectional modes and fast transfer protocols. Industry-supported modes enabled by chipset vendors such as Intel and Fujitsu included extended capabilities for handshaking and DMA coordination, paralleling development efforts by peripheral manufacturers like Hewlett-Packard and Epson to support enhanced graphics and raster data transfers.

Devices and Uses

The interface underpinned a wide array of peripherals: impact and laser printers by Epson, Hewlett-Packard, and Canon; scanners and imaging devices by Kodak and Canon; tape backup systems by Iomega; and specialty instrumentation by Tektronix and National Instruments. It also saw use in networking adapters and storage bridges from firms like 3Com and Adaptec, and hobbyist expansions for microcomputers such as the Commodore 64 community. Embedded systems and industrial controllers from manufacturers including Siemens and Schneider Electric exploited parallel signaling for legacy I/O.

Legacy, Decline, and Modern Alternatives

Decline accelerated with the rise of high-speed serial interfaces promoted by Intel and Microsoft such as Universal Serial Bus and by multimedia-focused standards from Apple Inc. including FireWire. Printers and peripherals migrated to network protocols supported by IEEE 802.3 Ethernet and wireless standards driven by Wi‑Fi Alliance participants. Legacy documentation, maintenance ecosystems, and specialty adapters persisted from vendors like StarTech.com and Belkin while archival and industrial systems continued to use the interface where determinism and existing cabling from suppliers like 3M remained valued. Category:Computer buses