An early parallel interface was developed by Dr. An Wang, Robert Howard and Prentice Robinson at Wang Laboratories. The now-familiar connector was selected because 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 Centronics Model 101 printer was introduced in 1970 and included this parallel interface for printers. The Centronics parallel interface quickly became a de facto industry standard. The IEEE 1284 standard superseded the Centronics interface in 1994. The IEEE 1284 standard allows for faster throughput and bidirectional data flow with a theoretical maximum throughput of 4 megabits per second, with actual throughput around 2 megabits, depending on hardware. IEEE 1284 can operate in five modes:
- Compatibility Mode, also known as Centronics, standard or SPP, is a uni-directional implementation with only a few differences from the original Centronics design.
- Nibble Mode is a uni-directional interface that allows the device to transmit data four bits at a time using status lines for data. This is the Bi-tronics mode introduced by HP and is generally used for enhanced printer status.
- Byte Mode allows the device to transmit eight bits at a time using data lines.
- Enhanced Parallel Port (EPP) is a half-duplex bi-directional interface designed to allow devices like printers, scanners, or storage devices to transmit large amounts of data. EPP can provide up to 2 Mbit/s bandwidth, approximately 15 times the speed achieved with normal parallel-port communication with far less CPU overhead.
- Extended Capability Port (ECP) is a half-duplex bi-directional interface similar to EPP, except that PC implementations use direct memory access to provide even faster data transfer than EPP. Many devices that interface using this mode support RLE compression. ECP can provide up to 2.5 Mbit/s of bandwidth
Cables and connectors
An IEEE-compliant cable must meet several standards of wiring and quality. Three types of connectors are defined:
- DB-25 (type A) for the host connection.
- Centronics (officially called "Micro Ribbon", type B) 36 pin for the printer or device connection.
- Mini-Centronics (type C, MDR36) 36 pin, a smaller alternative for the device connection that has not proven popular.
Pinouts for a parallel port connectors are:
|IEEE 1284 connector configuration|
|Pin # (DB25)||Pin # (36 pin)||Signal name||Direction||Register - bit||Inverted|
|table reproduced from wikipedia|
IEEE-1284 cables have a maximum length of 10m (31 feet) and a maximum data rate of 2.5Mbs.
The IEEE-1284 interface has been used in the laboratory for many years as the interface of choice to printing devices, but it is being phased out in favor of more modern interfaces, such as USB. It has never been widely used for general device or instrument interfacing.
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