PARALLEL PORT STANDARD IEEE 1284

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IEEE 1284 is a standard that defines bi-directional parallel communications between computers and other devices. In the 1970s, Centronics developed the now familiar printer parallel interface that soon became a de facto standard. The standard became non-standard as enhanced versions of the interface were developed, such as the HP Bitronics implementation released in 1992. In 1991 the Network Printing Alliance was formed to develop a new standard. In March of 1994, the IEEE 1284 specification was released.

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. In the printer venue, this allows for faster printing and back-channel status and management. Since the new standard allowed the peripheral to send large amounts of data back to the host, devices that had previously used SCSI interfaces could be produced at a much lower cost. This included scanners, tape drives, hard disks, computer networks connected directly via parallel interface, network adapters and other devices. No longer was the consumer required to purchase an expensive SCSI card—they could simply use their built-in parallel interface. These low-cost devices provided a platform to leapfrog the faster USB interface into its present popularity, displacing the parallel devices. However, the parallel interface remains highly popular in the printer industry, with displacement by USB only in consumer models.

Specifications

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

Most recent computers that include a parallel port can operate the port in ECP or EPP mode, or both simultaneously.

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 (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.

IEEE 1284-I device uses IEEE 1284-A and IEEE 1284-B connectors, while IEEE 1284-II device uses IEEE 1284-C connectors.

In IEEE 1284 Daisy Chain Specification, up to 8 devices can be connected to a single parallel port.