- The IEEE 1284 standard allows for faster throughput and bidirectional data flow with a theoretical maximum throughput of 4 megabytes per second; actual throughput is around 2 megabytes/second, 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.
IEEE 1284 can operate in five modes:
1. Compatibility Mode, also known as Centronics standard or SPP, is a uni-directional implementation with only a few differences from the original Centronics design. This mode is almost exclusively used for printers. The only signals that the printer can send back to the host are some fixed-meaning status lines that signal common error conditions, such as the printer running out of paper.
2. Nibble Mode is an interface that allows the device to transmit data four bits (a nibble) at a time, (re)using four of the status lines of Compatibility Mode for data. This is the Bi-tronics mode introduced by HP and is generally used for enhanced printer status. Although never officially supported with these, Nibble Mode works with most of the pre-IEEE-1284 Centronics interfaces as well.
3. Byte Mode, also known as “Bi-Directional” (although all modes except Compatibility Mode are in fact bi-directional), is a half-duplex mode that allows the device to transmit eight bits at a time using the same data lines that are used for the other direction. This mode is supported on a minority of pre-IEEE-1284 interfaces as well, such as those built into the IBM PS/2 computers; because of this, it is sometimes unofficially called the PS/2 mode.
4. 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 while quickly being able to switch channel direction. EPP can provide up to 2 MByte/s bandwidth, approximately 15 times the speed achieved with normal parallel-port communication with far less CPU overhead.[1]
5.Extended Capability Port (ECP) is a half-duplex bi-directional interface similar to EPP, except that PC implementations use direct memory access (usually ISA DMA on channel 3) to provide even faster data transfer than EPP by having the ISA DMA hardware and the parallel port interface hardware handle the work of transferring the data instead of letting the CPU do this work. Many devices that interface using this mode support RLE compression. ECP can provide up to 2.5 MByte/s of bandwidth, which is the natural limit of 8-bit ISA DMA.[2] An ECP interface on a PC can improve transfers to pre-IEEE-1284 printers as well, by reducing the CPU load during the transfer ; however, the transfer in that case is unidirectional.
Most recent computers that include a parallel port can operate the port in ECP or EPP mode, or both simultaneously.
IEEE-1284 requires that bi-directional device communication is always initiated in Nibble Mode. If the host receives no reply in this mode, it will assume that the device is a legacy printer, and enter Compatibility Mode. Oth
