The recently released standard, "IEEE Std.1284-1994 Standard Signaling Method for a Bi-directional Parallel Peripheral Interface for Personal Computers", is for the parallel port what the Pentium processor is to the 286. The standard provides for high speed bi-directional communiction between the PC and an external peripheral that can communicate 50 to 100 times faster that the original parallel port. It can do this and still be fully backward compatible with all existing parallel port peripherals and printers.
The 1284 standard defines 5 modes of data transfer. Each mode provides a method of transfering data in either the forward direction (PC to peripheral), reverse direction (peripheral to PC) or bi-directional data transfer (half duplex). The defined modes are:
All parallel ports can implement a bi-directional link by using the Compatible and Nibble modes for data transfer. Byte mode can be utilized by about 25% of the installed base of parallel ports. All three of these modes utilize software only to transfer the data. The driver has to write the data, check the handshake lines (ie: BUSY), assert the appropriate control signals (ie: STROBE) and then go on to the next byte. This is very software intensive and limits the effective data transfer rate to 50 to 100 Kbytes per second.
In addition to the previous 3 modes, EPP and ECP are being implemented on the latest I/O controllers by most of the Super I/O chip manufacturers. These modes use hardware to assist in the data transfer. For example, in EPP mode, a byte of data can be transfered to the peripheral by a simple OUT instruction. The I/O controller handles all the handshaking and data transfer to the peripheral.
Overall, the 1284 standard provides the following:
In summary, the 1284 parallel port provides an easy to use, high performace interface for portable products and printers.
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