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The Basic I/O Ports
This page covers the parallel port, the serial ports and the USB ports.
The Parallel Port
Most of us rely on the parallel channel for fast access to printers and other devices, such as external tape and CD-ROM drives.
A standard parallel cable consists of 25 wires that transfer information one byte at a time. Each wire terminates in a connector called a pin,
and is often referred to as a 25-pin connector. The 8 bits that make up each byte travel in parallel across 8 of the 25 wires. One limitation of the standard parallel channel is the distance between the device and the computer. The parallel
port is best suited for distances of 15 feet or less. Longer parallel cables are available, but the extra length may corrupt the data unless the signal is amplified.
The parallel port, has changed four since the first IBM PC:
Unidirectional, 4-bit Original IBM PCs and compatibles;
Bidirectional, 8-bit IBM PS/2s and compatibles;
Type 3 direct memory access (DMA) PS/2s and compatibles;
Enhanced parallel port (EPP) 386-based computers;
Enhanced capabilities port (ECP) Pentium-based computers.
The original IBM PC incorporated a unidirectional parallel port, which can send information in only one direction. In the late 1980s, IBM and other computer manufacturers
introduced bidirectional parallel ports. The new ports used an additional 8 of the 25 wires in the parallel cable for data traveling in the other direction.
Bidirectional ports enabled printers to send status messages about print jobs back to the computer.
The next parallel port was type 3 direct memory access (DMA)
ports, allowing much faster bidirectional performance. This technology relied on the computer setting aside a block of memory to
cache data written to the parallel port. Type 3 DMA ports improved performance dramatically, since the CPU no longer needed to regulate the flow of information.
The enhanced parallel port specification
(EPP, or fast mode port), depends on peripheral devices that can dynamically manage the information traveling across the cable. The current EPP standard is also
known as IEEE 1284.
The latest parallel port is the enhanced capabilities port (ECP). Parallel ports using this technology
have better performance than EPP ports, but
require software designed to take advantage of direct memory access and data compression capabilities.
The Serial Port
The serial channel is mainly used to connect modems and mice to the serial port.
Serial data is sent in series, one bit at a time, over a single wire. This is significantly slower than sending 8 bits at a time via a parallel channel, but
can travel much farther without needing amplified.
Serial ports use a universal asynchronous receiver-transmitter (UART) chips. This
chip, at the core of a serial port operation, converts data heading out through the serial port from the computer’s
parallel stream to a serial stream. The UART is also responsible for reassembling data coming in through the serial port and converting it back into
a parallel stream. The UART chip 16550 is found in all of todays ports. One of the major new features of the 16550 is that it has a 16-byte buffer and uses the first in, first out (FIFO) buffering technique. This means that the 16550 can continue to receive incoming characters and store them in the buffer while the CPU is busy handling other tasks. Then, when the CPU turns its attention back to the UART, it can pass on the entire contents of its buffer to the CPU for processing. Since it can continue to work while the CPU is busy, the 16550 has brought major performance gains in serial communications.
The chip was an improvement over the previous version, the 16450, but in the early 16550s
there were various limitations and bugs that can bottleneck the performance of serial devices.
Since, the 16550 has improved and released as the 16550A, 16550AN, and then the 16550AFN.
First in, first out (FIFO) is a method of processing a queue in which items are removed from the queue in the same order in which they were added.
The first item in is the first item out.
The USB Port
The Universal Serial Bus, using a single cable, allows connecting keyboards, mice, joysticks, scanners, printers, monitors, telephones, modems, ISDN modems, and an assortment of other devices to
a computer. 127 devices can theoritacally be daisy chained together from a single port, since each length of USB cable can be as long as
15 feet.
The USB channel sends more than just data through the cable. It also supplies a 5-volt power line to the peripherals connected to it.
Compared to parallel and serial ports, USB ports are very fast and has two data
speeds. 1.5 Mbs for devices such as keyboards, mice, and joysticks and 12Mbps for
devices such as scanners, printers, monitors, and modems. Almost the speed of a 10BaseT Ethernet
network. When used with an operating system that supports USB, devices can be added and removed
without powering down or reconfiguring the computer. When a new device is added or removed, the system automatically detects the change and then loads or unloads the appropriate driver. |
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