computer ports and how they work
Post on 12-Nov-2014
DESCRIPTIONA Quick guide to computer ports and how they function.
A guide to Ports This is a quick guide to computer ports and how they function. I/O Ports The Input/Output (I/O) ports are used to connect various devices to the computer and hence enable communication between the device and the computer. These ports are typically present at the backside of the computer. They are clearly marked. Example: External Modem is connected to serial ports; Printers are connected to parallel ports. These ports help in transmitting the data from the device to the computer and vice versa. Each port has a specific way of communicating and this depends upon the speed and size of the port. Example: Serial, Parallel and USB. In the following lesson, you will learn to identify these ports and list their functionality. Serial Port In a serial port, the communication takes place through a single line or a circuit. The data is transferred one bit at a time from sender to receiver. The serial port takes 8, 16, or 32 parallel bits from the computer bus and converts it as an 8, 16, or 32 bit serial stream. The name serial communication comes from the fact that each bit of information is transferred in series from one location to another. Serial communications are of two types: Synchronous and Asynchronous. Synchronous: In this type, the sending and receiving ends of the communication are synchronized using a clock that precisely times the period separating each bit. By checking the clock the receiving end can determine if a bit is missing or if an extra bit (usually electrically induced) has been introduced in the stream. Asynchronous: In this type, the bit stream is marked to help keep track of each bit. The start bit indicates the start of the data stream and the timing of each bit helps in finding the position of each bit. Here, timing is not an important aspect but the speed of the ports should be the same. Serial Cable There are different types of serial cables. However, we will discuss about the two most common types for our purposes which are
null-modem cables and standard (straight) RS-232 cables. Null Modem Cables A null-modem cable passes some signals, like signal ground, straight through, but switches other signals. For example, the send data pin on one end goes to the receive data pin on the other end RS-232 Cable All the RS-232C signals are passed straight-through in a standard serial cable which means the send data in on one end of the cable goes to the send data pin on the other end. This is the type of cable that is used to connect a modem to the system, and is also suitable for some terminals. It is an accepted industry standard for serial communication connections. Adopted by the Electric Industries Association, this Recommended Standard (RS) defines the specific lines and signal characteristics used by serial communication controllers to standardize the transmission of serial data between devices. The letter 'C' here signifies the third in a series. DB-9 A DB-9 connector is a 9 pin serial connection which is mostly used on laptop and desktop computers, as well as token ring connections . The DB-9 connector is commonly used for the 1st serial port (COM1). The serial ports are called "COM ports" and are labeled as COM1, COM2, etc. The COM1 on a PC, is typically DB-9 type connector and is commonly connected to the mouse. DB 9 Female connector These connectors are used either with shielded or un-shield cables with up to nine signal wires. However, all the pin positions may not be used. This is typically used for connection to nine-pin serial ports on PCs. DB-9 Male Connector: These connectors are used either with shielded or un-shielded cables with up to nine signal wires. Here again, all pin positions may not be used. Its typical usage is for extension cables. DB-25 Most of the Serial ports follow a standard called the RS-232 specification. RS-232 defines the meaning of the different serial signals and their respective pin assignments on a standard 25-pin (DB-25) serial connector. DB-25 connectors can either be male, with the pins sticking out, or female, with matching holes. Often there is very small number marking for the pins or holes 1 through 25. The female connector is just a mirror image of the male, so
that pins and holes match. RJ-11 The standard telephone line connector that is plugged into a jack in the wall to receive phone service uses RJ-11 connectors. This connector is also used to connect computers to internet or private networks. It is the standard connector used on 2-pair (4-wire) UTP (Unshielded Twisted Pair) and flat-satin untwisted cable. It is also the standard connector used in telephone wiring. The RJ-11 connector actually has 6 connector positions. However, only 4 are normally wired and in many cases only 2 are utilized. RJ-45 RJ-45 (Registered Jack - Type 45) connectors look almost like standard telephone connectors. They house eight wires as against four wires in a telephone connector. The actual concept was that the central two pins would be one pair, the next two the second pair, and so on until the outer pins of an eight-pin connector would be the fourth twisted pair. Also, signal shielding was optimised by alternating the "live" and "earthy" pins of each pair. This pattern for the eight-pin connector resulted in a pinout where the outermost pair are then too far apart to meet the electrical requirements of high-speed LAN protocols.
Parallel Ports Parallel ports are probably the easiest to use and work with as they are fast and mostly problem free. Parallel ports have remained unchanged for a long period of time but they have extended their capabilities to exchange data faster and more reliably. The functioning of a parallel port takes place by sending its information in parallel signal wires. By sending the signal through 8 separate wires faster communication can be achieved than with a serial port that send its information through 2 wires. Theoretically, it looks like the parallel port would be 8 times faster, but in practice, the increase in speed is much greater, since there is no wait times for encoding and decoding of the serial signal. Simple parallel ports typically send their data at 115,200 bits per second and new enhanced parallel ports will go up to 100 times faster. DB-25 Connector:
The original parallel port in a computer was a 36 pin interface developed by Centronics, a printer manufacturer. This was the standard port for connecting printers to the computer. However, when IBM introduced its personal computer it used a new parallel port which was called DB-25. DB-25 has 25 pins and this is the standard port on any IBM compatible computer. Now, the Centronics interface is used only on printers. A special printer cable is needed to connect printer to the computer and this is available as a standard accessory. Parallel Connector DB25 Strobe: A strobe line is the heart of the parallel port which tells the printer as to when to sample the information of the data lines. Usually it is high and it goes low when a byte of data is transmitted. Data: These 8 lines carry the information to be printed along with special printer codes which sets the printer in different modes like bold, italics etc. Acknowledge: This lets the computer know that the character was successfully received and that it's been dealt with. Busy: This pin is set to high when the printer receives the data from computer and is processing it. Once the printer processes the byte, this pin is set to low indicating the computer to send data. Paper End: This line will go high when you run out of paper, similar to the paper out light on the printer. This is the way by which the computer will know and lets you known of the problem. Select: When the select line is at high, the printer is online which means it is ready to receive data. When it's at low the computer will not send any data. Error: This is a general error line. There is no way of knowing the exact error from this line. Ground: This is the regular signal ground which is used as a reference for the low signal or logical 0.
IEEE 1394(firewire) IEEE 1394 is a high-speed, serial input/output port for connecting computer peripherals and consumer electronics. IEEE 1394 has two types of ports they are IEEE 1394A and IEEE 1394B. IEEE 1394A is capable of transferring data at speeds of up to 400 megabits per second and IEEE 1394B is capable of transferring data at speeds of 800
megabits per second. It is the fastest external input/output bus available and can support up to 63 devices. It is optimal for bandwidth-hungry devices such as digital video cameras and external storage devices. IEEE 1394 also has the added advantage of delivering data both asynchronously and isochronously. In asynchronous mode data transfers can be interrupted but in isochronous mode data is transferred without interruption. While transferring data asynchronously, IEEE 1394 allocates bandwidth to devices as needed. When one device needs to communicate with another, it sends out a signal. There is a possibility that the devices may get the bus with the desired bandwidth or they might have to wait before getting the requested bandwidth. However, this procedure doesn't work for applications such as streaming video. Here, if another device requests space on the bus, the video data stream will be interrupted which results in degrading the image quality. To overcome this, IEEE 1394 offers isochronous mode, which guarantees that video and similar devices consistently get the bandwidth they require. IEEE 1394 is a personal computer and digital video serial bus interface standard that offers high-speed communications and isochronous real-time data services. Its capabilities are sufficient to support various high-end digital audio/video applications such as the consumer audio / video device control and signal routing, home networking, nonlinear DV editing and 32-channel (or more) digital audio mixing. FireWire 400 is capable of transferring data between devices at data rates of 100, 200, or 400 Mbit/s. Within this six-pin cable, two shielded twisted-pair wires transmit data while another acts as a ground and supplies power. Data is sent along the twisted pairs in packets that contain both the data and the addressing information. The 1394 protocol achieves its fast data-transmission rates by reducing noise along the wires, that allows the signal to move along the line quickly and without any error. However, in order to support these high speeds, the cable can only be 4.5 meters long. The Standard bus interconnections are made with a 6-conductor cable containing two separately-shielded twisted pair transmission lines for signaling, two power conductors and an overall shield. The two twisted pairs are crossed in each cable assembly in order to create transmit-receive connection. The power conductors (8 to 40 v, 1.5 a max.) supply power
to the physical layer in isolated devices. PS/2 Mini Din The PS2 port was developed by IBM for interface keyboards and pointing devices. It uses synchronous serial signals to communicate between the keyboard and mouse to the computer. All the signals are TTL logic level voltages i.e., 0 volts for logical 0 and +5 volts for logical 1. It also supports bi-directional communications. When a PS2 mouse sends its information it must send 3 consecutive data packets in a row. Each packet sent has different information i.e., for button pressed, movement and direction of movement. PS/2 PinSIGNAL NAME 1Data 2Reserved 3Ground 4+ 5 Volts 5Clock 6Reserved Data: Mouse data packets / Keyboard scan codes are sent serially from the mouse or keyboard to the computer on this single wire. Clock: This signal is sent from the mouse or keyboard to synchronize the data signal. Ground: This is a common ground signal used as a return path for data and is a reference to logical 0.
Infrared Port In the early 1800's Sir William Hershel discovered Infrared. He found that on moving a thermometer across the spectrum of colors, the heat would increase towards red. As he continued to move and when the thermometer passed the red, the heating continued to rise. This area is what we call the Infrared or Below Red. Today the Infrared port is also commonly referred to as the Infrared Data Association (IRDA). This is a port much alike the one we would find on a TV/VCR remote. We need to configure only the Infrared ports on both source and destination and there is no need to physically connect the ports using wires. Infrared port provides high-speed digital exchange through the typical PC UART/Serial port at 9600-115200 bits/s, and in some cases it is compatible to high-speed extensions of up to 1Mb/s and 4Mb/s.
USB A single USB host is capable of connecting 127 devices, either directly or through USB Hubs. Individual USB cables can run as long as 5 meters; with hubs and devices it can go up to 30 meters in length. A USB cable has two wires for power (+5 volts and ground) and a twisted pair of wires to carry the data. On the power wires, the computer can supply up to 500 milliamps of power at 5 volts. Devices such as Mouse which need low-power can draw their power directly from the bus. High-power devices such as Printers have their own power supplies and hence they draw minimal power from the bus. Hubs too can have their own power supplies to provide power to devices connected to them. The devices that are connected to a USB port depend on the USB cable to carry power and data. The USB standard uses A and B connectors to avoid confusion. A connectors head upstream toward the computer. B connectors head downstream and connect to individual devices. By using different connectors on the upstream and downstream end, makes it impossible to ever get confused. Example, if you connect any USB cable's B connector into a device, you know that it will work. Similarly, you can plug any A connector into any A socket and be sure that it will work too. USB is also faster than ports on older technologies such as serial and parallel ports. Transfer Rates: USB 1.1: Up to 12Mbps USB 2.0: Up to 480 Mbps A USB system has an asymmetric design which consists of a host controller and multiple devices connected in a tree-like fashion with the use of special hub devices. 127 devices per host controller can be connected. These signals are transmitted on a twisted pair of data cables, labeled D+ and D-. These collectively use half-duplex differential signaling to combat the effects of electromagnetic noise on longer lines. D+ and Doperate together and are not separate connections. USB supports three data rates. The first one is data rate of 1.5 Mbits per second which is used to connect devices like mouse, keyboard etc. The second one has a data rate of 12 Mbits per second and the devices use the entire bandwidth with this connector which operates on first come
first serve basis. The third one, called the USB 2.0 has a Hi-Speed rate of 480 Mbit per second. The advantage with USB 2.0 is that it supports low speed USB devices by reducing its bandwidth.
Created by BobbyR1234 on 4/5/08 and uploaded to scribd.