A+ Essentials Tutorial

1.0 Personal Computer Components

  • 1.1 Identify the fundamental principles of using personal computers
    • Identify the names, purposes and characteristics of ports and cables for example: USB 1.1 and 2.0, parallel, serial, IEEE 1394 / firewire, RJ45 and RJ11, PS2 / Mini-DIN, centronics (e.g. mini, 36) multimedia (e.g. 1 / 8 connector, MIDI, Coaxial SPDIF)

Cables and Connectors

Serial Ports:

The serial port is an Asynchronous port which transmits data one bit of data at a time. Serial port hardware usually consists of a UART (Universal Asynchronous Receiver/Transmitter).

Most commonly used serial ports are given below:

1. DB9:

DB9 adheres to the RS-232c interface standard. It has 9 pins as shown in the figure. The connector is "D" shaped, and easy to recognize. The function of each pin is described below.

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Outline Diagram of DB-9

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DB-9 Female Connector

Pin Description:

Pin # Pin Description
Pin 1 Data Carrier Detect DCD
Pin 2 Received Data RxData
Pin 3 Transmitted Data TxData
Pin 4 Data Terminal Ready DTR
Pin 5 Signal Ground Gnd
Pin 6 Data Set Ready DSR
Pin 7 Request To Send RTS
Pin 8 Clear To Send CTS
Pin 9 Ring Indicator RI

2. DB25:

DB25 adheres to the RS-232C interface standard. It has 25 pins as shown in the figure. The connector is "D" shaped, and easy to recognize. DB-25 is normally used in older computers, and not much used in modern day computers.

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3. RJ-11:

RJ-11 is a 4-wire connector, commonly used with a modem. It should not be confused with bigger RJ-45 cable and connector. RJ-45 is commonly used for Ethernet network interface card (NIC).

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Schematic of RJ-11 Connector

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An RJ-11 Cable with Connector

Pin # Function
A1 Ground
A2 Rx (Data Input)
A3 Tx (Data Output)
A4 Vc (Power)

4. RJ-45:

An RJ-45 connector has 4 pairs of wires as shown in the schematic diagram below. Note that an RJ-11 is a 4-wire connector, where as RJ-45 is an 8-wire connector.

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RJ-45 connector schematic

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RJ-45 connector crimped to a cable.

RJ-45 connector is commonly used for Ethernet Networking ports. Devices that normally use RJ-45 ports include NICs, Hubs, Switches, and Routers.

There are basically two types of cables. One is Straight-through cable, and the other is Cross-over cable. Straight-through cables are used for connecting a network device to a work station. Cross-over cables are used for connecting a hub to a switch or a hub to another hub.

1. USB

USB stands for Universal Serial Bus. The most important features of USB bus include the following:

  • Plug'n'Play: USB is a truly plug-n-play port. The port is automatically detected by the system, and its ready to use.
  • Hot plug and unplug: It is possible to insert an USB device and and unplug without affecting the functioning of a computer. You dont need to power-off the computer to remove or insert a USB device.
  • 127 physical devices: A USB can support up to 127 devices. This is much more than what is normally required.
  • Low cost cables and connectors

USB connectros can be broadly divided into USB A, and USB B. The difference between the two is in the physical layout of pins in the connector. Both are shown in the figure below.

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Pin # Function
Pin 1 +5V DC
Pin 2 Data-
Pin 3 Data+
Pin 4 Ground

USB"A" plugs are used towards the host system and USB "B" plugs are used towards the USB device.

2. FireWire/IEEE1394

FireWire/IEEE1394 port provides data rates up to 400 Mb/sec. The standard is most suitable for transferring high volumes of information including video, and voice data.

Given below are some of the important features of IEEE 1394 standard:

  • Based on open licensing system.
  • Digital transmission with data rates of 100, 200, or 400 mega bits per second (Mb/sec).
  • Plug and play: Automatically detected by the operating system during start-up or during usage.
  • Hot pluggable: Cables can be connected and disconnected without turning off power to the computer.
  • Flexible: You can connect FireWire cables in a daisy-chain or in branching cable configurations.
  • Peer-to-peer functionality
  • Compatibility, lower speed USB devices can be connected with higher speed devices.
  • Supports both asynchronous (Based on request-and-acknowledge form of computer communication, used typically with dial-up modems). and isochronous data transfer.( Continuous, guaranteed data transmission at a pre-determined speed. Useful for video and audio).

4 PIN IEEE1394 (without Power)

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A IEEE 1394 Connector schematic

Pin Function
Pin #1 Twisted-pair B, differential signals, TPB-
Pin #2 Twisted-pair B, differential signals, TPB+
Pin #3 Twisted-pair A, differential signals, TPA-
Pin #4 Twisted-pair A, differential signals, TPA+

6 PIN IEEE1394 (with Power):

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IEEE1394 6-pin connector schematic.

Pin # Function
1 Power, 18-28v no load.
2 Ground
3 Twisted-pair B, differential signals, TPB-
4 Twisted-pair B, differential signals, TPB+
5 Twisted-pair A, differential signals, TPA-
6 Twisted-pair A, differential signals, TPA+

Parallel Ports:

1. DB-25

DB-25 connector is most commonly used in conjunction with a parallel printer. It has an 8 bit data bus as shown in the figure below.

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Pin # Function
Pin 1 Strobe
Pin 2 Data Bit 0
Pin 3 Data Bit 1
Pin 4 Data Bit 2
Pin 5 Data Bit 3
Pin 6 Data Bit 4
Pin 7 Data Bit 5
Pin 8 Data Bit 6
Pin 9 Data Bit 7
Pin 10 Acknowledge
Pin 11 Busy
Pin 12 Paper End
Pin 13 Select
Pin 14 Auto Feed
Pin 15 Error
Pin 16 Initialize Printer
Pin 17 Select Input
Pin 18 - Pin 25 Ground (return for pins 0-7)

The length of Parallel Printer cable usually limited to a maximum of 15 feet

Other Type of Parallel Ports:

Enhanced Parallel Port (EPP): The Enhanced Parallel Port (EPP) operates close to ISA bus speed and can achieve transfer rates up to 1 to 2MB/sec of data.

Enhanced Capabilities Port (ECP): The Enhanced Capabilities Port (ECP), is an additional enhanced Parallel port.

PS/2:

The PS/2 port is most frequently used for connecting a computer mouse or keyboard to a PC. The PS/2 port is a mini DIN plug and contains six pins. Below is a picture of an actual PS/2 plug and pin description:

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MIDI:

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The MIDI protocol lets computers and electronic musical instruments (like digital keyboards and sequencers) share performance data with each other.

S/PDIF

Short for Sony/Philips Digital InterFace, is a serial interface for transferring digital audio from CD and DVD players to amplifiers and TVs. S/PDIF uses unbalanced 75 ohm coaxial cable up to 10 meters with RCA connectors or optical fiber terminated with a Toslink (Toshiba link) connector.

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An S/PDIF connector cable.

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