Computer Basics

Link to What You See 10 - What You See: On the Back

Link to Home - Jan's Illustrated Computer Literacy 101

 External devices connect to the computer in the back. The diagram below shows the most common connections. But the arrangement of these is quite varied from machine to machine. The name-brand computers often have unique designs with special connectors for the peripherals that are sold with the computer.

diagram of back of computer


Note that many of the connectors come in two types: male and female.
The male has the pins while the female has the holes.

Male serial connector Female serial connector
Male Female

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JegsWorks > Lessons > Computer Basics

  1. Computer TypesArrow: Subtopics
  2. ApplicationsArrow: Subtopics   
  3. InputArrow: Subtopics
  4. ProcessingArrow: Subtopics  
  5. OutputArrow: Subtopics
  6. StorageArrow: Subtopics
  7. Computer to ComputerArrow: Subtopics
  8. System SoftwareArrow: Subtopics
  9. ProgrammingArrow: Subtopics

Link to What You See 10. What You See Arrow: Subtopics list open
    On the Front:
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    On the Back:
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    On the Inside:
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     Power Protection
  11. Hands On!Arrow: Subtopics
  12. On Your OwnArrow: Subtopics




Power Power plugs

The power cord connects the computer to electrical power. It is a thick, round cable with a three-prong plug on one end and a three-hole plug on the other.
Here is a User Warning.If you are ever working on the inside of the case, be SURE that this cord is disconnected, else you might find out what it feels like to stick your finger in an electrical socket. Zap!

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Fan Fan grill on computer back

The fan that you can see on the back of the computer is not a connection, but it is critical to the health of your computer. It cools the power supply.

There is at least one other fan inside the computer, which keeps air flowing inside the case to remove the heat that all this processing generates. If things get too hot inside the casing, the CPU will fail to calculate accurately. You will get wrong answers, the wrong commands will be executed, there may be unpredictable crashes of your programs. This could be annoying or disastrous, depending on exactly what happens. If you have trouble only after the computer has been on for awhile, you can put HEAT on the top of your list of suspects.

Here is a User Warning.Never block the vent holes in your computer's case. Blow the dust off the blades from time to time.
Noise: When your computer first starts up, most of the noise you hear comes from the fans.
Here is a User Warning.If you ever fail to hear a fan running, don't operate the computer until it is fixed or you'll be risking serious damage from heat.

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Keyboard PS/2 keyboard portPS/2 keyboard cableKeyboard cable

The keyboard plugs into the computer with a round connector, which can only fit one way. Recent keyboards may use a PS/2 connector, which is smaller than the old style keyboard connector. The standard color coding scheme uses purple for the keyboard connector and port, to make it easy to find the right port for each connector. Not all manufacturers use the standard scheme.

PS/2 keyboard portPS/2 keyboard connectorHere is a User Tip.The barrel of the connector usually has a mark or channel or a flat area to show where the "top" of the plug is. That spot should match the "top" of the computer-side port. In tower cases the "top" is not toward the top of the case when it is in use, but toward the "top" as it is laying open for being worked on.

WarningConnecting or disconnecting a device with a PS/2 connector can cause a power surge to your motherboard, which can ruin it. The damage will not be visible, but it will definitely not be repairable.
Here is a User Tip.Some cases make it a little hard to get the keyboard plugged in firmly. When computer boots, it checks for the presence of a keyboard and will not continue if it can't find one. If this happens while your keyboard is plugged in, first shut down the computer and unplug the keyboard, then plug it up again. Try to be sure that the plug is fully seated. Then reboot.
Here is a User Warning!Keyboards can fail, especially after a session with spilled liquids or crumbs. So keep such away from your computer working area. Your keyboard may recover from a swimming session after it dries out if the liquid did not leave anything behind like sugar or tea leaves.
Heavy use will eventually wear out the electrical contacts in the keyboard, as in any electrical device.

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PS/2 Mouse PS/2 mouse port

Newer mice use a PS/2 connection instead of a serial connection. The port and the connector are light green, if the manufacturer has followed the standard color scheme.

WarningConnecting or disconnecting a device with a PS/2 connector can cause a power surge to your motherboard, which can ruin it. The damage will not be visible, but it will definitely not be repairable. (I didn't get this warning myself until AFTER I discovered it by expensive, accidental experiment!)

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USBUSB connector

USB ports on the back of a computerThe Universal Serial Bus will soon be used for nearly all peripherals instead of the variety you see in the diagram. The computer chip on the main board can automatically recognize any USB device and assign the resources and power that it needs. This avoids the hair-pulling sessions that commonly go with the installation of a new device.

A USB device can be connected or disconnected at any time without having to shut down or reboot the computer.

A USB device can send data at 12 Mbps for devices like scanners and printers or at 1.5 Mbps for keyboards and joysticks.

If you connect a hub to the USB port on the back of the computer, you can then connect up to 127 other USB devices to the hub. They will have to share power and resources, of course, but many can work at the same time. No more problems with running out of connectors!

Most USB cables are 5 meters long (16.5 feet).  This length allows for the signal to get through properly. If you need a longer cable, you can hook up several 5 m. cables and some USB hubs in a chain - up to 25 meters.

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Serial Ports

Serial ports come in two sizes, 9 pin and 25 pin. The computer-side connector will be male. (Older video types use the female 9-pin type.) Often there will be one of each size showing in the same slot on the back of the computer. Notice that the connector has angled sides so that the plug can fit only one way. Many devices use a serial port, including the computer mouse and external modems. A serial port sends data one bit at a time.

PS/2 mouse portThere is another kind of port that newer mice used, called a PS/2 port or mouse port. A mouse port should be colored green. Purple is for the keyboard.

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Parallel Port Parallel portParallel cable

[Becoming obsolete] A parallel port is used primarily by printers. Scanners and external storage devices of many types also connect to the parallel port. USB connections are replacing the parallel port.

A serial port sends data one bit at a time while parallel ports can send 8 bits at a time. The parallel port uses a 25 pin female connector.

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Video Video connector with screws video connector pins

The monitor cable plugs into a port on the video card with places for 15 pins, but there are not 15 pins on the cable connector. The sides of the plug are sloped so there is only one way to insert the plug.

Digital monitors have a different connector without pins and require a digital video card. 
Here is a User Tip.The video connector seems to be easier to knock off than the other connectors. There are screws on either side to fasten it down. Keeping it fastened down will protect the pins in the plug from getting bent. It is easy to bend the pins by pushing too hard when the pins and holes are not quite lined up. You may think that you have a good connection. If the color is not right on the monitor after you've connected it back up, you have probably bent the pin that carries the instructions for red. It seems to be the one bent most often.
Here is a User Tip.You can straighten a pin that is out of alignment by carefully using a small flat blade of a knife or screwdriver to move the pin back in place. If it's really kinked, take hold of the pin with needle-nosed pliers and gently straighten it. Be VERY gentle. You don't want to get into the problem of replacing the video plug.

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Sound Card  Headphones

A sound card has holes (ports) for connecting a microphone, speakers or headphones, and an outside sound source with a single prong plug. There is also a serial port for connecting devices like musical keyboards and synthesizers.
Recent devices are color-coded to help you match the connector to the correct port. Audio line IN is blue,  microphone is pink, Audio line OUT (speakers) is lime green.

Some sound cards do not have the plug-in holes colored or even marked as to which is which. If you can't find the documentation that came with the sound card, you'll have to experiment to see which one your speakers go in.
Here is a User Tip.Once you figure it out, mark the holes with fingernail polish or something so you won't have the experience of working for hours to "fix" your sound when the only problem is that the speakers are plugged into the wrong hole. (Personal experience is talking here!)
Some sound cards have a volume control wheel but others rely on software volume controls. Some kinds of speakers have volume control knobs or slides.
Here is a User Tip.If you want a manual control and your speakers don't have one, you can buy a device that you can reach easier than the back of the computer and faster than on-screen volume controls. It doesn't seem to have a particular name. The speakers plug into this simple device which is basically a knob to turn. Then the device plugs into the sound card's hole for speakers. Stick the knob device onto the side of the monitor or your desk and you're all set.

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Modem telephone connector telephone connector

An internal modem has connectors for phone lines, both "in" and "out". The "in" line runs from the telephone wall outlet. The "out" line runs from the computer to another device, usually either a FAX machine or a regular telephone.

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Network Network cable

Network cable may be a coaxial cable. It's round, insulated, and has a single wire in the middle. There is a collar to screw down to make the connection firm.

A network patch cable is round but the connector is similar to a telephone connector but wider.

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Last updated: 30 Apr 2012

Power plugs Fan Serial ports Serial ports Parallel port Parallel port Sound card Sound card Video card Video card Network connector Network connector Modem Modem Keyboard connector Keyboard connector PS/2 connector USB connector