Motherboard:  The motherboard ("MB") is the main electronic circuit board in the computer, to which all other components are connected.  It holds the CPU, RAM memory, expansion cards, and has connectors into which component cables are connected (i.e., cables for the CD-ROM, hard drive, floppy, etc.).  Motherboards come in two basic "form factors", or shapes:  AT - which fit inside AT cases, and ATX - which fit in ATX cases:

PHOTOS:   AT socket MB from above, AT socket MB from side, ATX slot MB from side, ATX slot MB from above, 486 MB from side, 486 MB from above (Note: 486 motherboards are old, and basically obsolete.  They are included here for the sake of completeness, since they may still be found in use), ATX slot MB from above #2, ATX slot MB from side 2

The motherboard is secured to the bottom of a desktop case, or to the side of a tower case.  Mounting threads are screwed into the case so that they align with holes in the motherboard.  The motherboard is secured with screws, which are kept from touching the motherboard by small round insulators (usually colored red).

PHOTOS:  MB screw and insulator closeup, screws and insulators

Sometimes, plastic mounting screws or brackets are used instead of metal screws.  It is very important that the screws are insulated, and that the motherboard be mounted securely - so that it is not touching the metal of the case.  Motherboards which touch the case metal can be electrically shorted to that point that they work improperly or are destroyed.

VIDEOS:  AT Motherboard install, ATX MB install, ATX MB install #2

PHOTOS:  AT MB in case, ATX slot MB in case, AT MB mounting brackets, AT MB mounting brackets in case

The MB receives power from the case power via a standard AT or ATX connector:

PHOTOS:  AT power cables, AT power cables #2 (Note: the two black wires in AT power supply cables should always be next to one another when plugged into the MB!!! - see video below), AT power supply connector on MB, AT power supply plugged in, AT power supply plugged in #2, ATX MB power connector, ATX power supply connector on MB, ATX power connected, ATX power connected #2

VIDEOS:   AT power supply plug in, AT power supply plug in 2 (Note: the two black wires should always be next to one another when plugged into an AT Motherboard!!!),  ATX power supply connect, ATX MB power connector install, ATX power cable install (loose)

The motherboard contains "Bus" slots into which expansion cards (video card, modem, network adapter, etc.) are connected.  Older expansion cards fit into "ISA" bus slots, which are usually colored black.

PHOTOS:  ISA Card, ISA PCI and AGP cards, ISA PCI and AGP connectors, ISA card connector

VIDEOS:  ISA card install (ATX case), ISA card install CLOSE UP, ISA card install (AT case), ISA, PCI, and AGP connectors, ISA card install ATX #2.

Most computer cases come with the expansion card slot openings covered with a small piece of metal.  In order to install a card, you may need to break off that piece of metal in order to expose the opening and install the card (metal pieces which have been broken away may be discarded). 

VIDEO:  back panel slot breakout

Most modern motherboards have a number of  "PCI" bus slots which hold newer, faster PCI type cards.  PCI slots are typically colored white, and are smaller than ISA connectors.

PHOTOS:  PCI card connector, ISA PCI and AGP cards, ISA PCI and AGP connectors

VIDEOS:  PCI card install ATX, PCI card install AT, ISA, PCI, and AGP connectors

Finally, most new motherboards also contain 1 "AGP" slot, which is reserved particularly for AGP video cards.  They are typically colored brown, and are smaller than PCI slots.

PHOTOS:  AGP card connector, ISA PCI and AGP cards, ISA PCI and AGP connectors

VIDEOS:  AGP Card install - bare MB, ISA, PCI, and AGP connectors

NOTE:  When installing expansion cards, be absolutely sure to put the card in the right type of slot!  Installing a PCI card into an ISA slot, for example, can destroy the system hardware!

VIDEO:  BE CAREFUL that you install cards in the correct slots!

Motherboards also contain the connectors for keyboards, mice, printers, speakers, USB, serial, and other devices which hook up to jacks on the back of the computer (i.e., joysticks, cameras, and other peripherals).  ATX motherboards typically contain all of these devices, with the connectors built right into the motherboard:

PHOTOS:  Back panel connections, ATX case back panel , AT case back panel, Back panel connections #2

VIDEOS:   back panel KB connector, back panel mouse AT-SERIAL, mouse PS2 back panel (NOTE: The mice which plug into ATX motherboards, like the one in this video, are sometimes called “PS/2” mice), printer cable back panel, KB AT + ATX back panel (plug in ATX) (NOTE:  AT Keyboards are approximately 1/2 inch wide - ATX keyboards and mice (somtimes called PS/2) are approximately 1/4 inch wide.  Take a look at this video to see the difference in size (the ATX keyboard is the one being plugged in), USB back panel, speaker cable back panel, back panel all plugged in, back panel monitor, joystick into back panel, phone cable into modem back panel, USB cam back panel connection

AT motherboards always contain at least a keyboard connector.  Serial and parallel devices often have a cable running from a pin pad on the motherboard to a connector which screws into an opening on the back of the case.  In some systems, all such connectors are added as expansion cards:

VIDEO:  AT peripheral connectors (NOTE: this is one of the longest videos in this set of lessons.  ATX motherboards have ALL of these connectors built in - if you want to avoid having to do ALL of that work when setting up a MB, buy ATX).

Speaker wires and wires which lead to LEDs on the outside of the case (to show the user that the power is on, the hard drive is running, etc.) are labeled, and should be connected to the pins on the motherboard which have matching labels.

PHOTOS:  Case LED connectors AT MB, Case LED connectors AT MB - plugged in, Case LED connectors on MB, Case LED connectors ATX

VIDEO:  Case LED MB connectors, Case LED connect ATX