Hardware components of the PC
The System Board
Other Types of Motherboard

The motherboards described are examples of the normal motherboards fitted to machines, or available for purchase, they are not intended to be representative of any specific motherboard.
There are other versions of these motherboards:

• Baby AT - There are smaller versions that have less Input/Output (I/O) slots called the Baby AT (BAT). This type of board has the processor at the front and the longer expansion cards extend over it. This all right for some of the early processors, but as the speed of processors became faster they required more and more cooling, which meant larger heatsinks and bigger fans. This restricted the length of the cards that could be fitted into the I/O slots. Some of these types of boards had ATX power supply connections and fitted into some of the ATX cases.
• LPX - This type of motherboard is a variant of the BAT that has a central socket for a daughterboard called a "Riser" to be fitted that contain the I/O slots. The expansion cards sat parallel with the motherboard. The reason for this was to lower the profile of desktop machine to give them a slimline look. The riser card with its attached cards does restrict the airflow somewhat and additional fans are generally required.
• Mini-ATX - This is simply a smaller version of the ATX motherboard. A Mini-ATX board will not fit into an AT case due to its positioning of the peripheral ports.
• NLX - This is an improved version of LPX design of low-profile desktop machines. The design also had an emphasis on ease of maintenance.The power supply connector, expansion cards and peripheral connectors are all fitted on to a riser card that fits with an edge connector to the rear of the main board.

Risers

Risers are boards that can accept I/O ports at right angles to the normal plane of the motherboard. Many riser boards will connect via a single socket on the motherboard and provide ISA and PCI slots that are parallel to it. In this way the height of a desktop computer unit can be reduced. Some recent technology has provided the functions of several expansion cards on to one riser card, which can also provide a slot equivalent to the slot it takes up on the motherboard. In the late 1980s a specification called Audio Modem Riser (AMR) was introduced which added an audio and modem chipset on to a riser card. The main problem with this specification was that it did not support the Plug and Play feature. The concept of using the riser architecture is to implement more function for a machine whilst taking up less I/O slots. There are 2 specifications, ACR and CNR, which are rivals for this type of riser card at the moment. They both incorporate the same features of audio, modem, Local Area Network (LAN), and xDSL (different types of Digital Subscriber Line technologies). CNR does this with standard riser architecture while ACR uses the same ideas as AMR by using riser architecture in a PCI slot giving it an open-standards context. ACR has provision for wireless networking, while CNR has provision for USB and could well support wireless networking via this. Which standard may become popular will depend on the motherboard manufacturers and how costly it is to implement