Cluster Nodes - Page 5

2.2 Motherboards and System Busses

In this section the different choices for motherboards will be given. Distinctions between workstation and server chasis will be presented. This will include an introduction to system busses. Most of the detailed information will be about the PCI bus. The two next generation of buses that will replace PCI will also be introduced.

2.2.1 Motherboards

The motherboard is the main circuit board inside the PC which holds the processor, memory and expansion slots and connects directly or indirectly to every part of the PC. It is made up of a chipset, some ROM code and various interconnections known as buses. The physical layout of the motherboard itself varies greatly from PC to PC, two different boards can have very similar performance even though they might be laid out completely differently. This is more true because of the large number of vendors available who manufacture a variety of motherboards. But the basic function of the motherboard is to provide a useful working place for all the components of the PC. The following sections give a brief overview of the basic functionality and layout of the motherboard.

Motherboard Form Factors
The form factor of the motherboard describes its general shape, the kind of power supply used, its physical organization and the kind of cses it uses. The two most common form factors in motherboards are the AT and the Baby AT form factors. These two forms differ mainly in the width, the older AT board being 12" wide, while the Baby AT board is 8.5" wide and nominally 13". The AT form is the much older version and is usually found in older machines (386 or older). Another troublesome feature of this board is that a good percentage of the board overlaps with the drive bays which makes installation and upgrading difficult and cumbersome. For the Baby AT form. the reduced width allows much less overlap with drive bays. IT has three rows of mounting holies, the first running along the back of the board where the bus slots and key connectors reside, the second running through the middle of the board and the third along the fron of the board near to where the drivers are mounted. One problem with the Baby AT is that many of its newer versions try and reduce cost by reducing the board size (for example 10" to 11" long). This often leads to mounting problems as the third row of holes might now line up with rows on the case. Both the AT and Baby AT form factors places the processor sockets, slots amd memory sockets at the front of the motherboard and long expansion cards were designed to extend over them. This design was introduced over a decade ago. However presently the processors need bigger heat sinks and fans mounted on them, the result is that the processor, heat sink and fan combination can often block as many as three of the expansion slots on the motherboard. Besides there are also SIMM/DIMM sockets. Although the newer Baby AT motherboards move the SIMM/DIMM sockets out of the way but the processors still remain a problem. The ATX was designed to solve this problem.

ATX and Mini ATX form factors
The ATX form was invented by Intel in 1995. The Pentium Pro and Pentium II are the most common users of this kind of motherboards. The ATX has many advantages over the older motherboards which include:

• Integrated I/O Port Connectors: Baby AT motherboards use headers which stick up from the board, and a cable that goes from them to the physical serial and parallel port connectors mounted on to the case. The ATX has these connectors soldered directly onto the motherboard. This improvement reduces cost, saves installation time, improves reliability (since the ports can be tested before the motherboard is shipped) and makes the board more standardized.
• Integrated PS/2 Mouse Connector: On most retail baby AT style motherboards, there is either no PS/2 mouse port, or to get one you need to use a cable from the PS/2 header on the motherboard, just like the serial and parallel ports. (Of course most large OEMs have PS/2 ports built in to their machines, since their boards are custom built in large quantities). ATX motherboards have the PS/2 port built into the motherboard.
• Reduced Drive Bay Interference: Since the board is essentially "rotated" 90 degrees from the baby AT style, there is much less "overlap" between where the board is and where the drives are. This means easier access to the board, and fewer cooling problems.
• Reduced Expansion Card Interference: The processor socket/slot and memory sockets are moved from the front of the board to the back right side, near the power supply. This eliminates the clearance problem with baby AT style motherboards and allows full length cards to be used in most (if not all) of the system bus slots.
• 3.3V Power Support: The ATX style motherboard has support for 3.3V power from the ATX power supply. This voltage (or lower) is used on almost all newer processors, and this saves cost because the need for voltage regulation to go from 5V to 3.3V is removed.
• Soft Power Support: The ATX power supply is turned on and off using signalling from the motherboard, not a physical toggle switch. This allows the PC to be turned on and off under software control, allowing much improved power management. For example, with an ATX system you can configure Windows 95 so that it will actually turn the PC off when you tell it to shut down.

LPX and Mini LPX
The primary design goal behind the LPX form factor is reducing space usage (and cost). This can be seen in its most distinguishing feature: the riser card that is used to hold expansion slots. Instead of having the expansion cards go into system bus slots on the motherboard, like on the AT or ATX motherboards, LPX form factor motherboards put the system bus on a riser card that plugs into the motherboard. Then, the expansion cards plug into the riser card; usually, a maximum of just three. This means that the expansion cards are parallel to the plane of the motherboard. This allows the height of the case to be greatly reduced, since the height of the expansion cards is the main reason full-sized desktop cases are as tall as they are.
LPX form factor motherboards also often come with video display adapter cards built into the motherboard. If the card built in is of good quality, this can save the manufacturer money and provide the user with a good quality display. However, if the user wants to upgrade to a new video card, this can cause a problem unless the integrated video can be disabled. LPX motherboards also usually come with serial, parallel and mouse connectors attached to them, like ATX.

NLX form factor
The need for a modern, small motherboard standard has lead to the development of the new NLX form factor. In many ways, NLX is to LPX what ATX is to AT: it is generally the same idea as LPX, but with improvements and updates to make it more appropriate for the latest PC technologies. Also like ATX, the NLX standard was developed by Intel Corporation and is being promoted by Intel. Intel of course is a major producer of large-volume motherboards for the big PC companies. NLX still uses the same general design as LPX, with a smaller motherboard footprint and a riser card for expansion cards.
The NLX form factor is, like the LPX, designed primarily for commercial PC makers mass-producing machines for the retail market. Many of the changes made to it are based on improving flexibility to allow for various PC options and flavors, and to allow easier assembly and reduced cost. For homebuilders and small PC shops, the ATX form factor is the design of choice heading into the future.

2.2.3 PCI Bus

PCI or Peripheral Component Interface is a 32 bit bus architecture (64 bit with multiplexing) developed by DEC, IBM, Intel and others, that is widely used in Pentium bases PCs. A PCI bus provides a high bandwidth data channel between system board components such as the CPU and devices such as hard disks and video adapters. The PCI superseded the VL-bus which as widely in use till the early 1990s. The essential purpose of introducing the PCI bus was to make expansion easier to implement by offering plug and play (PnP) hardware, i.e. a system that would enable the PC to adjust automatically to new cards as they are plugged in, thus making redundant the need to check jumper settings and interrupt levels. By 1994 PCI was established as the dominant Local Bus standard.

Unlike the VL-bus, which was essentially an extension of the bus that the CPU uses to access the main memory, the PCI is a separate bus isolated from the CPU but having access to the main memory. Besides the VL-bus was designed to run at system bus speeds, whereas since the PCI bus is linked to the system bus through special bridge circuitry, the speed of the PCI bus can be set synchronously or asynchronously depending on the chipset and the motherboard. In a synchronous setup (used in most PCs), the PCI bus runs at half the memory bus speed, which is usually 25 or 30 or 33 MHZ. In an asynchronous setup the speed of the PCI bus can be set independent of the memory bus speed, controlled through jumpers on the motherboard or BIOS settings. The PCI is also limited to five connectors, although each can be replaced by two devices built into the motherboard. It is also possible for a processor to support more than one bridge chip. The PCI is more tightly specified than the VL-bus and offers a number of additional features. For example it cab support cards running from both 5 volts and 3.3 volt supplies using different key slots to prevent the wrong card being put into the wrong slot.

In its original implementation the PCI ran at 33MHz, but was then raised to 66MHz by the later PCI 2.1 specification. As a result the theoretical thoroughput was increased to 266 MBps. The PCI can also be configured both as a 32 bit and a 64 bit bus and both kinds of cards can be used as well in either configuration.