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A Computer Power Supply (PSU) is vital to the operation of a computer. The Power Supply converts AC current to DC current and then sends power to all of the internal components in the computer system so they can function.

A Computer Power Supply is a metal box usually located inside the top backside of the computer case. The power supply is visible from the back of the computer.

It is easily identified by the presence of a port for the power cable. There are three typical voltages used in a power supply: 3.3 volts, 5 volts, and 12 volts. The 3.3 and 5 volt supplies are usually used by digital circuits, while the 12 volt supplies are more typically used to supply power to fans, motors, and disk drives.

The main specification of a power supply is in watts. Most PC’s today use a push button switch on the front of the computer case to power up the computer. This push button sends a 5 volt signal to the power supply letting it know it is time to send power to all of the internal computer components. To shut the computer down most computers have a “shut down” option located in a menu bar. When this is used the operating system sends a signal to shut the computer down. The Power supply also has a 5 volt circuit of “standby voltage”, known as VSB. This circuit is used so even when the computer is turned off, the push button to start up the computer will still work (enabling the computer to turn on). There are different types and styles of power supplies on the market today. Three of the basic types of desktop PC power supplies are AT, ATX and ATX-2.

AT Power Supply – Typically used in older PC’s

ATX Power Supply – Commonly used in PC’s today

ATX-2 Power Supply – New standard for power supplies today

Power supplies are easily changed and are generally cost effective. If you are going to change a power supply make sure you get one with room for expansion so you are prepared for the future.

IBM came back to the fore when color started to appear in computer displays. The CGA standard, introduced in 1981 and primative by today’s standards, was still color; even if only 16 of them. Because the first PCs were for business, the color did not first catch on and the MDA monochrome standard we more often used. As prices came down and clones of the IBM PC were introduced, CGA became more of a standard.

The CGA card came with 16 KB of video memory and supported several different modes:

Text mode which included 80×25 text (like the MDA system) in 16 colors. The resolution, however was lower as each character was made up of 8×8 pixels instead of the MDA’s 9×14 pixels. A 40×25 text mode was also supported in 16 colors. In both, the foreground and background colors could be changed for each character.

Monochrome graphics mode which displayed graphics at 640×200 pixels. This was lower than the Hercules card but seemed to serve the purpose for an initial release and this was quickly replaced with the EGA standard.

Color graphics mode which came in two flavors: a 320×200 pixel mode with four colors and a lesser-used resolution of 160×200 in 16 colors. The four-color mode only had two official palettes to choose from:

Magenta, cyan, white and background color (black by default).
Red, green, brown/yellow and background color (black by default).
The 16-color graphic mode used a composite color mode instead of the 16 colors of the CGA text above. Because the color technique was not supported in the BIOS there was little adoption of that mode except by some games.

The CGA color palette was based on the Motorola MC6845 display controller. Red, green, and blue were created by the three cathode rays with black being an absence of cathode rays. The other colors were mixes of two different colors and white used all three color beams. An “intensifier” bit gave a brighter version of the basic 8 colors for a total of 16. There was one exception to this. In the normal RGB model color #6 should be a dark yellow (#AAAA00) however IBM changed the monitor circuitry to detect it and lower its green component to more closely match a brown (#AA5500) color. Other monitor makers mimiced this which is why the intense version of #6, brown, turned out to be a bright yellow as the intense version was not so modified. There is no clear reason expressed why IBM did this but it’s speculated they wanted to match 3270 mainframe colors. So, the colors appeared as…

Color 0 – Black – #000000 and the intense version, color 8 – Dark Grey – #555555
Color 1 – Blue – #0000AA and the intense version, color 9 – Bright Blue – #5555FF
Color 2 – Green – #00AA00 and the intense version, color 10 – Bright Green – #55FF55
Color 3 – Cyan – #00AAAA and the intense version, color 11 – Bright Cyan – #55FFFF
Color 4 – Red – #AA0000 and the intense version, color 12 – Bright Red – #FF5555
Color 5 – Magenta – #AA00AA and the intense version, color 13 – Bright Magenta – #FF55FF
Color 6 – Brown – #AA5500 and the intense version, color 14 – Bright Yellow – #FFFF55
Color 6 in some clone monitors -Yellow – #AAAA00
Color 7 – Light Grey – #AAAAAA and the intense version, color 15 – Bright White – #FFFFFF (which you won’t see because the background here is white)
There were several tweaks to the CGA text and graphics systems which resulted in different default background colors, different colored borders, and other tweaks which gave the appearance of the CGA system having more than the graphic modes above; but, these were all tweaks and not changes to the basic system itself.

Refresh rate for CGA monitors was increased to 60 Hz as a result of eyestrain complaints from the MDA 50 Hz rate. (The higher the refresh rate the less likely pixels on the screen will flicker as the phosphor is refreshed at a faster rate.)

See here for more details and pictures.

But, the low resolution of CGA begged for higher resolutions. To fill those demands IBM developed EGA…

Just like the heart the motherboard supplies life to a computer, if it fails the computer will no longer function. The System board, main board or motherboard is the most important circuit board inside a computer besides being the largest it also contains the Central Processing Unit or CPU. All the devices inside a computer communicate with the motherboard. The motherboard is the PC’s center of activity. It hosts the largest single collection of chips of any PC component and serves as the “street system” for the grid of wires that link all the components, making it possible for them to communicate. The motherboard defines the computer’s limits of speed, memory, and expandability. A computer needs more than just a CPU and memory. To function properly it needs devices, such as a keyboard and a mouse. It also needs output devices, like monitors and sound cards, to cope with the powerful graphics and sound capabilities of the programs available today.

A computer also needs “permanent” storage devices, such as floppy disk drives and hard disk drives, to store data when it is turned off. It is the function of the motherboard to provide the connectivity for all these devices, as well as for the CPU, RAM, and support ICs. There are nine major components in a mother board.

1. The Central Processing Unit.

2. The chipsets Northbridge and Southbridge that support the CPU.

3. RAM random access memory to hold data and instructions.

4. Cache memory to speed up the memory accessed.

5. Communication on the system board via the BUS.

6. Slots to connect devices such as a video card, or modem.

7. BIOS which holds configuration to allow a computer to boot properly.

8. CMOS battery which acts as a backup when your computer is shutdown.

9. A power supply which enables the computer to boot and supplies power to all the devices directly connected to the motherboard.