Notes on Assembling a Basic "AT" Computer

Notes On Assembling A Basic "AT" Computer

Revision 1.2
05 Nov 2004
Bob Parker
Markup altered so the page could be viewed without horizontal scrolling.
Errol Hoffmann and Mark Holloway
Revision 1.1
24 April 2003
Marked up to HTML by Mark Holloway.
Revision 1.0
10 October 2002
Original document created by Errol Hoffman.Table of Contents

Components

1. Motherboard
2. Memory
3. Central Processing Unit (CPU)
4. Cooling Fan
5. Ports Serial (COM1, COM2), printer, keyboard
6. Display Adaptor (video card)
7. Hard Disk Drive
8. Floppy Disk Drive
9. CDROM
10. Power Supply
11. Local Area Network card (LAN card)

COMPONENTS

1. Motherboard.

Motherboards of different brands and vintages are all constructed differently and it is necessary to look carefully at the board in order to find the correct settings. There are two basic types of motherboard, the "AT" format and the "ATX" format. The AT is the older format and is the type you will most commonly encounter at Computerbank. This format has connectors for the external ports on the motherboard, but requires cables going to the back plane of the computer case. The ATX format has most of these ports hard wired to the motherboard and these project through the rear of the computer case.

The motherboard is the central part of the computer and has "chips" that control the whole operation of the computer, such as reading and writing information to the floppy and hard disk drives, sending information to the video display, sending information in and out of a local area network or modem and so on.

Two types of long sockets are usually on the AT motherboards. The black sockets are "ISA" sockets and the white shorter sockets are "PCI". These are simply different types of controller sockets for various types of devices such as video cards, local area network cards, sound cards and internal modems. Most of the network cards we have available at Computerbank are of the ISA variety, whereas most of the video cards will use a PCI socket.

2. Memory.

There are two types of memory in common use; these are (i) 72 pin "EDO" memory and (ii) 168 pin "SDRAM". Older motherboards tend to use just EDO memory; more modern motherboards use SDRAM. Some motherboards have slots for both types of memory, but usually, only one type can be used at any one time. EDO memory MUST be used in matched pairs, that is, you cannot put in a single "stick" of memory; there must be two sticks (minimum) of the same type. SDRAM can be used with a single stick.

Memory of both types can be inserted into the memory slots in only one direction. If the memory does not go in easily, do not apply excessive force, but try turning the memory the other direction. EDO memory is inserted at an angle and then rotated until it "clicks" into place; SDRAM is inserted vertically into its slot. Look at the spacing of the cutout in the bottom of the memory stick to see the direction in which it should be inserted.

At Computerbank, most of the memory you will encounter will be of the EDO variety. Each computer assembled is to have a minimum of 32 MB of random access memory (RAM). It is not always easy by inspection of the memory sticks to determine their capacity, only by starting up the computer is it possible to see how much memory has been inserted as the memory check is performed.

3. Central Processing Unit (CPU).

The CPU is the component of the computer that controls its whole operation. These are a fragile component and must be treated carefully to avoid bending of the pins and damage due to electrostatic charges. On the motherboard you will see a white square socket (usually) that is most commonly labelled "SOCKET 7". Again this is the most common form that you will see on computers donated to Computerbank. There are other types such as "SLOT 1" or "SOCKET 370", but these are fairly rare at Computerbank and require CPUs different to those used in SOCKET 7 motherboards.

CPUs have a given speed rating, that is, the speed at which information is processed by the computer. At Computerbank, we attempt to have a minimum speed of 133 megahertz (MHz), although this is not always possible. The settings of the computer motherboard must correspond to those of the CPU, if the motherboard settings are faster than the CPU can handle, then the computer will not operate. On the motherboard, you will generally find three settings that are relevant to the CPU. The first of these is the voltage that the CPU operates at. This is usually set by a series of "jumpers" or a "dip switch". It is important that the correct setting be made so that the CPU does not overheat. The next two settings are to set the speed of the CPU. You will find a switch or set of jumpers that sets the basic speed of the motherboard (the "front-side-bus" speed FSB). This is usually determined by the speed capability of the memory. Most of the memory you will use here is OK at 66 MHz, so the FSB speed should be set at this value. It is then necessary to set the "multiplier" ratio so that the product of the FSB speed and the multiplier is about equal to the CPU speed. For example, if you have a 133 MHz CPU and the FSB speed is 66 MHz, then you would select a multiplier of 2, since 2 x 66 = 132.

4. Cooling Fan.

CPUs generate heat and if this heat is too great, the CPU will be destroyed. Thus a cooling fan is used along with fins to dissipate this heat. Do not start a computer without a cooling fan in place and operating. An old computer may work for some time without cooling, or with just cooling fins; a modern computer CPU will burn out in matter of seconds if it is not adequately cooled. Some of the older CPUs have a fan attached to the chip, more recent CPUs require a fan to be clipped onto the CPU socket. You will see the attachment brackets on the CPU socket for this purpose.

5. Ports Serial (COM1, COM2), printer, keyboard.

These are the sets of pins on the motherboard that have cables attached to them that allow access to the external parts of the computer. In the AT format motherboards, it is necessary to attach cables from the motherboard to the rear face of the computer case. These cables are quite specific for each purpose. The first serial port ("serial" is a description of the way in which data is handled by the interface) is normally used by the mouse and the computer operating system will automatically detect the mouse at this location. The port COM2 is normally used for an external modem. There is also a port for the printer connection that must be installed, although we do not supply printers to clients. The keyboard is plugged into the keyboard port, which is in a standard location on all AT motherboards. The pins must be in the "downwards" position, relative to the motherboard, for insertion.

Cables for the serial and printer ports come in various combinations and it is necessary to select a combination that will give all ports. For example, sometimes the cable has a COM1 and printer together on the one assembly; at other times it is combined with a COM2.

6. Display Adaptor (video card).

The computer will not function at all without a video card attached and will generally give a series of "beeps" if the computer does not find an operational card. This is the component that organises and sends information to the monitor so that you can see what the computer is doing. The video card generally has its own "driver" which is a piece of software that organises the data and its presentation on the screen. The operating system (Linux) will generally detect the video card and install the correct driver. This is not always the case and it may be necessary to go the Internet and download an appropriate driver from the manufacturer's site.

7. Hard Disk Drive.

This component stores two major types of information. (i) The operating system LINUX in the case of the computers supplied by Computerbank and (ii) the programs used by the operator, such as the word-processor, data base, spread sheet, games etc. The capacity of the HDD is expressed either in megabytes (MB) or gigabytes (GB). Here "bytes" is a term used to describe the way in which data is coded on the disk; "Mega" means millions and "Giga" means thousands of millions. At Computerbank, we attempt to have a minimum of 1.5 GB for all supplied computers. This is not always possible with a single HDD, so sometimes the capacity is made up with two HDDs.

8. Floppy Disk Drive.

This is a standard component that uses a three and a half inch disk with a capacity to store 1.44 MB of information. It is commonly used during installation of some programs and for backing up information, such a word-processing files.

9. CDROM.

Not all computers supplied by Computerbank have CDROMs installed; it depends very much on availability as these tend to have a limited life.

10. Power Supply.

The power supply provides the necessary power to the motherboard and to the Floppy, Hard and CD drives.

11. Local Area Network card (LAN card).

For ease of installation of the operating system, the computers we build up at Computerbank all have a LAN card installed. This allows computers to be connected and to "talk" to each other.

Connecting the Components

Here we will learn how the motherboard is connected to each of the components. There is only one way in which this can be done so that the computer will work!

1. Power Supply.

With the AT power supply there are two cables, each with 6 wires, that must be attached to the motherboard. These two cables are attached to a single male plug on the motherboard that has 12 pins in a row. The way in which the cables are attached is critical, so that the correct voltages are applied to the motherboard at the appropriate places.

The plugs can only go into the socket in one way, that is, with the short projections against the vertical wall of the socket. It is absolutely essential that the two sets of black wires are next to each other when the two halves are inserted into the socket, thus there will be an orange wire at one end of the socket and a red wire at the other end of the socket.

The power supply for devices such as the various drives, cannot be confused as they can only be inserted in one way. The small connector is used for the floppy drive, the larger for the HDD and CD drives.

2. Hard Disk Drive.

On the motherboard you will find two connectors labelled ID1 and ID2. These, apart from the ISA and PCI slots, are generally the longest of the connectors on the motherboard. You will also see that the pins on these connectors have a label at one end or the other. The HDD cable should be plugged into the IDE1 socket. It is necessary to locate the end that is labelled "1", so that the cables can be inserted into these sockets in the correct direction. There are 40 pins in this socket and hence 40 pins in the cable that attaches the motherboard to the HDD. Insert the cable so that the RED side is at the PIN1 end. The cable is then plugged into the rear of the HDD so that the RED side of the cable is next to the power plug.

3. Floppy Disk Drive.

The floppy cable goes from the floppy socket on the motherboard to the rear of the FDD. It is essential that the twisted end of the floppy cable is at the drive end. Unfortunately there does not appear to be any fixed standard for the direction of the cable at the floppy drive. Some manufacturers have the red side near the power supply cable; others have this side at the other end. There is however a simple way of determining if the connection is correct; if the FDD light stays on
when the computer is booted up, you know that the direction is wrong. Simply plug it in the other direction!

4. CDROM.

Again the cable on the motherboard is inserted into IDE2 with the red side at the "1" pin end. The connection at the CDROM is then with the RED at the power cable end. An alternative to using a second cable is to use the same cable as the HDD, with a second connector. If you do this, it is necessary to have the HDD set as "primary-master" and the CDROM as "primary-slave" (see 6 below for details).

5. Cooling Fan.

Plug the cooling fan into one of the spare power connectors.

6. Setting the HDD and CDROM as "Master" or "Slave" in Primary and Secondary IDE ports.

If you are using both the IDE1 and IDE2 ports for the HDD and CDROM, then set the jumper connectors on the rear of the drives to the "Master" position (sometimes the jumpers for the HDD is on the underside of the drive). If however you are using a single cable for the HDD and CDROM, set the HDD to "Master" and the CDROM to "Slave". If you are using two HDDs, these are usually placed on the one cable and set to "Master" for the drive that contains the Operating System and "Slave" for the second drive. The CDROM would then be placed on the second IDE cable and could operate as either "master" or "Slave". In each case you would have to ensure that the set-up of the BIOS is such that the drives would be recognised.

Fitting the Motherboard into a Case

There are two types of cases, a flat "desktop" and a "tower". In each case the installation of the motherboard is similar, although it may be simpler in the case of the tower case, where the rear plate is removable. The base of the case will have several brass posts plus a series of slots for attachment of the motherboard. It is necessary to determine which of the posts and slots are relevant to the particular motherboard being used. The posts are used to firmly attach the motherboard with screws. The slots are to receive the plastic feet that are plugged into the motherboard. These are inserted into the motherboard only where there is an appropriate slot on the base to receive the foot. In some places it is necessary to provide a plastic foot just to give support against the base, where there is no slot for the foot. We shall demonstrate all this by dismantling and then reassembling a computer.

When the motherboard is attached within the case, the power supply cables to the motherboard and drives can then be attached. Following this, the connecting cables to the rear of the case for the COM and printer ports can be attached. Finally, the video and LAN cards are plugged into the ISA and/or PCI slots (as appropriate to the cards being used) and attached by screws to the rear of the case.

Inside the case at the front are a bundle of wires with plugs, some of which need to be connected to the motherboard. These are labelled (in very small print) RST for reset, HDD (or IDE/LED) for the HDD activity LED (light emitting diode), SPKR for speaker, plus several others. These three are however the only really useful ones. There are corresponding connections on the motherboard to which the plugs on these wires are attached. Note that the labelling and placement can be confusing. If you are unsure, ask a person more experienced at computer assembly.

This should complete construction of the computer and it is now ready for setting up and testing.

Do not close the computer case at this time as it may be necessary to make changes to cable connections and drive settings.

Connecting the External Cables (Power), Keyboard, mouse and monitor

The external connections now need to be made. These are (i) the keyboard is connected via the plug at the rear. (ii) The mouse is connected into the COM1 port (iii) the monitor is connected to the video card connector (iv) the 230 volt AC is connected to the rear of the power supply (v) a special cable is connected from the power supply to the monitor power. This connector ensures that the monitor is switched off when the computer power is turned off. Not all power supplies have this extra output and it may be necessary to have a standard power cable for the monitor.

Making the Bios Settings

Switch on the computer after you have checked all connections. Ensure that the FDD light does not stay on; if it does then switch off and reverse the FDD cable at the floppy drive end.

The computer will go through a memory check, you will see the amount of memory that is in the computer. If there is insufficient, then switch off, find further memory sticks and add these.

The computer will then note the drives that are in the computer; ie. it will list the contents of Primary Master, Primary Slave, Secondary Master and Secondary Slave.

You will see at this stage a note at the bottom of the screen telling you to press "delete" if you wish to change any of the settings. If the drives you expected to see are not shown, then press "DELETE" and a blue screen with a number of options will appear. This gives the BIOS contents and will allow you to specify the way in which you have set up the computer. The BIOS tells the computer on startup what to do prior to loading of the operating system.

Press "ENTER" and tab down to the information about drives. The simplest way to go at this stage is to use the Page Up and Page Down keys to put "AUTO" at both the first and last positions of the line. This will mean that the computer will attempt to automatically detect all the devices that are connected to the Primary and Secondary IDEs in either the Master or Slave positions. Leave this menu by pressing the sequence Esc, F10, Y, and the computer will re-boot with the new settings.

When you re-boot at this stage, the computer, after going through memory check and detection of drives, will give a summary of what it has found in the computer; things such as the amount of memory, drives and so on. Press "Pause" if you want to check this out and then "Enter" to continue with the boot.

At Computerbank, we use a program called "KILL DISK" that is inserted into the FDD. If this operates correctly, you can be reasonably sure that the computer is working OK and the HDD will be wiped of all information as a security measure for the previous owners of the HDD. The computer information is then filled out by you on several forms, the larger of which is attached to the computer. The necessary information about the video card and LAN card is usually obtained from the information printed on the card. For example, the video card may have a main chip that is labelled "S3 Trio64". This is the information recorded on the sheet. The machine is then passed on to the software people for installation of the operating system and other software.