PC Ports
Parallel ports:
  • 25 Pin female, "D" connector
  • Sends and receives 8 bits of data at a time
  • Sends data synchronously

Serial ports:

  • 25 Pin male, "D" connector or 9 Pin male, "D" connector
  • Sends and receives 1 bit of data at a time
  • Sends data asynchronously


  • DIN-5 or PS2


  • 9 Pin serial or PS2


  • HDA= 15 pin female, 3 rows of pins. (Current VGA and SVGA only)

Cables and Connectors
Tape, removable, hard, and optical drives along with scanners can all use SCSI connections.

Up to eight devices (including the controller) can be chained off of a SCSI port. (SCSI id 0-7)

18 feet is the maximum length that a SCSI cable can support.

Most external SCSI devices have Centronics-50 or female DB-25 connectors utilizing a male-to-male SCSI cable (although there are newer 68 pin connectors and other connectors as well).

Most SCSI Host Adapters must be set to SCSI ID 7.

Null modem cables or serial cables are used to transmit data between 2 DTE devices..

Because of interference, you have to reduce transfer rates the longer a cable is.

50 feet is the maximum length that a serial cable should be.

Phone lines (few network) cables use RJ11 or RJ12 connectors. Connector resembles a small phone jack.

Twisted pair cables use RJ45 connectors. Connector resembles a fat phone jack.

COM Ports
COM1 and COM3 use IRQ4.

COM2 and COM4 use IRQ3.

The majority of PC's have only 2 COM port connectors.

BIOS (Basic Input Output System) - Built-in software that contains low level software for configuring the system's capabilities with hardware.

System Assembly Basics
Phillips and Flat-Head screw drivers are the most common tools needed for disassembly.

Paper should be at hand for diagraming connections.

Egg cartons are helpful in keeping screws organized.

Expansion boards should be removed by gently rocking back and forth at each end.

Red and blue stripes on ribbon cables indicate pin 1.

Field replaceable units (or FRU's) are any computer parts that can be replaced without special equipment such as soldering irons.

System Board Repair
Mother boards, system boards, planar board, and main logic boards are all interchangable terms.

Older XT planar boards used DIP settings for floppy configuration.

Modern AT system boards use SETUP in the BIOS.

SETUP should always be run after motherboard replacement to reconfigure the system for customers.

ISA 8-bit or 16-bit
EISA 32-bit
PCMCIA 16-bit
AGP 32-bit
PCI 64-bit (Most implementations are 32-bit)

IRQ (Interrupt Requests)
IRQ 0 System Timer
IRQ 1 Keyboard
IRQ 2(9) Video Card or cascade to IRQ 9
IRQ 3 Com2, Com4
IRQ 4 Com1, Com3
IRQ 5 Available (Normally LPT2 or sound card )
IRQ 6 Floppy Disk Controller
IRQ 7 Parallel Port (LPT1)
IRQ 8 Real-time clock
IRQ 9 Redirected IRQ2
IRQ 10 Available
IRQ 11 Available
IRQ 12 PS/2 Mouse
IRQ 13 Math Coprocessor
IRQ 14 Hard Disk Controller
IRQ 15 Available (often used for second Hard Disk Controller)

I/O Ports
LPT1 378H
LPT2 278H

Floating-point numeric operations are dealt with by the math coprocessor.

The 80386SX uses the 80387SX as its fastest coprocessor, and has a 32-bit word size and 16-bit data path.

The 80386DX uses the 80387DX as its fastest coprocessor, and has a 32-bit word size and data path.

The 80486SX has a disabled coprocessor, and 32-bit word size and data path.

A 486DX2's external clock speed is half of the internal clock speed.

A 486DX4's external clock speed is 1/3 of the internal clock speed.

Protected mode is a processor feature that allows 2 or more programs run without interfering with one-another.

  • Superscalar - Two chips inside; one for parallel processing and the other for fault tolerance.

    Instructions / clock cycle - 4 instructions, 2 on each path.


      1) 57 instructions for manipulating video, audio and graphic data
      2) SIMD: Single Instruction Multiple Data
      3) More build-in cache on chip

    Dynamic Execution (Pro):

      1) Multiple branch prediction Predicts when instructions are to be processed
      2) Dataflow analysis Analyzes instructions
      3) Speculative execution

    Pentium II System Bus allows multiple simultaneous transactions.

  • SEC - (Pentium II) Single Edge Cartridge design. Core and L2 Cache in cartridge, designed for single and dual processor computers.


    • L1 - Internal Cache
    • L2 - External Cache

    CPU Connectivity:

    • Socket 7 - Pentium CPUs
    • Socket 8 - Pentium Pro 387-pin ZIF contact connector
    • Slot 1 - Pentium II 242 SEC contact connector

  • Memory
    ROM (Read-Only Memory):
    • ROM is a form of non-volatile memory.
    • Contains both POST and SETUP.

    NVRAM (Non-Volatile Memory):

    • Can maintain data without the use of power.


    • Contains the computer setup data used by BIOS.
    • Maintains its data with the use of a battery for periods when the machine is powered down.

    RAM (Random Acces Memory):

    • RAM is volatile memory and does not retain data without power.
    • RAM contains any active application, including the operating system.

    HMA (or high memory area) is the first 64K of extended memory.

    Conventional memory is the first 640K of memory.

    Upper memory is the memory between 640K and 1024K. Used to load DOS drivers to allow applications more conventional memory.

    Extended memory is the memory above 1024K.

    Expanded memory is addressed in pages of 16K.

    The suspend mode of portables often causes problems with the expanded memory manager.

    When transfering data, copying data from one drive to another is the best method of data protection.

    A cluster is a group of sectors.

    Sectors are aligned in tracks.

    The seek time is the time that is takes the head to reach the needed track.

    The latency period is the time that it takes the sector to move under the head.

    The overall time it takes a hard drive to find data is the access time.

    The data transfer rate is how fast the hard drive sends data to the PC.

    The BIOS in older systems may not support large hard drives, and must be upgraded before installation. With these BIOSes, you can upgrade (flash or replace) the ROM Bios chip, replace the motherboard, buy an EIDE controller card, or use DDO (disk drive overlay) software to support larger hard drives. The BIOS limit for older controllers was 504 megabytes (sometimes listed as 528 MB because 504 megabytes is 528,482,304 bytes).

    The physical or low level format is rarely needed.

    To prepare a hard disk for use, setup a partition on the disk, format the partition, then load the OS.

    FDISK and PART (found on older versions of DOS) are the programs used for setting up partitions.

    Always back up hard drives before formatting.

  • FORMAT.COM - Program used for DOS formatting.

    FORMAT /Q performs a quick format.
    FORMAT /S switch tells FORMAT to copy system boot files to the disk.

    Physical drives are actual separate drives while logical drives concern partitions on the physical drives.

    To write protect a 3.5 floppy, uncover the hole in the upper-right hand corner of the diskette.

    To write protect a 5.25 floppy, cover the notch on the side of the diskette.

    Computers detect HDD (high-density) 3.5 floppies by the hole in the upper left corner.

    FDD controllers use DMA channel 2 in most cases.

  • HDI = Head to Disk Interference

  • Controllers
    Controllers match speeds between sending and receiving devices, convert data between formats, and isolate the software from the hardware.

    A DTE device in general, is a device such as a computer or printer.

    A DCE device is a device such as a modem.

    A terminal sends the Ready To Send signal when it transmits to a DCE device such as a modem.

    A modem must have the Carrier Present signal before it is able to receive.

    RS232 is the standard for serial connections.

    DMAs (Direct Memory Access) work with the CPU, letting devices put data into memory rather than the CPU, therefore helping to speed transfer rates.

    The picture element (PIXEL) is the smallest point that can be controlled on a monitor display screen.

    The refresh rate is the number times display is redrawn every second.

    CGA 640 x 200
    EGA 640 x 350
    VGA 640 x 480
    SVGA 1024 x 768

    Coaxial, twisted pair, and fiber optic cables are all used in networking.

    A token ring network passes packets of data called tokens to each station in a network.

  • LAN - Local Area Network
  • WAN = Wide Area Network
  • MAN = Metropolitan Area Network
  • Fiber-Optic - Cables designed for high transfer rates over large disances; carry light pulse signals through glass core at speeds of between 100Mbps - 1Gbps.

    Ethernet can use coaxial and twisted pair wiring, and can support speeds of 10mbps - 100mpbs.

  • 10Base5 - 10 Mbps transfer rate with thick coaxial cable.
  • 10Base2 - 10 Mbps transfer rate with thin coaxial cable.
  • 10BaseT - 10 Mbps transfer rate, baseband transmission, with twisted pair wire.

  • 100BaseT - 100 Mbps transfer rate, baseband transmission, with twisted pair wire.

  • Troubleshooting
    When replacing a power supply, pay special attention to the physical characteristics, the voltage, and that the connectors are standardized.

    Rom addresses, I/O addresses, IRQs, and DMA channels can all cause conflicts.

    IRQ conflicts are the most common conflict, because there are so few IRQs compared to the many peripherals that can be installed into a system.

    If a battery tests fine after a boot configuration error, the most likely cause is the system board.

    Always carefully examine any new components before installing them and powering the system on.

    Preventative Maintenance and Safety
    Almost all computer equipment should be cleaned with a soft, damp cloth and a mild detergent.

  • ESD (Electro-Static Discharge) - The discharge of static electricity from skin or tools into computer components. Even the smallest static charge can damage components. This is prevented by being properly grounded before touching components.

    Compressed air is most effective for removing dust because of the pressure, directability, and small chance of ESD damage.

    Line analyzers are used for detecting line surges, sags, and spikes.

    Hard drives should be defragmented and backed up often.

    Hard drives should be protected from both magnetic fields and temperature extremes.

    Never lubricate the printhead pins of a dot matrix printer during preventative maintenence.

    Never wear a wrist strap while working on a monitor CRT because the stored voltage is capable of killing a person.

    ESD will degrade a component's integrity. Although not immediately seen, the effects could show up months later.

    Any amount of friction is enough to cause enough static to cause ESD.

    To avoid ESD use a wrist strap, and always ship items in ESD safe bags.

    Nylon, plastic, wood, and vinyl are not ESD safe materials.

  • Printers
    Laser Printers:
      The ozone filter needs to be replaced on laser printers when performing preventative maintenence.

    Dot Matrix:

      A tight ribbon can cause flecks and smudges on the paper.

      Missing or broken printhead pins cause incomplete characters.

      Never lubricate the printhead on a printer.

      If the print density is erratic, then there is probably an error with the ribbon advance.

    Modem Commands
    Command Function
    ATA Answer
    ATD Dial
    ATZ Reset
    ATF1 Half-Duplex Off (echo copy of receiving information)
    ATH0 Hang up
    ATH1 Off hook
    ATM0 Speaker Off
    ATM1 Speaker On
    ATSR Display register
    ATH Hangs up modem

    Type 1 3.3mm Memory
    Type 2 5 mm Modems, Network cards
    Type 3 10.5 mm Hard Disks

  • Socket Services - BIOS level software interface for hardware information.

  • Card Services - I/O, IRQ hardware interface with computers.

  • CIS (Card ID Structure) - Cards describe themselves to other devices.

    PCMCIA cards use very little power and can be hot-swapped.

  • Power Supplies
  • Yellow: +12

  • Blue: -12

  • Red: +5

  • White: -5

  • Voltage for motor: +/- 12 volts

  • Voltage for circuitry: +/- 5 volts