Index
PC Ports Parallel ports: 25 Pin female, "D" connector Sends and receives 8 bits of data at a time Serial ports: 25 Pin male, "D" connector or 9 Pin male, "D" connector Sends and receives 1 bit of data at a time External ports send data in an asynchronous fashion. Keyboard: 5 Pin DIN or PS/2 Mouse: 9 Pin serial or PS/2 Video: 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 seven devices can be chained off of a SCSI port. 18 feet is the maximum length that a SCSI cable can support. Most external SCSI devices have two 50 pin female ports. Most PC's are set to SCSI ID 7. Null modem cables (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. 15 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. Back To top 2. BIOS BIOS (Basic Input Output System) - Chip that contains low level software for configuring the system's capabilities with hardware. System Assembly Basics Phillips and Flat-Head screwdrivers are the most common tools needed for disassembly. Paper should be at hand for diagramming 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 Motherboards, system boards, planar board, and main logic boards are all interchangeable terms. Older XT planar boards used DIP settings for floppy configuration. Modern AT system boards uses SETUP in the BIOS. SETUP should always be run after motherboard replacement to reconfigure the system for customers. Busses ISA 8-bit EISA 16-bit PCMCIA 32-bit PCI 64-bit IRQ (Interrupt Requests) IRQ 1 Keyboard IRQ 2(9)Video Card 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 I/O Ports COM1 3F8H COM2 2F8H COM3 3E8H COM4 2E8H LPT1 378H LPT2 278H Processors The math coprocessor deals with floating-point numeric operations. 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. Superscaler - Two chips inside: one for parallel processing and the other for fault tolerance. Instructions / clock cycle - 4 instructions, 2 on each path. MMX: 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. Cache: 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. CMOS: Contains the computer BIOS. Maintains its data with the use of a battery for periods when the machine is powered down. RAM (Random Access 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. Drives When transferring 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. 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.EXE - Program used for DOS formatting. FORMAT /S is the fastest way to make a bootable disk. The /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 Back To top 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. Monitors Dot pitch is the space between pixels in millimeters. 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 Networking 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 distances; carry light pulse signals through glass core at speeds of between 100Mbps - 200,000Mbps. Ethernet use coaxial and twisted pair wiring, and can support speeds of 10mbps -100mpbs. 10Base5 - 10 Mbps transfer rate with coaxial wire. 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 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 test fine after a boot configuration error, the most likely cause is the system board. Always carefully examine any shipped in merchandise before installation and powering the system on. Preventative Maintenance and Safety Almost all computer equipment should be cleaned with only a soft, damp cloth with a mild detergent. ESD (Electro-Static Discharge) - Prevention of having electric currents damage system components. Compressed air it most effective for removing dust because of the pressure, directability, and less 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 maintenance. 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 components 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 maintenance. For dark spots on a page with a laser printer, it indicates loose toner particles. Running extra paper through will usually resolve it. The order of processes in a laser printer is: 1) Cleaning 2) Charging 3) Writing 4) Developing 5) Transferring 6) Fusing The primary corona has the highest negative charge in a printer. Always check where the leading edge of paper is when troubleshooting paper jams, because it can indicate which printer part is causing the jam. The photoconductive drum in a laser printer is charged negatively, and loses its charge when light hits it. The transfer corona can cause the print to be too light. Dot Matrix: A tight ribbon can cause flecks and smudges on the paper. Missing or broken printhead pins causes 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 ATDL Redial ATF1 Half-Duplex Off (echo copy of receiving information) ATHN Hang up ATMN Speaker On/Off ATSR Display register ATH Hangs up modem PCMCIA Type 1 3.3mm Memory Type 2 5mm Modems, Network cards Type 3 10.5mm 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
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