CompTIA A+ study notes on hardware

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20. Tools - Screw driver types:

• A Flat blade screw driver has Minus slot

• A Philips screw driver has X shape slot

• A Torx screw driver has star tip or head

• A Hex screw driver has hex head

21. Most commonly used modem commands:

• ATA- Answer the phone

• ATD-Dial the phone, ATDT for Tone dialing, ATDP for Pulse dialing

• ATH-Hang up

• ATZ-Reset

• AT&F-Reset modem parameters and settings to factory defaults

• AT&W- Write the current parameter values and settings

22. General PC error codes and probable causes:

• 100-199 : System board failures

• 200-299 : Memory failures

• 300-399 : Key board failures

• 400-499 : Monochrome video problems

• 500-599 : Color video problems

• 600-699 : Floppy disk errors

• 1700-1799: Hard disk problems.

Some of the frequently encountered error codes and their corresponding error messages are given below:

23. Cable lengths:

• Serial cable maximum length is 50 feet.

• Parallel cable maximum length is 10 feet.

• Longer cable lengths may lead to some errors & garbage characters.

24. When communicating across a serial cable, the most probable cause that the screen is dumping garbled characters is that the communication settings are not correct. Check the speed, parity, start/stop bits etc. If this is all correct, then you need to check the cable. Check whether you need a straight / cross cable, and the pin connections.

25. Power On Self Test (POST) happens after the computer cold boots.

6. The following devices require periodic cleaning:

• Floppy drives

• Tape drives

• Printers

• Mouse

27. Monitors and static charge:

1. Monitors accumulate very high static charges and need to be handled very carefully. Before attempting any repair, it is important to discharge any accumulated charges on the monitor. You can use a jumper, one end of which is grounded, and touch the other end of the jumper wire to the anode of the monitor. While doing so, ensure that you are not in direct contact with the jumper wire or the anode. You can use a screwdriver, or a nose pliers with rubber handle for this purpose. A "POP" sound can be heard when the static charges accumulated on the anode lead getting grounded through the jumper wire. Static charges accumulated on monitors may lead to severe burn or even fatal, if come into direct contact.

2. Never wear a wrist strap when working on monitors. Monitors contain very high voltages, sometimes fatal to human, even when the power is turned off. If you are wearing wrist strap, the human body works as a conduit to discharge the electric charge.

28. System board connectors of a PC:

• 9-pin male connector on system board is serial and usually COM 1 (mouse)

• 25-pin male connector on system board is serial and usually COM 2 (modem)

• 25-pin female connector on system board is parallel and usually LPT 1 (printer)

29. MS –DEFRAG utility: To defragment the hard disk, you can run Microsoft defragment utility by issuing a command "DEFRAG".

30. ESD:

• It is important to know that ESD damage may not be known immediately. A component may only suffer degradation in performance, which may show up only at a later date. In some cases, an ESD may result in the failure of the component instantly.

• When working on computers, use special ESD wrist strap. Do not directly ground yourself with a piece of wire. An ESD wrist strap has built-in resistor to prevent electric shock. Use specially designed grounded ESD mats. Do not wear synthetic clothing. Place all electronic components into anti static bags. Anti static bags can be reused. Keep your workplace clean.

• As the humidity decreases, static build up will increase. A level of 50% is considered safe. Below 50% humidity, static build up will be more.

• Electrostatic discharge (ESD) can damage the component at as little as 110 volts. CMOS chips are most susceptible to ESD.

31. Uninterrupted Power Supply (UPS):

An UPS is desired to be purchased for any critical and un-interrupted use of computers. It has the following benefits:

• Provide protection against small surges

• Filters noise from entering the computer

• Provide power to the computer during line power failure

• Give stable power to computer, even when the line power is unstable.

Note that the UPS can give un-interrupted power only for a fixed amount of time under a given load, in the absence of line voltage.

32. The following are types of PCMCIA (PC cards) cards:

• Type I: 3.3 mm thick. Used for memory upgrade cards.

• Type II: 5 mm thick. Used for modem and network cards. Some are combination Modem/NIC cards.

• Type III: 10.5 mm thick. Used in PC card hard drives

33. Computer Cache Memory: Level 1 cache is internal to the processor, and level 2 cache is external to the processor, it resides on the motherboard. In case of Pentium II, L2 cache is built into the cartridge.

34. CPUs and features:

* 32-bit operating systems like Windows XP can support only 32 bits of addressing space, and hence only 4GB of memory can be used. By using 64-bit operating systems, 16 Exa bytes (EB) of RAM can be used. 36 bits of address space can access up to 64GB of memory.

CPU models 80486SX, and above contain on board cache memory.

35. Bits and Bytes:

1 KB = 1024 bytes
1 MB = (1024 X 1024) bytes 
= (1024X 1 KB) bytes 
= 1,048,576 bytes 
1 GB = (1024 X 1024 X 1024 )bytes 
= (1024 X 1 MB) bytes 
= (1024 X 1024 X 1 KB) bytes 
= 1,073,741,824 bytes 
KB stands for Kilobyte 
MB stands for Megabyte 
GB stands for Gigabyte 
Byte is represented by "B" as in MB 
Bit is represented by "b" as in kbps 
One byte is 8 bits
		

36. Printer parallel ports come in the following varieties:

• Unidirectional: Here, the data travels only from the computer to the peripheral (printer) device.

• Bi-directional: Here, the data travels both from the computer to the peripheral device and vice-versa.

• ECP (Extended Capability Port): ECP mode offers bi-directional data transfer, as well as DMA for data transfer.

• EPP (Enhanced Parallel Port): In addition to bi-directional features, it offers an extended control code set

37. Laser printers:

The following are the 6 steps in the ElectroPhotographic (EP) print process of Laser Printer:

• Cleaning: Cleaning the photosensitive drum includes residual toner left on the drum and removing the electrical charges left out on the drum. The physical cleaning is done with a rubber blade and the electrical charge cleaning is done with erasure lamps.

• Charging: The next step in printing, is to charge the photo sensitive drum with high negative charge, this is done with the help of a corona wire.

• Writing: A laser (type 3) sweeps the entire length of the drum, creating the static image of the matter to be printed. The places where the laser travel, the highly charges are neutralized. Other places of the drum, it remains highly negatively charged.

• Developing: Now drum gets in close proximity to the toner. Because the toner is negatively charged, it gets attracted to the areas where the drum is neutral. It will not be attracted to the places where the drum is highly negatively charged. Thus the image of the page to be printed formed on the photosensitive drum.

• Transferring: Now, the toner on the drum gets attracted toward the paper, by using highly positive charges developed on the surface of the paper. The "transfer corona" is used to generate highly positive charge on the paper surface and to attract the toner from the drum. Thus the image of the page to be printed formed on the paper. But still, the toner is loose and can get easily smeared.

• Fusing: In order to permanently bond the toner particles to the paper, the paper is passed through rollers. One of the rollers, the non stick roller is heated by a high intensity lamp, generating the heat necessary to bond the toner to the surface of the paper.

Some of the frequently encountered problems using laser printers and probable causes are as given below:

• Speckled pages: The causes for this may be

  • The failure to clean the drum after printing properly, or

  • The drum might have developed scratches.

• Blank pages: The causes for white pages may be, A. The toner would have dried out, replace the toner. B. The transfer corona, that is responsible for transferring the toner to the drum might have failed. C. The High Voltage Power Supply (HVPS) failure will also result in white pages.

• Ghosted Images: Ghosting occurs when previously printed pages are printed again, though much lighter than the present image. The most likely cause is that the erasure lamp might not be working properly, thus leaving some charges representing the earlier image left on the photosensitive drum before new image is written. Also check the cleaning blade, which is responsible for scaping the residual toner.

• Smudged images: If the fusing fails, the toner will not bond with the paper. Check the halogen lamp responsible for heating.

38. Impact printers are capable of printing multipart forms, since they can give necessary impression to print to multiple forms simultaneously. ECP (Extended Capability Port) has less control overhead and best suited for transferring large chunks of data, such as between the computer and laser printer.

39. The interface cables used widely have the following pin count:

• floppy-34 pin,

• IDE-40 pin,

• SCSI-50 pin,

• SCSI Ultra wide-68 pin

40. One good way of determining a bad power supply is that the fan will not rotate. Also, the computer will not boot and the LED s indicating the speed will be OFF.

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