CompTIA® A+ Hardware Exam Cram Notes : Modem Commands, Laptops, Dos Errors Codes

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21. Most commonly used modem commands:

The following are most commonly used modem commands:

Modem commands-Command function

  • 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:

Error Code Error Message
161 CMOS battery failure: Replace the CMOS battery
164 Memory size error : If the error occurs after memory upgrade, run SETUP program.
201 Memory test failed : RAM chips failed, one or more may need to be replace.
301 Keyboard error: You may have to check the key board

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.

26. The following devices require periodic cleaning:

  • Floppy drives
  • Tape drives
  • Printers
  • Mouse

27. Monitors and static charge:

  • 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.
  • 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.

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