CompTIA® A+ Hardware Exam Cram Notes : Irq, Connectors, And Scsi Type

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11. Key board connectors:

Key boards come with two types of connectors. They are:

1. DIN-5, DIN-5 has 5 pins and used with AT style keyboards.

2. Mini DIN-6, also known as PS/2 connector, has around port with 6 pins, one of which being a square pin used for alignment.

12. It is important to know IRQs, and IRQ/ IO conflicts.

IRQ Standard Device Assignment I/O Port Address
0 System timer 40Hex
1 Keyboard 60Hex
2 Cascade to IRQ9. Can't be used.
3 COM ports 2 and 4 COM4: 2E8-2EF COM2: 2F8-2FF
4 COM ports 1 and 3 COM3: 3E8-3EF COM1: 3F8-3FF
5 Parallel Port LPT2. Very often used for sound cards. LPT2: 278-27F
6 Floppy drive controller 3F0-3F7
7 Parallel Port, LPT1 LPT1: 378-37F
8 Real time clock 70Hex
9 Unassigned (Also redirected from IRQ2)
10 Available
11 Available. SCSI adapter will usually use this IRQ.
12 Available
13 Math co-processor. F0Hex
14 Primary hard-drive IDE controller Primary Hard Drive Controller: 1F0-1FF
15 Secondary hard-drive IDE controller. Secondary Hard Drive Controller: 0170-0177
Monochrome Graphic Adapter 3B0-3BF
Color Graphic Adapter 3D0-3DF

To determine the COM port assignments, or which COM ports are being used for what, you can use any of the following commands:

1. MODE command

2. DEBUG command

3. MSD

4. Corresponding device applet in the control panel

13. AT Computer interrupt controllers:

An AT computer will have two interrupt controllers. The second interrupt controller need to deliver the interrupts through the primary interrupt controller. IRQ2 had been identified for this purpose on the primary and IRQ9 on the secondary interrupt controllers. In otherwords, IRQ2 and IRQ9 are cascaded.

14. Serial Interfaces:

COM1, COM2 on a computer uses serial cable. The pin assignments are shown below. The computer acts as a DTE(Data Terminal Equipment) and the Modem acts as DCE (Data Communication Equipment).

Signal Name DB9 Pin
RX Received Data 2
TX Transmitted Data 3
DTR Data Terminal Ready 4
GND Ground 5
DSR Data Set Ready 6
RTS Request to Send 7
CTS Clear to send 8

Normally, a DTE device connects to a DCE device. If you want to connect two DTR devices, as you would do to hook up two computers via the serial line to exchange files, the cable itself must have cross connections. Such a cable is called null modem cable.

15. RJ - cables:

1. RJ-11: These connectors are used to link modem / phone to the phone line. They have only two pins that get into the modem.

2. RJ-14: RJ-14 connectors are dual-line phone jacks that can accommodate up to 2 telephone line.

3. RJ-45: RJ-45 cable is widely used for attaching UTP cable in LAN environment. These connectors have eight pins.

16. SCSI Types:

SCSI Type Transfer speed bus
SCSI-1 5MBPS 8 bit bus
Fast Wide SCSI 20MBPS 16 bit bus
Wide Ultra SCSI 40MBPS 16 bit bus
Ultra2 SCSI 40MBPS 8 bit bus
Wide Ultra2 SCSI 80MBPS 16 bit bus
Ultra3 SCSI or Ultra 160 160MBPS 16 bit bus
Ultra320 320 MBPS 16 bit bus

SCSI ID - 0=bootable drive, 7=controller, 1-6=any other devices

Wide-Ultra SCSI - 16 devices, 0=bootable drive, 15=controller

17. Each device in a SCSI chain need to have unique ID. For a 16-bit Wide SCSI, there are 16 possible SCSI Ids, 0-15. A standard 8-bit SCSI can support only 8 devices (including SCSI card), the Ids allowed are 0-7.

18. SCSI bus termination:

If the termination is not done, a SCSI devices on the bus will not function properly. This is due to reflection of the signals at the end of the bus. To prevent this, both ends of the SCSI bus need to be terminated. If one end of the SCSI bus is terminated, you may find intermittent problems. Never terminate the bus at a device connected in between.

19. DC voltages commonly found in a PC:

  • +5, and +12 volts are the DC voltages commonly found on PCs.80386 / 486 operate at +5v
  • Pentium and above operate at +3.3v
  • Power supply wires - yellow=+12v, blue=-12v, red=+5v, & white=-5v

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

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