Modems
Modems are used to for low-speed long-distance connections
over telephone lines. Modems convert parallel digital data to
serial analog data and vice versa.
There are two main types of modems:
- Internal
Expansion cards (e.g. ISA, PCI) or 'On-board' (integrated in
mainboard)
- External
Modems that connect to the serial RS-232 or USB port and often
have their own power supply.
PCMCIA/PCcard modems can also be considered external modems
but these are becoming less common since most modern portables
are equipped with an integrated modem.
|
Installing
and Connecting Modems
Install an internal modem by inserting the expansion card in
to an ISA or PCI expansion bus, and if the OS or the modem is
not PnP assign an IRQ and I/O address.
External modems connect to an RS-232 or USB port on the computer.
Most modern PCs are equipped with one or two 9-pin serial ports
(DB-9), also commonly known as COM ports, older PCs often also
have a 25-pin serial port (DB-25)
The following picture is an example of a serial cable used to
connect an external modem to a COM port on the PC:
As you can see one end of the cable is a female DB-9, which
connects to a male COM port:
The other end that connects to the external modem is often a
DB-25 (male) connector connecting to the female DB-25 on the
modem:
|
| The following table lists the signals
of the pins of a RS-232 DB-9 connector: |
| Pin |
Signal |
| 1 |
Data Carrier Detected (DCD) |
| 2 |
Receive Data (RD) |
| 3 |
Transmit Data (TD) |
| 4 |
Data Terminal Ready (DTR) |
| 5 |
System Ground |
| 6 |
Data Set Ready (DSR) |
| 7 |
Request To Send (RTS) |
| 8 |
Clear To Send (CTS) |
| 9 |
Ring Indicator (RI) |
| |
COM ports
COM ports are predefined combinations of IRQs and I/O addresses.
The following table lists the COM ports resource assignments:
(be sure to memorize these for the exam) |
| Port |
IRQ |
I/O address |
| COM1 |
4 |
3F8 |
| COM2 |
3 |
2F8 |
| COM3 |
4 |
3E8 |
| COM4 |
3 |
2E8 |
| |
RJ-11
A telephone line is connected to the modem using a RJ-11 connector
displayed below:
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Modem speed
Although the baud rate does dictate how fast modem
can transmit data and older modems do use the baud rate to measure
their speed, the speed of modern modems is measured in bits
per second (bps). This is where the difference between
Baud and bps comes in:
the baud rate indicates how many frequency changes per second
can occur, with early modems the baud rate equaled the bits
per seconds. For example a 2400 bps modem ran at 2400 baud,
but so does a 14,400 bps modem, the difference is that with
14,400 bps modems use 6 times as many different frequencies,
simultaneously passing 4 bits per change.
|
| The following table
lists the modem speeds designated by the CCITT: |
| CCITT Term |
Speed in bps |
| V.21 |
300 |
| V.22 |
1200 |
| V.22bis |
2400 |
| V.23 |
1200--> 75<-- |
| V.29 |
9600 |
| V.32 |
4800 and 9600 |
| V.32bis |
14,400 |
| V.32fast |
28,800 |
| V.34 |
28,800 |
| V.42bis |
38,400 |
| V.90 |
56,600 |
| |
FAX
Most modems today have built-in fax capabilities, these
so called fax modems allow you to use the modem as
a fully capable fax machine.
|
Troubleshooting
Modems
The first thing you should do when troubleshooting modems is
check the telephone cable connection and with external modems
also the serial cable. Be sure the Line in (telephone
line) and the Line out (a telephone or fax for example)
are not mixed up.
Another common problem, especially with older non-pug and play
modems with jumpers/dip switches, is having IRQ conflicts. Reserve
the IRQ in the BIOS if you encounter problems with modems that
require a particular IRQ.
If the modem is installed correctly and all cables are connected
properly but the mode still isn't working, check basic configuration
settings such as: Tone/Pulse, (Hardware/Software compression),
modem speed.
And last but not least, always make sure you have the latest
drivers from the manufacturer.
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AT Modem Commands
Modems can be manually operated using AT commands,
the following table list a couple of common ones: |
| Command |
Description |
| AT |
Responds with OK of the modem is
turned on and connected. |
| ATA |
Answers an incoming call |
| ATD |
Dial (e.g. ATDT555-4321 the T following
the ATD stands for Tone, can also be P for Pulse) |
| ATH, ATH0 |
Hangs up, disconnect |
| ATM0 |
Mute the speaker |
| ATM1 |
Turns on the speaker |
ATE0
|
Do not echo commands. |
| ATE1 |
Echo commands back to the terminal
as they are entered. |
| ATZ |
Reset Modem Configuration. |
| |
Modems and
Serial Port related exam objectives for the 2002 A+ Core exam.
|
1.1 Identify
basic terms, concepts, and functions of system modules, including
how each module should work during normal operation and during
the boot process.
Examples of concepts and modules are:
- Modem
- Ports
1.3 Identify available IRQs, DMAs,
and I/O addresses and procedures for device installation and
configuration.
Content may include the following:
- Standard IRQ settings
- Modems
1.4 Identify common peripheral ports,
associated cabling and their connectors.
Content may include the following:
- Cable types
- Pin connections
Examples of types of connectors:
- DB-9
- DB-25
- RJ-11
1.7 Identify proper procedures for
installing and configuring peripheral devices.
Content may include the following:
- Modem
2.1 Identify common symptoms and problems
associated with each module and how to troubleshoot and isolate
the problems.
Content may include the following:
- Modems
Click
here for the complete list of exam objectives.
|
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