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CompTIA A+ Core TechNote: Modems & Serial Ports
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

A telephone line is connected to the modem using a RJ-11 connector displayed below:

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
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.

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
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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