Modems and Serial Ports

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:
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.
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.