MODEMS (Converts analog signals to digital and vice-versa) Lecture 7 The second semester: 2017\1438 T. Elham Sunbu

OUTLINE: Modulator – converts digital information to analog Demodulator – converts analog information to digital RS-232 Interface

MODEMS A modem is a device or program that enables computer to transmit data over telephone lines. Computer information is stored digitally, whereas information transmitted over telephone lines is transmitted in the form of analog waves. A modem converts between these two forms.

Basic Principle 0101

Basic Structure

How Modulation is used for data transfer ? 10100101011001010 - the transmitted symbol sequence will be: 6 4 5 1 2.

Classification of Modems Range Line Type Operation Mode Synchronization Modulation Data Rate

Range Line Type Dial-up Short Haul Leased Voice Grade Private Wide Band

Operation Mode Modulation AM FM PM TCM Simplex Half-Duplex Full-Duplex

CABLE MODEM Cable Modems make access to TV cable, and let you get  high-speed internet access. Availability is still limited in many areas, but is growing rapidly.

EXTERNAL MODEM It usually plug in through a serial port, or  a USB port.  External modems cost the most, but have the benefits of being easily installed. Also plugs into telephone line. External power supply is needed. Holder case is required. Often they have the  indicators that tell the status of your connection there is one standard interface for connecting external modems to computers called RS-232.

TYPICAL INDICATERS ON EXTERNAL MODEMS FRONT PANAL HS = HIGH SPEED AA = AUTO ANSWER CD =CARRIER DETECT OH =OFF HOOK RD =RECIVED DATA SD =SEND DATA TR =TERMINAL READY MR = MODEM READY

INTERNAL MODEM It can insert into a vacant expansion slot. No external power supply is needed. No holder case is required. Least expensive at given speed. Incompatible with different kind of PCs. These modems go inside the computer, and usually plug directly into the motherboard.

ISDN MODEM ISDN is an abbreviation for Integrated Services Digital Network. There is no such thing as a noisy or fuzzy line.  always runs at 64kbps. Ability to bond multiple "B" channels to get speeds of 128kbps. Call Setup is very fast (dialing, handshake and authentication), usually under 2 seconds (compared with 15 seconds at least for most analog modems). Ability to get 2 calls at the same phone number (one line is for voice, and the other line for your internet connection).

WIRELESS MODEM Convert digital data into radio signals and back. Support a maximum data transmission rate of 19.2Kbps (kilobits per second). Wireless modems can be used up to 20-30 miles (30-50 KM). Operation is in the 900 MHz and 2.4 GHz.  Modem names include – BreeseLINK, LATNET-Radio Data Links, AirLink Wireless Modems, and Open Minds.

DSL MODEM DSL Modems: DSL (Digital Subscriber Line)  service is usually offered through  the phone the company/ISP. You can leave your Internet connection open and still use the phone line for voice calls. The speed is much higher than a regular modem (up to 1.5 Mbps ) DSL doesn't necessarily require new wiring; it can use the phone line you already have

Modem Speed The Modulation Speed The Serial Port Speed Note: Modem speed is rated in bps (bits per second).  So a 56k modem transmits at 56,000 bits per second in theory. While 8 bits make a byte, and 1024 bits equal one kilobits, a 56k modem will transmit a maximum of approximately 7 kilobytes of information every second.   A modem will never connect higher than what it is rated for

Modem Installation Modem Cable Selection Selection of Port

What is RS232 RS means Recommended Standard. RS 232 uses both synchronous and asynchronous communication. In microcontrollers like ATmel 89C51 asynchronous communication is used and it is called as UART – universal asynchronous receiver and transmitter PCI Express x1 card with one RS-232 port

Interfacing RS232 We need a driver to use the RS232 with microcontroller That driver turned out to be MAX232 It is a16 PIN Ic which is used to interface RS232 with any other devices. The transmission speed in the RS232 is called as baud rate.

Setting serial port In microcontroller there are some special function registers which are allocated to control the serial communication. The list of registers which is used to control the serial port are SCON PCON SBUF