1 Fall 2005 Local Serial Asynchronous Communication Qutaibah Malluhi Computer Science and Engineering Qatar University

2 Bit-wise data transmission  Data transmission requires: –Encoding bits as energy –Transmitting energy through medium –Decoding energy back into bits  Energy can be electric current, radio, infrared, light  Transmitter and receiver must agree on encoding scheme and transmission timing

3 Using Electric Current to Send Data  Simple idea - use varying voltages to represent 1s and 0s  One common encoding use negative voltage for 1 and positive voltage for 0  In following figure, transmitter puts positive voltage on line for 0 and negative voltage on line for 1

4 Encoding Details and Standards  Encoding scheme must specify the details. For example: –How long will voltage last for each bit? –How soon will next bit start? –How will the transmitter and receiver agree on timing?  All details specified by standards  Allow interoperability of devices adhering to the standard  Several organizations produce networking standards »International Telecommunications Union (ITU) »Electronic Industries Association (EIA) »Institute for Electrical and Electronics Engineers (IEEE)

5 Types of Data Transmission

6 Parallel versus Serial Transmission Parallel Serial

7 Asynchronous Communication  Asynchronous and synchronous communications  In broad sense, communication may be called asynchronous if transmitter and receiver do not explicitly coordinate before each data transmission –Sender can wait arbitrarily long between transmissions. Sends when data becomes ready –Used, for example, when sender may not always have data ready to send (E.g., keyboard, mouse) –Receiver does not know when a character will arrive. May wait forever  In more technical sense, Asynchronous may also mean no explicit information about where individual data bits begin and end

8 Asynchronous Vs. Synchronous  Synchronous: –May use a separate clock signal to indicate duration of bits.  Asynchronous –How do we know beginning of a bit and the ending of it? –One way to ensure meaningful exchange »Sender and receiver agree on bit duration »Start bit before character »One or more stop bits after character »1s when idle Line idle Clock Data e.g., Ch1 Ch2 Ch3 Idle

9 Asynchronous Transmission

10 The RS-232C Standard  Standard specified by EIA  For transfer of characters across copper wire  Full name is RS-232-C common name is RS-232  Defines serial, asynchronous communication –Serial - bits are encoded and transmitted one at a time (as opposed to parallel transmission) –Asynchronous - characters can be sent at any time and bits are not individually synchronized

11 More Details about RS-232  Components of standard: –Data represented by voltage between +15 and –15 –Cable limited to ~50 feet –25-pin connector, with specific signals such as data, ground and control assigned to designated pins –Specifies transmission of characters between, e.g., a terminal and a modem  Transmitter never leaves wire at 0v; when idle, transmitter puts negative voltage (a 1) on the wire

12 RS-232 Character Transmission  Transmitter indicates start of next character by transmitting a zero –Receiver can detect transition as start of character –Extra zero called the start bit  Transmitter must leave wire idle so receiver can detect transition marking beginning of next character –Transmitter sends a one after each character –Extra one called the stop bit  Thus, character represented by 7 data bits requires transmission of 9 bits across the wire RS-232 terminology: MARK is a negative voltage (== 1) SPACE is a positive voltage (== 0)

13 Duration of a Bit  Transmitter and receiver must agree on timing of each bit  Agreement accomplished by choosing transmission rate –Measured in bits per second –Detection of start bit indicates to receiver when subsequent bits will arrive  Hardware can usually be configured to select matching bit rates –Switch settings –Software –Autodetection

14 Bit Rate And Baud Rate  Baud rate measures number of signal changes per second  Bits per second measures number of bits transmitted per second  In RS-232, each signal change represents one bit, so baud rate and bits per second are equal  If each signal change represents more than one bit, bits per second may be greater than baud rate –Bit rate = Baud rate * the number of bits represented by each signal unit –Example: An analog signal carries 4 bits in each signal change. If 1000 signal changes are sent per second, then baud rate = 1000 bauds per second, bit rate = 1000 * 4 = 4000 bps

15 Character Framing  Start and stop bits represent framing of each character  If transmitter and receiver are using different speeds, stop bit will not be received at the expected time  Problem is called a framing error  RS-232 devices may send an intentional framing error called a BREAK –E.g., ASCII keyboard BREAK key –Deliberately create a framing error. Applications noticing a framing error as a request to abort

16 Full-Duplex Communication  Simultaneous two-way communication  Requires each side to have transmitter and receiver  Requires an electrical path in each direction  Transmitter on one side connected to receiver on other  Separate wires needed to carry current in each direction  Common ground wire

17 DB-25 Connection Standard  RS-232 specifies use of 25 pin connector (DB-25)  Pins are assigned for use as data, ground and control: –Pin 2 - Receive (RxD) –Pin 3 - Transmit (TxD) –Pin 4 - Ready to send (RTS) –Pin 5 - Clear to send (CTS) –Pin 7 – Ground  Commonly DB-9 is used.

Swap  Cable must cross-over wires to connect pins 2 and 3 on receiver and transmitter  RS-232 specifies that modems (DTE) transmit on pin 2 and receive on pin 3, while computers (DCE) transmit on pin 3 and receive on pin 2 –DTE: Data Terminal Equipment (modem) –DCE: Data Computer Equipment (computer)  To connect two DCEs (Computers), RS-232 cables between two computers must have 2-3 swap –Also called null modem cable  To connect DCE to DTE, use a straight through cable –Standard modem cable

19 Electric Transmission  In real world –Electric energy dissipates as it travels along –Wires have resistance, capacitance, and inductance which distort signals –Magnetic or electrical interference distorts signals –Distortion can result in loss or misinterpretation

20 Distorted Signal For A Single Bit  In practice –Distortion can be much worse than illustrated

21 Consequences  RS-232 hardware must handle minor distortions –Take multiple samples per bit –Tolerate less than full voltage  Can not use electrical current for long-distance transmission –Use carrier signals

22 Newer Standards  USB (Universal Serial Bus) –Hot pluggable –Device powered by bus –Upto 12 Mbps –Newer USB2 speed is 480 Mbps  Firewire: a high speed serial bus –400/800 Mbps

23 Conclusions  Asynchronous communication - data can start at any time; individual bits not delineated  RS EIA standard for asynchronous character transmission  Bit rate and baud rate  Bandwidth limits maximum data transmission rate  Newer standards such as USB and Firewire are replacing the role of RS 232 in PCs