Chapter 5 Chapter 5 Asynchronous Asynchronous

CommunicationCommunication

Sender and receiver do not synchronize before data transmission

Receiver does not know when individual bits begin and end

RS-232 StandardRS-232 StandardUsed in async serial transmission

– Eg. Dumb terminals, terminal server, analog modems, console ports

sender transmit character anytime arbitrarily delay between charactersBinary 1 represented by –15 voltsBinary 0 represented by +15 voltsIdle voltage is –15 volts with minimum

duration of one bitStart bit (binary 0) sent before characterRS-232 voltage diagram (fig 5.2)

RS-232 (cont.)RS-232 (cont.) Baud rate

– number of changes in the signal per second.

Bits per second may be equal to or greater than baud rate

Supports full duplex DTE equipment (eg. computer) transmits on pin 2

and receives on pin 3 DCE equipment (eg. modem) transmits on pin 3

and receive on pin 2 Minimum 3 wires (fig 5.3) DTR,DSR,DCD,RTS,CTS,TxD,RxD,GND RTS/CTS used in hardware flow control

BandwidthBandwidth maximum rate (Hertz) that a transmission

system can change a signal

Maximum theoretical data rate D (bits/s) for a transmission system with bandwidth B and uses K unique values of voltages:

D = 2Blog 2 K (Nyquist’s theorem)