Download Chapter 5 Asynchronous Communication

Chapter 5
Asynchronous Communication


Sender and receiver do not synchronize
before data transmission
Receiver does not know when individual
bits begin and end
RS-232 Standard

Used in async serial transmission
– Eg. Dumb terminals, terminal server, analog
modems, console ports

sender transmit character anytime
 arbitrarily delay between characters
 Binary 1 represented by –15 volts
 Binary 0 represented by +15 volts
 Idle voltage is –15 volts with minimum
duration of one bit
 Start bit (binary 0) sent before character
 RS-232 voltage diagram (fig 5.2)
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
Bandwidth

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)