Download Electrical Signaling: Serial Ports

Electrical Signaling: Serial Ports
• EIA/TIA-232 Serial Ports (Chapter 4)
–
–
–
–
One is a low voltage (-3 to -15 volts)
Zero is a high voltage (+3 to +15 volts)
300 bps to 115.2 kbps
Length of clock cycle is 1/bit rate
0
1
0
0
1
Electrical Signaling: Loss of
Synch
• Problem of Long String of Ones or Zeros
– No transition to resynchronize receiver’s clock
– Receiver may interpret bit N as N-1 or N+1
– At 10 Mbps or 100 Mbps, bit periods are so brief that
synchronization must be very exact
Sender
1
Receiver
1
2
3
2
4
3
5
4
6
5
Electrical Signaling: 10Base-T
• Manchester Encoding
– Used in 10Base-T only
– Two voltage levels
• High: TD+ (Pin 1) is 2.2 to 2.8 volts higher than
TD- (Pin 2)
• Low: TD+ is 2.2 to 2.8 volts lower than TD-
High
Low
1
1
0
1
Electrical Signaling: 10Base-T
• Manchester Encoding
–
–
–
–
Used in 10Base-T
Transition in middle of each bit period
One ends high; zero ends low
Resynchronizes receiver’s clock every bit
Transition
in mid-bit
1 ends
high
1
1
0
1
Electrical Signaling: 10Base-T
• Manchester Encoding is Inefficient
– Baud rate is number of possible transitions per
second
– Baud rate is the limiting factor technically
– 20 Mbaud to deliver only 10 Mbps
8 possible
transitions
4 bits
1
1
0
1