Electrical Signaling: Serial Ports

1
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
2
Electrical Signaling Loss of Synch

• Problem of Long String of Ones or Zeros
• No transition to resynchronize receivers clock
• Receiver may interpret bit N as N-1 or N1
• At 10 Mbps or 100 Mbps, bit periods are so brief
  that synchronization must be very exact

Sender
1
2
3
4
5
6
Receiver
1
2
3
4
5
3
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
4
Electrical Signaling 10Base-T

• Manchester Encoding
• Used in 10Base-T
• Transition in middle of each bit period
• One ends high zero ends low
• Resynchronizes receivers clock every bit

Transition in mid-bit
1 ends high
1
1
0
1
5
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