PC 232 Serial Ports

1
PC 232 Serial Ports

• Ports
• Connectors at back of PC
• Plus related internal electronics to send/receive
• PC 232 Serial Port
• Follows EIA/TIA 232 standards

2
PC 232 Serial Ports 9-Pin and 25-Pin Ports

• 9 pins or 25 pins
• Parallel ports have 25 holes

Pins
9-pin Serial Port
25-pin Serial Port
25-pin Parallel Port
Holes
3
232 Serial Ports Send on One Pin Each Way

• 9-Pin 232 Serial Ports
• PC sends on Pin 3 (modem receives)
• PC receives on Pin 2 (modem sends)
• Pin 5 is a signal ground defining zero volts

PC
Modem
4
232 Serial Ports Send on One Pin Each Way

• 9-Pin 232 Serial Ports
• Other pins are control signals to tell other side
  when it may transmit
• Or tell PC what modem is hearing on the line
  (ringing, modem carrier signal)

PC
Modem
5
Serial and Parallel Transmission

• Serial one wire, one bit per clock cycle
• Second (ground) wire needed for circuit is not
  shown

1
0
One Bit in Clock Cycle One
One Bit in Clock Cycle Two
For simplicity, we assume binary transmission (2
possible states/clock cycle)
6
Serial and Parallel Transmission

• Parallel
• N bits per second on N wires
• Parallel is faster than serial

1
1
1
1
0
0
Eight Bits In Clock Cycle One
Eight Bits In Clock Cycle Two
1
1
1
1
0
0
0
0
0
0
7
Serial and Parallel Transmission

• Parallel Transmission
• N bits per second on N wires
• N8 in this example, but this is not the only
  possibility
• N can also be 4, 16, 32, etc.

1
1
1
1
0
0
1
1
1
1
0
0
0
0
0
0
8
Serial and Parallel Transmission

• Parallel Transmission is Only for Short Distances
• Usually up to about 2 meters (6 feet)
• Wire propagation speeds vary
• Over long distances, bits from different clock
  cycles overlap

1
1
1
1
0
0
1
1
1
1
0
0
0
0
0
0
9
PC 232 Serial Ports Voltages

• For sending data
• One is -3 to -15 volts
• Zero is 3 to 15 volts
• Binary (only two possible states)

15v
0
0
1
-15v
10
PC 232 Serial Ports

• PC 232 serial ports are binary because there are
  only two states (voltage levels)
• PC 232 serial ports are serial because data is
  sent on only one wire at a time
• These are separate things
• One does not require the other