Transcript PC 232 Serial Ports

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
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
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
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
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)
Serial and Parallel Transmission
• Parallel
– N bits per second on N wires
– Parallel is faster than serial
1
1
0
1
1
0
0
0
Eight Bits
In Clock
Cycle One
1
1
0
1
1
0
0
0
Eight Bits
In Clock
Cycle Two
Serial and Parallel Transmission
• Parallel Transmission
– N bits per second on N wires
– N=8 in this example, but this is not the only
possibility
– N can also be 4, 16, 32, etc.
1
1
0
1
1
0
0
0
1
1
0
1
1
0
0
0
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
1 cycles
1
overlap
1
1
0
1 1
1 1
0 0
0 0
0 0
0
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
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