Simplex is one
direction. A good example would be your keyboard to your CPU. The CPU never
needs to send characters to the keyboard but the keyboard always sends
characters to the CPU. In many cases, Computers almost always send characters
to printers, but printers usually never send characters to computers (there are
exceptions, some printers do talk back). Simplex requires only one lane (in the
case of serial).
Half-Duplex is like the
dreaded "one lane" road you may have run into at construction sites. Only one
direction will be allowed through at a time. Railroads have to deal with this
scenario more often since it's cheaper to lay a single track. A dispatcher will
hold a train up at one end of the single track until a train going the other
direction goes through. The only example I could think of for Half-Duplex is
actually a Parallel interface. Even though parallel is eight lanes, data
travels through the lanes in the same direction at the same time but never in
both directions at the same time. The IEEE-1284 allows printers to send
messages to the computer. The printer cannot send these messages while the
computer is sending characters but when the computer stops sending characters,
then the printer can send messages back. It's kind of like some roads that head
into downtown. In the morning, they're one way roads, allowing traffic to go
into downtown. In the evening their one way roads, allowing traffic to head out
of downtown. The only advantage that Half-Duplex would have is the single lane
or single track is cheaper then the double lane or double track.
Full-Duplex is like the
ordinary two-lane highway. In some cases, where traffic is heavy enough, a
railroad will decide to lay a double track to allow trains to pass in both
directions. In communications, this is most common with networking. Our fiber
optic hubs have two connectors on each port, one for each lane of a two-lane
roadway. Full-Duplex fiber is two cables bundled or tied together to form the
two-lane roadway. In 100Base-TX, the two lanes are housed in the same jacket.
RS232 was also designed to handle Full-Duplex but some of our short haul modems
and converters give the user the option to go Half-Duplex or Simplex to reduce
the number of conductors needed to connect between them.