Ports which communicate the information in a serial basis; i.e. one bit at a time.  Examples include:

One-bit-at-a-time (transmits either "0 bit" or "1 bit" signal at any instant):

* RS-232 (probably the most ubiquitous of all serial ports)
* RS-485
* UniversalSerialBus (USB)
* I2C (also IIC, InterIntegratedCircuit) Bus
* SerialAta (new replacement for parallel IDE)
* Ethernet
* PS2
* 300 bit/s and slower dial-up modems
* 10Base-T Ethernet (Manchester modulation)
* FireWire
* Many types of signaling on FiberOptic cables are serial
* <add examples here>

Only one or 2 wires (definitely not parallel, but not one-bit-at-a-time either):

* 100Base-T Ethernet: 3-level modulation (100BASE-T4 uses 8B/6T) (100BASE-TX and 100BASE-FX use ... something else ?)
* 1000Base-T Ethernet (GigaBitEthernet): 5-level modulation on each pair of wires
* 1200 bit/s and faster dial-up modems: up to 255-level modulation (limited by the telephone network)

Constellation diagrams show all the possible symbols that may be transmitted at any instant, using phase-amplitude modulation.
The first group has exactly 2 symbols in their constellation diagram ("2-level modulation"), so they transmit one bit at a time.
The second group has more than 2 symbols, so at any instant, they are transmitting one "symbol" that conveys more than one bit of information.
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Doesn't 100 Base-T Ethernet also combine bits in a more complex manner? (using phase and amplitude changes, just as are used by modern modems)

''Yes, but not quite as complicated.''
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See also: http://en.wikibooks.org/wiki/Programming:Serial_Data_Communications http://en.wikipedia.org/wiki/Serial_communications
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CategoryHardware