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I know what is difference between parallel and serial interface. I know also what does it mean half/full duplex. However, I don't know why most parallel interfaces are half-duplex, and most of serial interfaces are full-duplex.

Can anyone explain?

Transistor
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  • RS485, for example, is a asynchronous serial interface physical layer that is half duplex. – Harvard Aug 06 '16 at 20:10
  • RS-485 is not half-duplex, it _can_ be operated in half-duplex mode, if required. – pipe Aug 06 '16 at 20:12
  • To point out several exceptions: I2C (slow but cheap) and CAN (relatively slow but reliable) are serial and half-duplex. – Nick Alexeev Aug 06 '16 at 20:16
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    I'm voting to close this question as off-topic because the observation it requests an explanation of is itself false, and as a result it continues to attract bad answers. – Chris Stratton May 08 '19 at 13:53
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    @Something like "_All_ parallel interfaces are half-duplex, while _many_ serial interfaces are full-duplex" would probably be closer to the truth. – pipe May 08 '19 at 14:47

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For a true full-duplex interface, you really need two wires for each signal - one in each direction. This quickly becomes impractical for a parallel bus.

If you have a parallel bus, the bandwidth is often important. If you must add twice the number of wires, you can almost always benefit more by sending twice the amount of data instead of reserving half the wires for one direction and half for the other.

pipe
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  • Gigabit Ethernet is no true full-duplex interface then? https://electronics.stackexchange.com/questions/132486/how-does-bidirectional-transmission-on-gigabit-ethernet-work – bot47 May 08 '19 at 15:42
  • @MaxRied "you really need" is weaker than "you need". You can do full duplex on one wire but you get into voodoo territory and is not very common. Note that you don't even need wires at all for full duplex communication - just use two antennas! – pipe May 08 '19 at 15:45
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I think it's due to the fact that most parallel interfaces are buses. they are shared media that allow more than 2 devices connected at the same point.

If devices have to share the same medium. they have to use it for both TX and RX on the same wire, making half-duplex the only possibility

user221458
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