> USB 1/2 has a single differential pair so you can only transmit or receive but not at the same time
>
> USB 3 has differential pairs for RX and TX so you can do both at the same time.
Just in case this leaves some readers thinking it's only possible to transmit or receive on a differential pair, not so:
Copper gigabit ethernet (1000Base-T) transmits and receives at the same time over all four differential pairs, using adaptive equalisation and echo cancellation to separate the outgoing and incoming signals on each pair.
The signal processing is quite power hungry, and doesn't work so well at higher speeds. So it makes sense that PCIe wouldn't do this.
Just in case this leaves some readers thinking it's only possible to transmit or receive on a differential pair, not so:
Copper gigabit ethernet (1000Base-T) transmits and receives at the same time over all four differential pairs, using adaptive equalisation and echo cancellation to separate the outgoing and incoming signals on each pair.
The signal processing is quite power hungry, and doesn't work so well at higher speeds. So it makes sense that PCIe wouldn't do this.