There are two kinds of duplex connections :

1- Half Duplex : That is a one-way connection. You are sending data or receiving some at a time. Some wireless access points are half-duplex. Talki Walkie is a good example of this kind of connection.

2- Full Duplex : That is a two-way connection. You can send and receive date at a same time. Telephone is the best example of it, as you can listen and talk at a same time.

CAN is half-duplex. One differential pair means one message at a time.

duplex duplex

TCP is usually used as full duplex, but in some case, half duplex can also be used in TCP.

Full Duplex