Half duplex means data can be relayed in only one direction at a time. Two-way transmission is possible, but the transmissions must be alternate. A walkie-talkie is half duplex-when one person is speaking, she cannot also listen A telephone is full duplex-information can go both ways simultaneously; both ends can talk and hear all the time-so does a modem.
Your communication software may have a half duplex setting, but this doesn't actually cause your modem to alternate between sending and receiving. Instead, it just sends the characters you type directly to your screen and to the other modem, too. In this sense, half duplex is the same as "local echo" or "local echo on." If you can't see what you type when you're telecommunicating (because the other modem isn't sending the characters back as you type them), switch to half duplex, or turn local echo on.
If you see two copies of every character you type, switch to full duplex, or turn local echo off. In full duplex mode, your modem sends the characters you type only to the other modem, not to your screen. But you will still see the characters because the other computer is sending them back to you.