Nullmodem adapter with DB25 male / female connector. Sending RTS.

With type 1, the equipment itself ensures that it receives the "handshake signal" (CTS) that many communications software requires.

With type 3, the counterparty must deliver the handshake signal. This means that the counterparty can start and stop the data flow using its RTS signal. In some cases, type 3 can replace type 1 - see leg connections in the drawing on the left