| Constant | Description |
| _none (0) | No handshaking. This is the default value. |
| _CTS (1) | CTS (Clear To Send) hardware handshaking. When the computer is ready to send a block of data, it sets the CTS signal. The computer must then wait for a DTR signal from the other device, before sending the data. |
| _DTR (2) | Sets the DTR (Data Terminal Ready) signal on. |
| _DTRToggle (-2) | Sets the DTR signal off. |
| _XON (-1) | XON/XOFF software handshaking. Each device sends an XON character when it's ready to receive data, and sends an XOFF character if its buffer fills up. |