Dual protocol transceivers support the two protocols RS-232 and RS-485. RS-232 is a single-ended interface requiring a ground wire between signal source and sink. The interface uses two conductors for transmitting and receiving data, TxD and RxD, respectively. Because RS-232 drivers have no tristate capability, the interface can be used for point-to-point links only. Bus voltages can range from +3 V to +15 V for a logic low, and from -3 V to -15 V for a logic high, with reference to signal ground. RS-232 operates in full-duplex mode, meaning data can be transmitted and received at the same time. The standard specifies a maximum slew rate of SR = 30 V/µs, which at large voltage swings limits the data rate to 20 kbps, while at low voltage swings the data rate can be increased to 1 Mbps. RS-232 is often used to exchange control and configuration data between a host controller and associated equipment. The maximum distance of a data link is limited to around 20 m (60 ft.).