In dual-tone multi-frequency (DTMF) selective calling, the radio is alerted by a string of digits. Systems typically use 2- to 7-digits. These can be dialed from a traditional telephone dial connected to a radio or may be generated as a string of DTMF digits by an automatic encoder. In some systems, a dispatching computer is connected to a DTMF encoder via a serial (RS-232) cable: the computer sends commands to the encoder that generates a pre-defined digit string that is then sent to the transmitter.