The Maxim MAX-232 converts the TTL/CMOS input/output levels to the nominal 10V RS-232 input/output levels. On power-up, the data set ready (DSR) signal is set by the MAX-232's DC to DC converter. The incoming data terminal ready (DTR) signal enables the transmitter keying signal. The request to send (RTS) and clear to send (CTS) signals are level shifted from the RS-232 interface to TTL/CMOS signal levels. When the modem (DCE) receives an RTS, the RF transmitter and the MSK tone are keyed immediately and a 30 ms to 100 ms timer is started. At the completion of the timing, a CTS signal is generated for the data terminal (DTE) to allow serial data flow.