The device operates in half duplex,

The device operates in half duplex, so when the device is in transmit mode the receive path (discriminator and audio output amplifiers) should be disabled, and can be powered down, by the host μC.A single modulator output with programmable gain is provided which combines both the audio and sub-audio signals to facilitate single or two-point modulation.To avoid spurious transmissions when changing from Rx to Tx the MOD output is ramped to the quiescent modulator output level, VBIAS before switching. Similarly, when starting a transmission, the transmitted signal is ramped up from the quiescent VBIAS level and when ending a transmission the transmitted signal is ramped down to the quiescent VBIAS level. The ramp rates are set in the Programming register P4.6 and enabled by bits 0,1 of the Analogue Input Gain register. When the modulator output is disabled, their outputs will be set to VBIAS. When the modulator output driver is powered down, its output will enter a hi-Z state (high impedance), so the external RF modulator should be disabled to avoid unwanted transmissions.For all transmissions, the host μC must only enable signals after the appropriate data and settings for those signals are loaded into the C-BUS registers. As soon as any signalling is enabled the CMX138A will use the settings to control the way information is transmitted.A programmable gain stage in the microphone input path facilitates a host controlled VOGAD capability.See:o Audio Control – $C2 writeo Analogue Input Gain - $B0 write.