Huawei ETS2000 Series Maintenance Manual page 8

Figure1-5 Output power level conversion circuit
Data receiving signal power level conversion circuit. The power level conversion
of signal UART1_RX is accomplished by clamp circuit. The reason lies in that the
signal power-level needs to be lowered to detect whether or not the serial port line
is connected when there are no data lines.
Figure1-6 Data receiving signal power level conversion circuit
VDD1_RS power supply circuit:
Figure1-7 VDD1_RS power supply circuit
The circuit may have the fault that the serial port is NOT through. Then you can
connect the serial port line in the startup status to check whether or not the
VDD1_RS voltage is within the range from 3 to 4.4V. If it is not, check Q209 and
Q208.
2，Audio interface
The ETS2000 series FWT audio interfaces include handle interface (microphone
and earphone are included), speaker interface and hands-free microphone interface.
Since the power of the speaker is quite high, an audio amplifier is adopted and the
MIC interface and the handle interface are directly connected.
Figure 1-8 shows the circuit of the handle microphone interface. Bead and
capacitor constitute the π-type filter circuit.
8