Hitachi RAC-25YH4, Service Manual, Page: 66 / 121

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3. Indoor/Outdoor Interface Circuit
The interface circuit superimposes an interface signal on the DC 35V line to perform communications
between indoor and outdoor units. This circuit consists of a transmitting circuit which superimposes an
interface signal transmit from the microcomputer on the DC 35V line and a circuit which detects the
interface signal on the DC 35V line.
Communications are performed alternatively transmitting and receiving.
3-1 Communication signal from outdoor microcomputer to indoor microcomputer.
At first outdoor microcomputer will send a request signal (SDO) to indoor microcomputer.
38 KHz of carrier signal is generated and modulated by the request signal (SDO) from the outdoor
microcomputer pin 11.
This signal is superimposed to DC 35V line via C801 and L801.
To prevent erroneous reception, the outdoor microcomputer is designed so that it cannot receive a
signal while it is outputting a request signal.
The receiving circuit in the indoor unit consists of a comparator and transistor. The interface signal from
the outdoor unit on the DC 35V line is supplied to C821, where DC components are eliminated, and is
then shaped by the comparator. The shaped signal is detected by diode, amplified by amp, and output
to pin 49 of the indoor microcomputer.
Fig. 3-2 shows the waveforms at each component when data is transferred from the outdoor
microcomputer to the indoor microcomputer.
3-2 Communication signal from indoor microcomputer to outdoor microcomputer.
The request signal (SDO) generates by indoor microcomputer is output to pin 50 , and amplifies by
Q801.
I/F signal approx. 38 kHz is generated by comparator, then modulated by the signal from pin 50 of
indoor microprocessor.
This modulated I/F signal is then amplified and superimposed to DC 35V line via L801 and C802 of
indoor interface circuit.
Fig. 3-3 shows the waveforms at each component when data is transferred from outdoor microcomputer
to indoor microcomputer.
The circuit operation of the outdoor receiving circuit is same as indoor receiving circuit.
RAS_25_35_AW_055-086 2003.12.31, 10:59 AM 64