Indoor/Outdoor Interface Circuit - Hitachi RAK-25NH4 Service Manual

2. Indoor/Outdoor Interface Circuit

The interface circuit superimposes an interface signal on the DC 35V line supplied from the outdoor unit
to perform communications between indoor and outdoor units. This circuit consists of a transmiting circuit
which superimposes an interface signal transmit from the microcomputer on the DC 35V line and a
transmiting circuit which detects the interface signal on the DC 35V line and outputs it to the microcomputer.
Communications are performed by mutually transmiting and receiving the 4-frame outdoor request signal
one frame of which consists of a leader of approx. 100 ms., start bit, 8-bit data and stop bit and the
command signal with the same format transmit from the indoor unit.
Communication signal from outdoor microcomputer to indoor microcomputer. At first outdoor microcomputer
will send a request signal (SDO) to indoor microcomputer. A high-frequency IF signal approx. 38 KHz is
generated and modulated by the request signal (SDO) inside the outdoor microcomputer then output to
pin of microcomputer. This modulated IF signal is output to pin
11
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 of the indoor microcomputer.
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Fig. 2-2 shows the voltages at each component when data is transferred from the outdoor microcomputer
to the indoor microcomputer.
Communication signal from indoor microcomputer to outdoor microcomputer. The request signal (SDO)
generates by indoor microcomputer is output to pin
is generated by comparator, then modulate by the request signal from pin
This modulated IF signal is then amplified and superimposed to DC 35V line via L801 and C802 of indoor
interface circuit.
Fig. 2-3 shows the voltages 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.
, and amplifies by C801. IF signal approx. 38 kHz
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– 69 –
of HIC and amplified by amp. This
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of indoor microprocessor.
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