Hitachi RAS-25CNH2 Service Manual page 41

2. Indoor/Outdoor Interface Circuit
The interface circuit superimposes an interface signal on the 36V DC line supplied from the outdoor unit
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to perfom communications between indoor and outdoor units. This circuit consists of a transmiting circuit
which superimposes an interface signal transmit from the micro computer on the 36V DC line and a
transmiting circuit which detects the interface signal on the 36V DC line and outputs it to the micro
computer.
Communications are performed by mutually transmiting and receiving the 4-frame outdoor request signal
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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.
From outdoor microcomputer to indoor microcomputer
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The request signal output from microcomputer pin 70 is input to the transmitting circuit. The transmitting
circuit outputs an approx. 38-kHz high-frequency signal via pin 11 and continues the output intermittently
according to the request signal. This high-frequency signal is amplified by a transistor, superimposed on
the DC 36V line via C801 and L701, and supplied to the indoor unit.
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 36V line is supplied to C811, where DC components are eliminated, and is
then shaped by the comparator. The shaped signal is detected by diode, amplified by amp, and supplied
to receiving input 19 of the indoor microcomputer.
Fig. 2-2 shows the voltages at each component when data is transferred from the outdoor microcomputer
to the indoor microcomputer.
Indoor micro computer to outdoor micro computer
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The communications from the indoor micro computer to the outdoor micro computer are the same. Fig.
2-3 shows the voltages and waveforms at each circuit.
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