Hitachi RAS-50WX8 Service Manual page 71

2. Indoor/Outdoor Interface Circuit
The Interface circuit superimposes an interface signal on the 35V DC line supplied from the outdoor unit
to perform communications between indoor and outdoor units. The circuit consists of a transmiting circuit
which superimposes an interface signal transmit from the microcomputer on the 35V DC line and a
transmiting circuit which detects the interface signal on the 35V DC line and outputs it to the micro
computer.
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.
The request signal (SDO), output from pins and of the outdoor microcomputer, is input to pin of
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IF-HIC. IF-HIC creates a high-frequency signal from an approx. 38kHz signal generated by an oscillator
circuit using a comparator, and outputs an intermittent signal corresponding to the request signal, via pin
. The signal at pin is superimposed on the DC 35V line through the IF transformer and is supplied
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to the indoor unit.
The indoor interface circuit has the same basic configuration as that of the outdoor unit. It uses C801 to
eliminate DC components from the request signal supplied from the outdoor unit, and then supplies the
signal to pin of HIC uses a comparator to convert the approx. 38kHz harmonic signal to digital
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signals (approx. 5V, 0V) which can be recognized by a microcomputer, and then supplies them to pin of
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the indoor microcomputer.
The indoor unit is interfaced with the outdoor unit in the same way. The operation command (SDO) from
the indoor microcomputer is input to the same circuit as IF-HIC, and the approx. 38kHz harmonic signal
is superimposed on the DC 35V line in the same way as with the outdoor unit and is supplied to the
outdoor unit.
The outdoor unit receives the signal at pin of IF-HIC through the IF transformer and converts it to
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digital signals (approx. 5V, 0V) which are supplied to pins and (SDI) of the microcomputer.
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