Indoor/Outdoor Interface Circuit - Hitachi RAS-25CNH1 Service Manual

2. Indoor/Outdoor Interface Circuit

The interface circuit superimposes an interface signal on the 36V DC line supplied from the outdoor unit
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 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.
Outdoor micro computer to indoor micro computer
Request signal output from IF transmitting output pin of outdoor microcomputer input to transmitting
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circuit. Transmitting circuit and receiving circuit of the outdoor unit are provided inside HIC. Transmitting
circuit intermits high frequency oscillation circuit of about 36kHz with comparator according to the request
signal. This high frequency signal is applified by the transistor and is ourput from HIC = pin, then
superimposed with DC 35V line via C701 and L701.
To prevent mis-operation, outdoor microcomputer does not accept receive signal whil outputting request
signal.
The receiving circuit of the indoor unit consists of COM4. The interface signal from the outdoor unit, whose
DC component is cut from DC35V line at C801, is waveform-shaped and rectified amplified in HIC, then
input to receive input pin ( of 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
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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