Hitachi RAM-50CNHZ2, Service Manual, Page: 65 / 107

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2. Indoor/Outdoor Interface Circuit
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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. 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.
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From outdoor microcomputer to indoor microcomputer
The request signal output from microcomputer pins
X
is input to the transmitting circuit. The transmitting circuit
outputs an approx. 38kHz high-frequency signal via pin
!
and continues the output intermittently according to
the request signal. This high-frequency signal is amplified by a transistor, superimposed on the DC 35V line
via C801, C811 and L801, L802 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 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 supplied to receiving
k
input of the indoor microcomputer.
Fig. 2-2 shows the voltages at each component when data is trnsferred 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.
(5) C001 to C005, NF COIL 1
There absorb electrical noise generated during
operation of compressor, and also absorb external
noise entering from power line to protect eletronic
parts.
(6) Sugar absorber, varistor 1,2,3
These absorbs external power surge.
(7) Inrush protective resistor (R002, R003)
This works to protect from overcurrent when power
is turned on.
❈
Be sure to ground outdoor unit.
If not grounded, noise filter circuit does not operate
correctly.
❈
If outdoor unit is not grounded, “sugar absorber”,
“varistors 1 and 3” do not operate.
Be sure to perform grounding.
<Reference>
When inrush protective resistor is defective, diode
stack may malfunction. As a result, DC voltage is not
generated and no operation can be done. In this case,
3A fuse may have been blown.