Hitachi RAM-90QH5, Service Manual, Page: 45 / 83

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(6) C001 to C014, C022 to C027, NF COIL1 to NF COIL3,
NF COIL4. These absorb electrical noise generated
during operation of compressor, and also absorb
external noise entering from power line to protect
electronic parts.
(7) Surge absorber, Varistor 1, 2, 3,
These absorbs external power surge.
(8) Inrush protective resistor (R002)
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, “surge
absorber”, “varistors 1 and 3” do not
operate.
Be sure to perform grounding.
< Reference >
When inrush protective resistor is
defective, D25VB60 may malfunction.
As a result, DC voltage is not generated
and no operation can be done. In this
case, 5A fuse may have been blown.
Take care.
There is using a multiplexer (IC15) between the microcomputer and the transmiting/receiving circuit for
indoor unit 1 to 4.
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. This 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
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.
From outdoor microcomputer to indoor microcomputer.
The request signal output from microcomputer pin
^
,
&
is input to the transmitting circuit. The transmitting
circuit modulates this signal by approx. 38kHz high-frequency. This high-frequency signal is amplified
by a transistor, superimposed on the DC 35V line via C801 (or C811, C821, C831, C841) and L801
(or L802, L803, L804, L805) 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 input 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 microcomputer to outdoor microcomputer.
The communications from the indoor microcomputer to the outdoor micro computer are the same.
Fig. 2-3 shows the voltages and waveforms at each circuit.