Kyocera KM-1820

3JG
2-2-5
2-2-2 NCU PWB
Noise
fileter
circuit
C038, C039
Noise
fileter
circuit
FCPWB
Surge
absorption
section
V10, V12
V11
Relay
circuit
Q10
DCCONT
LINE1
LINE2
DC/DC converter on/off
circuit Q48, Q49
DC/DC converter circuit
Q47
Off-hook detection circuit
photo coupler PC40
External telephone
off-hook detection
circuit
U11
Accounting
pulse
elimination
circuit
C041, C042
C043
Ground start
circuit
DB20, PC20
(modular
connector)
Line
(modular
connector)
Tel
TELOFHKn
EARTH
TEL SEL
TEL_DET1
TEL_DET2
Figure 2-2-3 NCU PWB block diagram
The NCU PWB (NCUPWB) mainly controls the connection to the telephone line. It consists of the circuits shown in the block
diagram.
The external telephone off-hook detection circuit U11 detects the off-hook state of the telephone connected.
In the DC/DC converter ON/OFF circuit (Q48 and Q49) , when the DCCONT signal reaches high level, Q48 becomes ON
and supplys the electric current to the the DC/DC converter circuit Q47.
In the off-hook detection circuit PC40, by carrying out the off-hook of the external telephone, a loop is formed between
TEL_DET1 signal and TEL_DET2 signal, when 3.3 V DC flows to a photo coupler, a TELOFHKn signal is set to low level and
an off-hook is detected.
The accounting pulse elimination circuit (C041, C042 and C043) removes signals representing the communication charge
information (accounting pulses) before they reach the modem when telephone line is used. This is because accounting
pulses obstruct fax communications.
The ground start circuit (DB20 and PC20) requests an outside connection to the private branch exchange (PBX) when
calling via the PBX.