Canon fax basic 2000 サービスマニュアル
FACSIMILE SYSTEM
6–6
The following describes the functions of the NCU board divided into basic
functions.
Fig. 6-4 Block Diagram of NCU Board Functions
3.1
Off-hook Detection
The off-hook of the telephone connected to the facsimile is detected when
the telephone is in standby state or during facsimile communications. The
circuit’s built-in telephone set is designed so that terminals to be connected
with the telephone line have an infinite (DC) resistance when the handset
is placed down (on-hook) and several hundred ohms of DC resistance
when it is lifted (off-hook).
3.2
Formation of DC Loop
As a DC voltage (0 to several tens of volts) is supplied across L1 and L2 at
all times from the
, DC current flows between the facsimile and
the exchange when the telephone becomes off-hook or when the CML
relay
turns ON. This current flow is called “formation of DC loop.”
By formation of this DC loop, the exchange recognizes the response from
the telephone (reception terminal), and performs connection of the tele-
phone line to enable communications. For this reason, the DC loop must
be retained during facsimile communications or during a conversation on
the telephone.
In Fig. 6-5, DC voltage is supplied to the telephone as DC voltage is being
supplied to terminals L1 and L2 on the facsimile from the exchange and
L1 and L2 are connected internally on the NCU board. Yet, as the HS is
VH
MODEM
L2
T21
T12
T11
T22
L1
System
controller
Off-hook
detection
Detection of CI
H relay
Line signal
monitor
Exchange
CML relay
S relay
P relay Formation of
DC loop
2-wire/4-wire
conversion
Vref
generation
NCU
board
SCNT
board