Canon fax basic 2000 サービスマニュアル

FACSIMILE SYSTEM

6–12

3.6

2-wire/4-wire Conversion

Normally, telephone lines are 2-wire. However, 2-wire must be converted

to 4-wire so that the send data can be processed separately from the

received data internally on the facsimile. The 2-wire/4-wire conversion cir-

cuit performs this conversion.

Fig. 6-13 2-wire/4-wire Conversion Circuit

3.7

Protective Circuits

An overvoltage protection function is provided for protecting elements on

the NCU board when lightning, for example, causes a high voltage to be

applied from the telephone line or when something causes an overvoltage

to be applied. An arrestor is provided as a countermeasure for lightning

between L1 and L2 on the primary side, and between L1, L2 and the frame

ground (FG). Though arrestors normally do not allow current to pass,

when a high voltage is applied, they allow current to pass for the amount

that the rated voltage is exceeded to lower the voltage across L1 and L2,

and across L1, L2 and FG. The arrestor lowers the voltage to the rated volt-

age by converting electrical energy to optical energy.

In some countries, the arrestor between L1, L2 and FG is connected to an

arrestor ground, and the arrestor ground is connected to the power supply

unit and in some cases is earthed via the power supply outlet.

Fig. 6-14 Overvoltage Protection (arrestor)

Transmission

Reception

2-wire/4-wire

conversion

hybrid circuit

Arrestor
(across L1 and L2)

CML relay

P relay

S relay

DC seize

circuit

Arrestor (across L1, L2 and FG)
Current is allowed to pass when
an overvoltage is applied.