Hook Button - Canon FACSIMILE BASIC 2000 Service Manual

BASIC OF TELEPHONE
2.4

Hook Button

When a hook button is closed by picking up a handset, direct current flows
to the telephone circuits.
The purpose of this is twofold (i) so that direct current is made to flow to
the transmitter to provide current for sending voice when the handset is
picked up, and (ii) so that the exchange detects this direct current to recog-
nize that the handset has been picked up.
Picking up a handset is called "making a DC loop". A DC loop lets the
exchange know that:
(1) The calling party (the party making the call) is ready to call someone.
(2) The called party answered. (The party receiving the call)
(3) The handset is on-hook.
1–8

Hook button

Exchange
Direct current (DC) loop
Direct
current
detector
Closed when a handset is
Detects direct
picked up. (Off-hook state)
current.
Fig. 1-9 Hook Button
Telephone
HS
T
Fig. 1-10 DC Loop
Hook button
T: Transmitter
R
R: Receiver
HS: Hook switch