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When the Off hook status, pulse dial transmission is executed by alternating On hook and Off hook. The status of On hook or Off
hook is controlled by the switching transistor Q10. 

  It is insulated by the capacitors in a COL circuit, and the call ID signal inputted from outside line is transmitted to a CALLID
option card.
The flow of a CALLID signal 
Tip

→L1→C29→R54→IC4

Ring

→L1→C29→R55→IC4

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This circuit consists of bidirectional amplifier function for communication between the extensions and COL, returns loss com-
pensation for conference, shunt function and mute function.
(Composition)
For transmitting signals from the extensions to COL, this circuit consists of R34, R35, R26, R27, R18, R29, C14, C15, C10, C16
and the operational amplifier (IC1).
For transmitting signals from the COL to extensions, this circuit consists of R24, R25, R31, R28, R37, R30, C12, C13, C17, C18,
and the operational amplifier (IC1).
COL side-tone suppression circuit which includes a balanced network BN1 consists of R22 R23, C11 and R16, R17, R20, R21.
EXT side-tone suppression circuit which includes a balanced network BN2 consists of R41, R42 and R40, R43, R38, R39.
Also extension side-tone suppression circuit which includes a balanced network BN3 consists of R45, R46 for supplementing
side-tone suppression during the conference communication.
The analog switch (IC2) is used for the following:
(1) Conference (Pin 6, 8 and 9)
(2) Shunt (Pin 12, 10 and 11)
(3) Mute (Pin 13, 1 and 2) 

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Normally, pin 6 of the analog switch IC2 is low level, but during conferences, this pin becomes high level. Because, during con-
ferences, it should compensate the return loss by connecting the balance network BN2 and the balance network BN3 in parallel.

The mute switch consists of pin 13, 1 and 2 of the analog switch(IC2). This switch has the following functions.
a) When a dial signal (DP) is sent to the COL, signals from the extensions are blocked.
b) When the hold on music is sent to the COL, signals from the extensions are blocked.
c) When the COL interface circuit is in the idle state, oscillation of COL bidirectional amplifier is inhibited. When pin 13    of IC2
changes to low level, the interval between pins 1 and 2 of the analog switch turns off, and signals are blocked.

The shunt switch consists of pin 12, 10 and 11 of analog switch IC2.It is used to prevent the pulse dialing signal which is trans-
mitted to the extensions. When pin 12 of IC2 changes to high from low, the analog switch becomes ON (the interval between pin
10 and 11), and GAIN of the COL cross point operational amplifier becomes zero.

Condition of COL interface amplifier circuit and analog switches.

Shunt SW

Mute SW

Conference SW

pin no. of analog switch

12

13

6

No connection (idle)

H (on)

L (off)

L (off)

Two party call

L (off)

H (on)

L (off)

Conference

L (off)

H (on)

H (on)

Hold on music transmission

H (on)

H (on)

L (off)

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The cross point circuit composed of three cross point switch IC's (IC601:8X16, IC602 and IC603:4X8).
1) Cross Point Switch IC Operation

The cross point SW (IC601) contains a 8X16 array of cross point switches along with a 7 to 128 line decoder and latch circuits.
Any one of the 128 switches can be addressed by selecting appropriate seven address bits. The selected switch can be turned
on or off by applying either logical one or zero to the DATA input. Chip select allows the cross point array to be cascaded for
matrix expansion. 
The cross point SW's (IC602, IC603) contain 4X8 array of cross point switches along with a 5 to 32 line decoder and latch cir-
cuits. Any one of the 32 switches can be addressed by selecting appropriate five address bits. The selected switch can be
turned on or off by applying either logical one or zero to the DATA input. Chip select allows the cross point array to be cascaded
for matrix expansion.