6

Functional Diagram

21

RX

Receive Input, 4-Wire Side - A high impedance analog input which is internally biased. Capacitive coupling to this

input is required. AC signals appearing at this input differentially drive the Tip feed and Ring feed terminals, which in

turn drive tip and ring through 300

Ω of feed resistance on each side of the line.

22

Capacitor #4 - An external capacitor to be connected between this terminal and analog ground. This capacitor

prevents false ground key indication and false ring trip detection from occurring when longitudinal currents are

induced onto the subscriber loop from near by power lines and other noise sources. This capacitor is also required

for the proper operation of ring trip detection. Typical value is 0.5

µF, to 1.0µF, 20V. This capacitor should be

nonpolarized.

23

AG

(Note 5)

Analog Ground - To be connected to zero potential and serves as a reference for the transmit output (TX) and receive

input (RX) terminals.

24

TX

Transmit Output, 4-Wire Side - A low impedance analog output which represents the differential voltage across Tip

and Ring. Transhybrid balancing must be performed (using the SLIC microcircuit’s spare op amp) beyond this output

to completely implement two to four wire conversion. This output is unbalanced and referenced to analog ground.

Since the DC level of this output varies with loop current, capacitive coupling to the next stage is essential.

NC

No internal connection.

24 PIN

SOIC

SYMBOL

DESCRIPTION

LOOP

MONITORING

RING

CONTROL

2 WIRE

LOOP

SECONDARY

PROTECTION

LINE

DRIVERS

TIP

RING

PD

POWER DENIAL

150

Ω

RING

150

Ω

RF

BG

150

Ω

TF

TIP

RD

RC

RS

RING SYNC

RING COMMAND

RING

TRIP

SHD

GKD

OUT

RX

RECEIVE

INPUT

SLIC MICROCIRCUIT

1/2 RING

RELAY

RING

VOLTAGE

RFS

1/2 RING

RELAY

150

Ω

SWITCH HOOK

DETECTION

GROUND KEY

DETECTION

BATTERY

FEED

LOOP

CURRENT

LIMITER

DIFF

AMP

TX

TRANSMIT

OUTPUT

+

-

+1

-1

OP

AMP

+IN

-IN

+

-

HC-5504B