Line Interface: HC-5504B SLIC

The HC-5504B Subscriber Line Interface Circuit (SLIC) is the critical component that connects the Ma Bell Gateway to a standard analog telephone. It implements nearly all essential BORSCHT functions (Battery feed, Overvoltage protection, Ringing, Supervision, Codec hybrid, and Test), emulating the classic Bell System line card in a single chip.

Overview

  • Provides DC loop feed (-48V) and line supervision for the phone.

  • Handles 2-wire to 4-wire hybrid audio conversion.

  • Drives ringing voltage to the subscriber line via relay control.

  • Detects off-hook, ring trip, and ground key conditions.

  • Integrates protection features for robust operation.

Pinout Summary (DIP-24)

Pin

Name

Description

1

TIP

Analog subscriber loop (tip)

2

RING

Analog subscriber loop (ring)

3

RFS

Ring Feed Sense (supervision, ringing)

4

VB+

Positive supply (+5V or +12V)

5

C3

Loop current/timing capacitor

6

DG

Digital ground

7

RS

Ring synchronization input (tie to +5V if unused)

8

RD

Ring relay driver output (logic low = active)

9

TF

Tip Feed (to line via 150Ω resistor)

10

RF

Ring Feed (to line via 150Ω resistor)

11

VB-

-48V battery supply

12

BG

Battery ground

13

SHD

Switch hook detect (off-hook, logic output)

14

GKD

Ground key detect (logic output)

15

PD

Power denial (logic input)

16

RC

Ring command (logic input)

17

C2

Capacitor for ring trip/ground key filter

18

OUT

Uncommitted op-amp output (optional)

19

-IN

Op-amp inverting input

20

+IN

Op-amp non-inverting input

21

RX

Receive input (audio from codec/DAC)

22

C4

Capacitor for longitudinal balance/filter

23

AG

Analog ground

24

TX

Transmit output (audio to codec/ADC)

Typical Application Circuit

#.. figure:: /images/hc5504b-app-circuit.png # :alt: Typical application circuit for HC-5504B SLIC

Typical application circuit for the HC-5504B SLIC (see datasheet for details).

Note: - Requires external protection (bridge rectifier, MOV, fuses) for robust field operation. - Uses precision resistors (typically 150Ω) and filter capacitors as per the datasheet. - Relay or MOSFET driver may be needed for high-voltage ringing.

Interfacing with ESP32

  • SHD (Switch Hook Detect, Pin 13): Connects to ESP32 input GPIO for off-hook status.

  • GKD (Ground Key Detect, Pin 14): Optional; connects to ESP32 input if ground key features are needed.

  • RD (Ring Relay Driver, Pin 8): Can be monitored by ESP32 for relay status or used to trigger ringing logic.

  • RC (Ring Command, Pin 16): Controlled by ESP32 output to trigger ringing sequence.

  • PD (Power Denial, Pin 15): Controlled by ESP32 output if selective power shutdown is desired.

  • TX (Pin 24), RX (Pin 21): Connect to audio codec, then to ESP32 I2S interface for digital audio in/out.

Design Notes

  • Carefully manage ground domains: AG, DG, and BG should be star-connected as per best practices.

  • VB- (-48V) and VB+ (+5V/+12V) supplies must be isolated and protected.

  • See datasheet for specific application examples and protection recommendations.

References

Note

Update this page with your final schematic snippet, board pin numbers, and any specific integration notes as you build and test.