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) ----------------------- .. list-table:: :header-rows: 1 :widths: 8 18 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 ---------- - `HC-5504B Datasheet (Intersil/Renesas) `_ .. note:: Update this page with your final schematic snippet, board pin numbers, and any specific integration notes as you build and test.