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)

Application Circuit

The following shows the complete application circuit for the HC-5504B SLIC, including all external components, power connections, and interface to the ESP32.

Basic SLIC Connections

Telephone Line (TIP/RING)
        |
        v
┌───[ Protection Circuit ]───┐
│   (See Protection section) │
└──────────┬─────────────────┘
           │
           ├── TIP ──────────────────────┐
           │                              │
           └── RING ────────────────────┐ │
                                        │ │
                  ┌─────────────────────┼─┼───────────┐
                  │   HC-5504B SLIC     │ │           │
                  │                     │ │           │
Power Supplies:   │  Pin 1: TIP ────────┘ │           │
                  │  Pin 2: RING ──────────┘           │
+12V ─────────┬───┤  Pin 4: VB+ (+12V)                 │
              │   │  Pin 11: VB- (-48V) ───────── -48V │
Decoupling:   │   │  Pin 12: BG (Battery GND)          │
100nF + 10µF  │   │  Pin 6: DG (Digital GND)           │
              │   │  Pin 23: AG (Analog GND) ──┬───────┤
              │   │                             │       │
External      │   │  Pin 9: TF ──┬── 150Ω ─────┤       │
Components:   │   │  Pin 10: RF ─┴── 150Ω ─────┤       │
              │   │  Pin 5: C3 ──── 4.7µF ─────┤       │
              │   │  Pin 17: C2 ─── 2.2µF ─────┤       │
              │   │  Pin 22: C4 ─── 0.1µF ─────┤       │
              │   │                             │       │
Signals:      │   │  Pin 13: SHD ───────────── ESP32   │
              │   │  Pin 8: RD ────────────── GPIO 32  │
              │   │  Pin 16: RC ───────────── GPIO 33  │
              │   │                         GPIO 13    │
Audio:        │   │  Pin 24: TX ───────┐               │
              │   │  Pin 21: RX ───────┤── Codec       │
              │   │                     │   I2S         │
              │   └─────────────────────┴───────────────┘
              │
              └─ Star Ground Point (AG=DG=BG)

External Component Values

Based on HC-5504B datasheet recommendations:

  • R_TIP, R_RING (Pins 9, 10): 150Ω, 1% tolerance, 1/4W

    • Feed resistors for tip and ring

    • Critical for proper loop current limiting

  • C3 (Pin 5): 4.7µF electrolytic or ceramic

    • Loop current timing capacitor

    • Voltage rating: min 16V

  • C2 (Pin 17): 2.2µF

    • Ring trip and ground key filter

    • Voltage rating: min 16V

  • C4 (Pin 22): 0.1µF ceramic

    • Longitudinal balance capacitor

    • Improves common-mode noise rejection

  • Decoupling on VB+: 100nF ceramic (close to IC) + 10µF electrolytic

    • Place ceramic capacitor within 0.5” of pin 4

    • Electrolytic for bulk supply stabilization

Note: The SLIC requires external protection (bridge rectifier, MOV, fuses) for robust field operation. See the “Line Protection” section for complete protection circuit design.

ESP32 GPIO Interface

The HC-5504B SLIC interfaces with the ESP32 microcontroller through several GPIO pins for control and status monitoring. Audio signals flow through an external codec connected via I2S.

GPIO Pin Mapping

SLIC Pin

SLIC Function

ESP32 GPIO

Direction

Description

13

SHD (Switch Hook Detect)

GPIO 32

Input

Off-hook detection - Goes LOW when phone is off-hook

8

RD (Ring Relay Driver)

GPIO 33

Input

Ring relay status monitoring - Active LOW when relay engaged

16

RC (Ring Command)

GPIO 13

Output

Trigger ring sequence - Drive LOW to activate ringing

15

PD (Power Denial)

Not Connected

(Output)

Optional power control - Not used in this design

14

GKD (Ground Key Detect)

Not Connected

(Input)

Optional ground key detection - Not used in this design

24

TX (Transmit Audio)

Via Audio Codec

Input to SLIC

Audio from ESP32 → Codec → SLIC → Phone

21

RX (Receive Audio)

Via Audio Codec

Output from SLIC

Audio from Phone → SLIC → Codec → ESP32

Signal Level Notes

  • Logic Levels: All SLIC digital outputs (SHD, RD, GKD) operate at 3.3V logic levels, directly compatible with ESP32 GPIOs

  • Input Protection: No level shifting required - SLIC pins 13, 14, 8 can directly drive ESP32 inputs

  • Output Drive: ESP32 GPIO 13 can directly drive SLIC pin 16 (RC) for ring control

  • Pull-ups: Internal ESP32 pull-ups may be enabled on input pins (SHD, RD) for noise immunity

Key Interface Functions

Off-Hook Detection (SHD → GPIO 32): The SHD pin goes LOW when the telephone handset is lifted (off-hook condition). The ESP32 monitors this pin to detect when a call is initiated or answered.

Ring Control (GPIO 13 → RC): To ring the telephone, the ESP32 drives GPIO 13 LOW. This signals the SLIC to engage the ring relay and apply ringing voltage to the line. A typical ring pattern is 2 seconds ON, 4 seconds OFF.

Ring Status Monitor (RD → GPIO 33): The RD pin reflects the state of the internal ring relay driver. When active (LOW), the ring relay is engaged. This allows the ESP32 to confirm ringing is occurring.

Audio Path: Audio does not connect directly to ESP32. Instead, the SLIC’s TX and RX pins connect to an external audio codec, which performs analog-to-digital and digital-to-analog conversion. The codec then interfaces with ESP32 via I2S (see Audio Signal Path section).

Audio Signal Path

The SLIC’s analog audio outputs (TX and RX pins) require an external audio codec to interface with the ESP32’s digital I2S bus. The codec performs ADC (analog-to-digital) and DAC (digital-to-analog) conversion.

Signal Flow Diagram

┌──────────┐        ┌───────────┐       ┌─────────────┐       ┌───────────┐
│          │ Analog │           │ I2S   │             │  BT   │           │
│  Phone   ├────────┤   Audio   ├───────┤    ESP32    ├───────┤ Bluetooth │
│  (SLIC)  │        │   Codec   │       │             │       │  Device   │
│          ├────────┤           │       │             │       │           │
└──────────┘        └───────────┘       └─────────────┘       └───────────┘
 TX    RX            DAC   ADC          I2S Pins:
 Pin24 Pin21                            - GPIO 25 (LRCLK/WS)
                                        - GPIO 5  (BCLK)
                                        - GPIO 26 (DOUT to codec)
                                        - GPIO 35 (DIN from codec)

Transmit Path (ESP32 → Phone Earpiece): ESP32 → I2S → Codec DAC → SLIC RX (Pin 21) → Tip/Ring (RJ11: green=Tip, red=Ring) → Telephone handset earpiece

Receive Path (Phone Microphone → ESP32): Telephone handset microphone → Tip/Ring (RJ11: green=Tip, red=Ring) → SLIC TX (Pin 24) → Codec ADC → I2S → ESP32

Selected Audio Codec Configuration

The Ma Bell Gateway uses the PCM5100 (DAC) + PCM1808 (ADC) from Texas Instruments for production-quality audio in Bluetooth telephony applications.

Why This Configuration

  • “Wire and Go” Simplicity: Hardware-configured via pin strapping - no I2C initialization, no MCLK signal required. Just connect power, I2S signals, and audio - it works.

  • Reliable Integration: No codec driver code means no codec-related bugs. The chips configure themselves via pin strapping.

  • Overkill Specs (Intentional): The 106 dB SNR DAC and 98 dB SNR ADC far exceed 8kHz telephony requirements. These chips were chosen for integration simplicity, not optimal specification matching.

  • Trade-off Acknowledged: Using two separate chips instead of one integrated codec (like ES8388 or WM8960) costs slightly more in BOM, but eliminates firmware complexity entirely.

PCM5100 (DAC) - Digital to Analog Converter

Key Specifications:

  • 32-bit, 384 kHz stereo audio DAC

  • SNR: 106 dB (A-weighted)

  • THD+N: -93 dB

  • Hardware-configured (no I2C/SPI needed)

  • Integrated PLL for clock flexibility

  • 2.5V - 3.6V supply voltage

Pin Connections:

PCM5100 Pin

ESP32 Connection

Notes

LRCK (WS)

GPIO 25

I2S word select / frame sync

BCK (SCK)

GPIO 5

I2S bit clock

DIN

GPIO 26

I2S data from ESP32

XSMT

3.3V (via 10kΩ)

System mute (HIGH = unmuted)

FMT

GND

I2S format selection (see datasheet)

VDD

3.3V

Power supply (+decoupling)

GND

GND

Ground

OUTL/OUTR

AC coupling to SLIC RX

Analog output (use OUTL for mono)

PCM1808 (ADC) - Analog to Digital Converter

Key Specifications:

  • 24-bit, 96 kHz stereo audio ADC

  • SNR: 98 dB (A-weighted)

  • THD+N: -85 dB

  • Hardware-configured via pin strapping

  • Single-ended or differential input

  • 2.7V - 5.5V supply voltage

Pin Connections:

PCM1808 Pin

ESP32 Connection

Notes

LRCK

GPIO 25

I2S word select / frame sync (shared with DAC)

BCK

GPIO 5

I2S bit clock (shared with DAC)

DOUT

GPIO 35

I2S data to ESP32

FMT0

GND

I2S format selection (00 = I2S, 24-bit)

FMT1

GND

I2S format selection

MD0

3.3V

Mode selection: slave mode

MD1

GND

Mode selection

VDD

3.3V

Power supply (+decoupling)

GND

GND

Ground

VINL

AC coupling from SLIC TX

Analog input (use VINL for mono)

Signal Path Diagram

Transmit Path (Phone → ESP32 → Bluetooth):
┌───────────┐                           ┌────────────┐              ┌────────────┐
│  Handset  │     TX Pin 24             │  PCM1808   │     I2S      │   ESP32    │
│  (Mic)    ├───────────────┬─[10µF]───┤ VINL  DOUT ├──────────────┤ GPIO 35    │
└───────────┘               │           └────────────┘              └────────────┘
                SLIC        └─[10kΩ]─┬─GND
                                      └─AGND (Pin 23)

Receive Path (Bluetooth → ESP32 → Phone):
┌───────────┐              ┌────────────┐                           ┌────────────┐
│   ESP32   │     I2S      │  PCM5100   │     RX Pin 21             │  Handset   │
│ GPIO 26   ├──────────────┤ DIN  OUTL  ├───[10µF]──────────────────┤ (Speaker)  │
└───────────┘              └────────────┘                           └────────────┘
                                                     SLIC

Common I2S Clock Signals (Shared):
ESP32 GPIO 25 ────┬───► PCM5100 LRCK
                  └───► PCM1808 LRCK

ESP32 GPIO 5  ────┬───► PCM5100 BCK
                  └───► PCM1808 BCK

Bill of Materials

Component

Part Number

Qty

Notes / Supplier

DAC

PCM5100PWR

1

Texas Instruments, TSSOP-20

ADC

PCM1808PWR

1

Texas Instruments, TSSOP-20

AC Coupling Caps

10µF film/MLCC

2

Non-polarized, 16V+, X7R/C0G

Decoupling Caps (DAC)

0.1µF + 10µF

2

Ceramic + tantalum/MLCC

Decoupling Caps (ADC)

0.1µF + 10µF

2

Ceramic + tantalum/MLCC

Pull-up Resistor (XSMT)

10kΩ

1

1% metal film

Input Bias Resistor

10kΩ

1

For PCM1808 VINL to AGND

PCB Layout Recommendations

  • Placement: Keep codec ICs within 3-4 inches of ESP32 for clean I2S signals

  • Signal Integrity:

    • Route I2S clock signals (LRCK, BCK) as matched-length pairs

    • Keep I2S data lines (DIN, DOUT) short and direct

    • Avoid routing I2S signals near high-frequency switching (e.g., power converters)

  • Power:

    • Place 0.1µF decoupling caps within 5mm of each IC’s VDD pin

    • Add 10µF bulk caps near codec power pins

    • Use low-ESR ceramic capacitors (X7R or better)

  • Ground:

    • Connect codec grounds to AGND plane (SLIC Pin 23)

    • Avoid ground loops - use star grounding topology

    • Separate analog and digital ground planes if possible

  • Audio Coupling:

    • Place AC coupling capacitors close to SLIC pins

    • Use film or high-quality MLCC capacitors for audio paths

    • Keep analog traces short to minimize noise pickup

Prototyping Option: ESP32 Internal DAC

Note

For Prototyping/Testing Only

The ESP32 has two 8-bit internal DACs (GPIO25, GPIO26) that can be used for transmit-only testing during early development:

  • Use Case: Quick audio output testing without external hardware

  • Limitations:

    • 8-bit resolution (vs. 16-bit+ for external codecs) = significantly lower quality

    • Transmit-only - no ADC capability for receiving audio from phone

    • Cannot support full-duplex Bluetooth HFP calls

    • Higher noise floor and distortion

  • Connection: ESP32 DAC pin → 10µF AC coupling capacitor → SLIC RX (Pin 21)

  • Not suitable for production - Use PCM5100 + PCM1808 for production telephony applications

Alternative Codec Options (Reference)

For reference, alternative codec configurations are possible but not recommended for the Ma Bell Gateway:

WM8731 (Wolfson/Cirrus Logic) - Integrated Codec

  • Single-chip ADC+DAC solution, well-documented

  • Requires I2C configuration (additional firmware complexity)

  • Cost: ~$3-5

  • When to use: If PCB space is extremely limited and single-chip solution is required

UDA1334A (NXP) - DAC Only

  • Transmit-only (no ADC)

  • Cost: ~$2

  • When to use: If only ESP32→Phone audio is needed (no bidirectional calls)

Audio Coupling

Between the codec outputs and SLIC RX/TX pins, AC coupling capacitors are recommended:

  • Value: 10µF non-polarized (or 10µF electrolytic in series pair for bipolar)

  • Purpose: Block any DC offset from codec outputs

  • Placement: As close to SLIC pins as possible

Codec Output ──┤├── 10µF ──┤├── 10µF ─── SLIC RX/TX Pin
                (AC coupling, bipolar configuration)

Or use non-polarized film capacitors if available.

I2S Pin Connections (PCM5100 + PCM1808)

ESP32 I2S interface connections to PCM5100 (DAC) and PCM1808 (ADC), as defined in pin_assignments.h:

Shared Clock Signals:

  • GPIO 25 - PCM_FSYNC (I2S Word Select / LRCLK / Frame Sync)

    • Connected to: PCM5100 LRCK + PCM1808 LRCK

    • Common frame sync for both DAC and ADC

  • GPIO 5 - PCM_CLK_OUT (I2S Bit Clock / BCLK)

    • Connected to: PCM5100 BCK + PCM1808 BCK

    • Common bit clock for both DAC and ADC

Data Signals:

  • GPIO 26 - PCM_DOUT (I2S Data Out from ESP32)

    • Connected to: PCM5100 DIN

    • Audio data to DAC (for playback/receive path)

  • GPIO 35 - PCM_DIN (I2S Data In to ESP32)

    • Connected to: PCM1808 DOUT

    • Audio data from ADC (for recording/transmit path)

Connection Summary:

ESP32                       PCM5100 (DAC)         PCM1808 (ADC)
──────                      ─────────────         ─────────────
GPIO 25 (LRCLK)  ──────┬──► LRCK          ┌────► LRCK
GPIO 5  (BCLK)   ──────┼──► BCK           ├────► BCK
GPIO 26 (DOUT)   ───────┘─► DIN           │
GPIO 35 (DIN)    ◄─────────────────────────────── DOUT

All I2S signals should be routed as short, direct traces with matched lengths where practical.

Sample Rate and Format

Typical audio configuration for telephone-quality audio:

  • Sample Rate: 8 kHz or 16 kHz (8 kHz is standard for telephony)

  • Bit Depth: 16-bit

  • Format: I2S standard format

  • Channels: Mono (though I2S is stereo-capable, phone audio is mono)

Power Supply Integration

The HC-5504B SLIC requires multiple voltage rails and careful grounding to function properly. All power rails are derived from the main 48V supply as documented in the Power Supply Architecture (see power-supply.rst).

Voltage Rail Requirements

SLIC Pin

Rail

Voltage

Source/Notes

Pin 4 (VB+)

Positive Supply

+12V DC

From 48V→12V buck converter. Powers SLIC analog circuits

Pin 11 (VB-)

Negative Battery

-48V DC

From inverting DC-DC converter. Loop feed voltage for telephone line

Pin 12 (BG)

Battery Ground

0V (GND)

Battery/power supply ground reference

Pin 6 (DG)

Digital Ground

0V (GND)

Digital logic ground

Pin 23 (AG)

Analog Ground

0V (GND)

Analog/audio ground

Power Rail Connections

From Power Supply System:

48V Main Supply
     │
     ├─► Buck Converter ──► +12V ──┬──► VB+ (Pin 4)
     │                              │
     │                     Decoupling: 100nF + 10µF
     │                              │
     └─► Inverting DC-DC ─► -48V ──┴──► VB- (Pin 11)


                              Star Ground Point ◄─┬─ AG (Pin 23)
                                   (GND)          ├─ DG (Pin 6)
                                                  └─ BG (Pin 12)

Grounding Strategy

The SLIC has three separate ground pins that must be connected using a star grounding topology:

  1. AG (Analog Ground, Pin 23) - For audio and analog signals

  2. DG (Digital Ground, Pin 6) - For digital logic signals

  3. BG (Battery Ground, Pin 12) - For power supply return

Star Ground Implementation:

All three ground pins should connect to a single point as close to the SLIC as possible. This minimizes ground loops and prevents noise coupling between analog, digital, and power domains.

SLIC Pins:                           Common Star
                                     Ground Point
     AG (23) ─────────────────────────┐
                                      ├───► System GND
     DG (6)  ─────────────────────────┤
                                      │
     BG (12) ─────────────────────────┘

On PCB: Use a copper pour or wide trace for the star ground point, with short, direct connections from each SLIC ground pin.

Power Supply Decoupling

Critical for stable SLIC operation:

On VB+ (+12V, Pin 4):

  • 100nF ceramic capacitor - Place within 0.5 inches (12mm) of pin 4

  • 10µF electrolytic or ceramic - Place within 1 inch (25mm) of pin 4

  • Purpose: Filter switching noise from buck converter, stabilize analog circuits

On VB- (-48V, Pin 11):

  • 10µF electrolytic, 63V rated - Place near pin 11

  • Purpose: Stabilize negative supply, handle transients during ringing

Example PCB Layout

┌──────────────────────────────────────┐
│         HC-5504B SLIC Package        │
│                                      │
│   Pin 4 (VB+) ─┬─ 100nF (ceramic)   │
│     +12V       └─ 10µF (electro/cer) │
│                                      │
│   Pin 11 (VB-) ─── 10µF, 63V (electro)
│      -48V                            │
│                                      │
│   Pin 23 (AG) ──┐                   │
│   Pin 6  (DG) ──┤                   │
│   Pin 12 (BG) ──┴─► ★ Star GND      │
│                                      │
└──────────────────────────────────────┘

Cross-Reference

For complete power supply system design including:

  • 48V main supply specifications

  • Buck converter selection (+12V rail)

  • Inverting DC-DC design (-48V rail)

  • Ring generator (90V AC, 20Hz)

See: docs/source/implementation/circuit/power-supply.rst

Line Protection

The telephone line interface must include protection circuitry to safeguard the SLIC and connected electronics from overvoltage, reverse polarity, and fault conditions. The HC-5504B provides some internal protection, but external components are essential for robust field operation.

Protection Circuit Overview

Telephone                                          To SLIC
Line Jack                                       TIP/RING Pins
     │                                                │
     ├─ TIP ──┬─[ Fuse 0.5A ]──┬─[ Bridge Rect ]──┬──► TIP (Pin 1)
     │        │                 │    DB107 or      │
     │        │                 │    equiv.        │
     └─ RING ─┴─[ Fuse 0.5A ]──┴──────┬───────────┴──► RING (Pin 2)
              │                        │
              │                      [ MOV ]
              │                    V150LA10A
              │                    (150V rating)
              │                        │
              └────────────────────────┴──► GND


Protection Elements:
1. Fuses - Overcurrent protection (0.5A slow-blow)
2. Bridge Rectifier - Reverse polarity protection
3. MOV - Overvoltage/surge protection

Protection Components

1. Overcurrent Protection - Fuses

  • Type: 0.5A slow-blow (time-delay) fuse

  • Purpose: Protect against sustained overcurrent or short circuit

  • Placement: One fuse in series with TIP, one with RING

  • Rating: 250V AC minimum

  • Part Example: Littelfuse 0217.500 or equivalent

Why slow-blow? Telephone lines experience brief current surges during ringing. A fast-blow fuse would trip unnecessarily. Slow-blow fuses tolerate brief surges while still protecting against sustained faults.

2. Reverse Polarity Protection - Bridge Rectifier

  • Type: Full-wave bridge rectifier

  • Purpose: Protect SLIC from reversed TIP/RING connections

  • Rating: 1A, 200V minimum

  • Part Example: DB107 (1A, 1000V) or Vishay DF10S (1A, 1000V)

The bridge rectifier ensures the SLIC always sees correct polarity regardless of how the telephone line is connected. This is critical because telephone line polarity can vary in field installations.

Bridge Rectifier Operation:

TIP/RING ──┬─►|├──┬──► To SLIC TIP (always positive)
           │  BR  │
           └─►|├──┴──► To SLIC RING (always negative)

3. Overvoltage Protection - Metal Oxide Varistor (MOV)

  • Type: MOV (Metal Oxide Varistor)

  • Purpose: Clamp transient overvoltages (lightning, power line cross, inductive kicks)

  • Rating: 150V RMS / 200V DC

  • Part Example: Littelfuse V150LA10A or Bourns MOV-14D151K

  • Placement: Across TIP and RING, after fuses

The MOV acts as a voltage-dependent resistor. Under normal voltage (<150V), it has very high resistance. During a surge (>150V), it clamps to ground, shunting excess energy away from the SLIC.

Optional: Gas Discharge Tube (GDT)

For enhanced lightning protection in areas with high lightning activity:

  • Type: Gas discharge tube (GDT)

  • Rating: 90V - 230V breakdown

  • Placement: Parallel to MOV, or as first stage before MOV

  • Purpose: Handle very high energy transients (direct lightning strikes)

GDTs can handle much higher energy than MOVs but have slower response time. Using both (GDT as first stage, MOV as second stage) provides optimal protection.

Complete Protection Schematic

Phone Line Jack
     │
TIP ─┼─[ F1: 0.5A ]──┬────────┬───────┐
     │               │        │       │
     │             [ BR1 ]  [ MOV ] [GDT]   ← Protection
     │               │      150V   90-230V    Components
     │               │        │       │
RING─┴─[ F2: 0.5A ]──┴────────┴───────┘
                              │
                             GND
                              │
                 ┌────────────┴────────────┐
                 │                         │
          Rectified & Protected     To HC-5504B
          TIP/RING Output           Pins 1 & 2
                 │                         │
                 └─────────────────────────┘

Placement and Routing

  • Fuses: Place immediately after phone line connector

  • Bridge Rectifier: Place after fuses, before SLIC

  • MOV: Place between TIP/RING and ground, as close to fuses as practical

  • GDT (if used): Place in parallel with MOV, or as first protection stage

PCB Considerations:

  • Use wide traces (>20 mil) for TIP/RING to handle surge currents

  • Keep protection components physically close to phone jack

  • Route protected signals to SLIC with minimal trace length

  • Maintain adequate creepage/clearance for high voltage (>2mm for 150V)

Component Selection Notes

  • Fuse holders: Use panel-mount or PCB-mount holders for easy replacement

  • Bridge: Higher voltage rating (e.g., 1000V) provides extra margin

  • MOV clamping voltage: Choose 130-150V range for -48V telephone lines

  • All components: Ensure automotive/telecom grade for reliability

Safety Notes

  • This protection is for normal telephone line faults and surges

  • For direct lightning strikes, additional building-level protection (whole-house surge suppressors) is recommended

  • Always test protection circuits with controlled overvoltage sources before field deployment

  • Replace MOVs after major surge events (they degrade with use)

Component Selection & Bill of Materials

This section provides a complete BOM for the SLIC circuit, including all passive components, protection devices, and recommended part numbers.

SLIC External Components

Component

Value/Type

Part Number/Example

Notes

R_TIP (Pin 9 feed)

150Ω, 1%, 1/4W

Generic metal film

Tip feed resistor, precision critical

R_RING (Pin 10 feed)

150Ω, 1%, 1/4W

Generic metal film

Ring feed resistor, precision critical

C3 (Pin 5)

4.7µF, 16V+

Generic electrolytic/ceramic

Loop current timing capacitor

C2 (Pin 17)

2.2µF, 16V+

Generic electrolytic/ceramic

Ring trip/ground key filter

C4 (Pin 22)

0.1µF (100nF)

Generic ceramic

Longitudinal balance

C_VB+ (Pin 4 decoupling)

100nF ceramic

Generic X7R/X5R

Place <0.5” from pin 4

C_VB+ (Pin 4 bulk)

10µF, 16V+

Generic electrolytic/ceramic

Place <1” from pin 4

C_VB- (Pin 11)

10µF, 63V

Generic electrolytic

-48V supply decoupling

Protection Circuit Components

Component

Value/Type

Part Number/Example

Notes

F1, F2 (Fuses)

0.5A slow-blow, 250V

Littelfuse 0217.500

Overcurrent protection on TIP/RING

BR1 (Bridge)

1A, 200V+ bridge

DB107 (1A, 1000V)

Reverse polarity protection

MOV1

150V RMS MOV

Littelfuse V150LA10A

Overvoltage/surge protection

GDT1 (Optional)

90-230V GDT

Bourns 2036-15-SM

Enhanced lightning protection

Audio Coupling Components

Component

Value/Type

Part Number/Example

Notes

C_TX, C_RX

10µF non-polar or film

Generic film or dual electrolytic

AC couple codec to SLIC TX/RX

Ring Relay (if external ringing used)

Component

Value/Type

Part Number/Example

Notes

RLY1

DPDT, 12V coil

Omron G5V-2 or equiv

Switches 90V AC ring voltage

D1 (Flyback diode)

1N4148 or 1N4001

Generic signal/power diode

Protect relay driver from inductive kick

Main IC

Component

Value/Type

Part Number/Example

Notes

U1 (SLIC)

HC-5504B-5 DIP-24

HC-5504B-5 (Renesas/Intersil)

Main SLIC IC

Audio Codec (Choose One Option)

Option 1: Integrated Codec

WM8731 Codec

Wolfson/Cirrus WM8731SEDS/V

Stereo codec, I2S + I2C interface

Option 2: Separate DAC/ADC

PCM5100 DAC

Texas Instruments PCM5100PW

DAC for ESP32 → Phone path

PCM1808 ADC

Texas Instruments PCM1808PWR

ADC for Phone → ESP32 path

Option 3: Simple DAC (TX only)

UDA1334A DAC

NXP UDA1334ATS

Simple I2S DAC, TX path only

Connector & Miscellaneous

Component

Description

Notes

J1

RJ11/RJ45 phone jack

Standard 6P4C or 8P8C modular jack for telephone line

Fuse holders

Panel/PCB mount holders

For F1, F2 (facilitates replacement)

Sourcing Notes

  • Most passive components (resistors, capacitors) are generic and available from Digi-Key, Mouser, LCSC, or similar

  • SLIC IC (HC-5504B-5) may have limited availability; check Renesas, Digi-Key, or surplus distributors

  • Audio codecs (WM8731, PCM5100, PCM1808) are widely available

  • Use automotive/industrial-grade components where possible for reliability

Total Estimated Cost

Approximate component costs (USD, quantity 1, retail pricing):

  • SLIC IC (HC-5504B-5): $8-15 (if available)

  • Passives (resistors, caps): $2-5

  • Protection (fuses, bridge, MOV): $3-5

  • Audio codec: $3-6

  • Ring relay (if used): $3-5

  • Misc (connectors, PCB): $5-10

Total: ~$25-45 USD for SLIC circuit components (excluding ESP32 and power supply)

PCB Design Guidelines

The SLIC circuit requires careful PCB layout for reliable operation. Key considerations include:

  • Star ground topology for SLIC ground pins (AG, DG, BG)

  • High voltage clearances for -48V and 90V ring signals

  • Separation of analog audio paths from digital signals

  • Protection circuit placement near phone jack

For complete PCB layout guidelines including grounding, decoupling, layer stackup, and component placement, see PCB Design Overview.

Design Notes

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

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

  • See datasheet for specific application examples and protection recommendations.

References

Hardware Documentation:

Firmware Integration:

  • SLIC Interface Monitoring - Off-hook detection and phone hardware monitoring implementation

  • main/hardware/phone_hardware.c - Source code for SLIC interface monitoring

  • main/app/pin_assignments.h - GPIO pin definitions

Note

This documentation describes the complete SLIC circuit design. For firmware implementation details on how the ESP32 interfaces with the SLIC hardware (off-hook detection, ring control), see the firmware documentation linked above.