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2026-02-10 12:12:11 +01:00

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DCC Programming Track Implementation

Overview

The DCC-Bench uses the LM18200 H-Bridge for both normal DCC operation AND programming track functionality. Since this is a dedicated test bench with only one locomotive at a time, the same driver can handle both modes without issue.

Why This Works

Traditional DCC Systems

Main Track: High current (3-5A) for running multiple locomotives
Programming Track: Limited current (250mA max) with ACK detection

DCC-Bench Approach

Single Track: Only one locomotive under test
LM18200: Can handle both operation and programming
Current Limit: LM18200 has built-in current limiting
ACK Detection: Monitor current draw through sense resistor

Hardware Requirements

Essential Components

LM18200 H-Bridge (already in design)
- Dual-purpose: DCC signal amplification + programming
- Built-in current limiting and thermal protection
- Pins: GPIO 18 (Signal A), GPIO 19 (Signal B), GPIO 23 (Brake)
Current Sense Resistor (0.1Ω, 1W)
- Monitor programming track current
- Placed in series with LM18200 output
- Creates voltage drop proportional to current
ADC Input for ACK Detection
- ESP32 ADC pin (e.g., GPIO 35)
- Connected to current sense resistor voltage
- Detects 60mA+ ACK pulse from decoder

Optional Enhancements

Current Limiter Circuit: Additional 250mA fuse for extra safety
LED Indicator: Visual feedback during programming
Isolation: Optocouplers for additional protection

Wiring Diagram

ESP32 GPIO 18 ──────┐
                    ├──> LM18200 ──> Current Sense ──> TRACK
ESP32 GPIO 19 ──────┘                    │
                                         │
ESP32 GPIO 35 (ADC) <──── Voltage Divider ┘
                          (for ACK detect)

Current Sense Circuit:
                    0.1Ω, 1W
  Track+ ────┬──────╱╲╲╲───── LM18200 Output
             │
             ├─── 1kΩ ───┬──── GPIO 35 (ADC)
             │           │
             │         10kΩ
             │           │
  Track- ────┴───────────┴──── GND

DCC Programming Protocol

Service Mode (Programming Track)

The DCC-Bench implements NMRA DCC Service Mode:

Factory Reset (CV8 = 8)
- Resets decoder to factory defaults
- Standard NMRA reset command
Set Address
- Short Address (1-127): Write to CV1
- Long Address (128-10239): Write to CV17 + CV18
- Automatically updates CV29 for address mode
CV Read (Bit-wise Verify)
- Tests each bit (0-7) individually
- More reliable than direct read
- Requires ACK detection for each bit
CV Write (Write + Verify)
- Writes value with 3 retry attempts
- Verifies write with ACK detection
- NMRA-compliant packet structure

ACK Detection

How It Works:

Decoder receives programming command
If command is valid and matches, decoder draws 60mA pulse for 6ms
Current sense resistor creates voltage spike
ESP32 ADC detects voltage above threshold
ACK confirmed = command successful

Threshold Values:

ACK Current: 60mA minimum (NMRA standard)
ACK Duration: 6ms typical
Timeout: 20ms wait for response

Current Implementation Status

✅ Implemented

NMRA-compliant packet encoding
Service mode packet structure (22-bit preamble)
Factory reset command (CV8 = 8)
Address programming (short and long)
CV read (bit-wise verify method)
CV write (write + verify)
Programming screen UI with numeric keypad

⚠️ Needs Hardware

ACK Detection: Currently returns true (assumed success)
Current Sensing: ADC reading not yet implemented
Calibration: Threshold tuning for specific hardware

Adding ACK Detection

Step 1: Wire Current Sense

// Add current sense resistor (0.1Ω) in series with track output
// Connect voltage divider to ESP32 GPIO 35 (ADC1_CH7)

Step 2: Update `waitForAck()` Method

bool DCCGenerator::waitForAck() {
    #define CURRENT_SENSE_PIN 35
    #define ACK_THRESHOLD 100  // Adjust based on calibration
    
    unsigned long startTime = millis();
    
    // Wait up to 20ms for ACK pulse
    while (millis() - startTime < 20) {
        int adcValue = analogRead(CURRENT_SENSE_PIN);
        
        // If current spike detected (60mA+)
        if (adcValue > ACK_THRESHOLD) {
            Serial.println("ACK detected!");
            return true;
        }
        
        delayMicroseconds(100);
    }
    
    Serial.println("No ACK");
    return false;
}

Step 3: Calibrate Threshold

// Test with known-good decoder
// Measure ADC values during programming
// Adjust ACK_THRESHOLD to reliably detect 60mA pulse

Safety Features

Built-in Protection

LM18200 Thermal Shutdown: Protects against overheating
Current Limiting: Prevents excessive current draw
Brake Pin: Emergency stop capability (GPIO 23)

Software Safety

Power-Off on Mode Change: Prevents accidental high current
CV Range Validation: Only allows CV 1-1024
Address Range Check: Validates 1-10239
Write Verification: Confirms successful programming

Recommended Additions

250mA Fuse: Additional protection for programming track
Timeout Handling: Abort if no response after retries
Error Logging: Track failed programming attempts

Usage

From Touchscreen UI

Enter DCC Mode
- Press [MODE] button until "DCC" selected
- Press [POWER] to enable
Open Programming Screen
- Press [PROG] button (appears in DCC mode)
Factory Reset
- Press [FACTORY RESET] button
- Wait for confirmation (or timeout)
Set Address
- Enter address using keypad (field auto-selected)
- Press [SET ADDR] button
- Wait for verification
Read CV
- Enter CV number (tap CV field, then use keypad)
- Press [READ] button
- Value appears in CV Value field
Write CV
- Enter CV number and value
- Press [WRITE] button
- Wait for verification

From Serial Monitor

DCCGenerator dcc;

// Factory reset
dcc.factoryReset();

// Set address to 42
dcc.setDecoderAddress(42);

// Read CV7 (Version)
uint8_t version;
if (dcc.readCV(7, &version)) {
    Serial.printf("Decoder version: %d\n", version);
}

// Write CV3 (Acceleration) = 20
dcc.writeCV(3, 20);

Troubleshooting

No ACK Detected

Possible Causes:

Current sense not connected
Threshold too high/low
Decoder not responding
Wrong CV number/value

Solutions:

Verify current sense wiring
Test with multimeter (should see 60mA spike)
Calibrate ADC threshold
Try factory-reset decoder first
Check decoder is DCC-compatible

Programming Fails

Check:

Only one locomotive on track
Decoder supports NMRA DCC
Track connections solid
LM18200 powered and enabled
No shorts on track

Inconsistent Results

Causes:

Dirty track/wheels
Poor electrical contact
Noise on current sense line
Decoder in bad state