Adding Servo-Controlled Turnouts to Your Model Railway with DCC-EX and EXRAIL

Adding Servo-Controlled Turnouts with DCC-EX, EXRAIL and PCA9685

This support guide explains how to add reliable, low-cost servo turnout control to a working DCC-EX CommandStation using a PCA9685 16-channel I2C servo controller and EXRAIL.

It covers wiring, HAL setup, turnout definitions and basic servo calibration.

Which setup are you using?

Hardware You Need

To add servo-controlled turnouts, you will need:

• Working DCC-EX CommandStation
• PCA9685 16-channel I2C servo controller board
• Servos — one per turnout
• Regulated 5V power supply for the servos
• I2C connection to your CommandStation, such as Qwiic or SDA/SCL pins
• Common ground between the CommandStation, PCA9685 and 5V servo power supply

The PCA9685 is used as an expansion board. It allows one DCC-EX CommandStation to control multiple servos for points, turnouts, signals or accessories.

Note: many users with the Ready-to-Run system will only need to focus on the PCA9685, servo power and EXRAIL configuration steps below.

Wiring the PCA9685 Servo Controller

1. Connect SDA on the PCA9685 to SDA on the CommandStation.
2. Connect SCL on the PCA9685 to SCL on the CommandStation.
3. Connect VCC for logic power, usually 3.3V or 5V depending on your board and setup.
4. Connect GND between the CommandStation and PCA9685.
5. Connect your separate 5V servo power supply to the PCA9685 servo power input.
6. Connect the servo power supply ground to the common ground.

Common ground example

CommandStation GND  ─┬─ PCA9685 GND
                     └─ 5V Servo PSU GND

Without a shared ground, the servos may behave unpredictably or fail to respond correctly.

Checking I2C Detection

After wiring the PCA9685, power up the CommandStation and check the console log. DCC-EX should list detected I2C devices and show the PCA9685 address.

The most common PCA9685 I2C address is:

0x40

If the console shows:

< No I2C Devices found >

Check the following:

• SDA and SCL are connected correctly
• The PCA9685 has logic power
• Ground is shared with the CommandStation
• The board is connected the correct way round
  

Using More Than One PCA9685 Servo Controller

If your layout requires more than 16 servo outputs, multiple PCA9685 boards can share the same I²C connection to the CSB1 or other DCC-EX CommandStation.

However, each PCA9685 connected to the same I²C bus must have a unique I²C address.

A new PCA9685 normally uses the default address:

0x40

If two boards are connected while both are set to 0x40, the CommandStation cannot address them independently. Before connecting additional boards, change their addresses using the small A0-A5 address solder pads on the PCA9685.

The address pads work as a binary offset from the base address of 0x40. Bridge the required pad, or combination of pads, with a small amount of solder.

For most model railway installations, this provides considerably more servo capacity than will normally be required.

Example: Connecting Two PCA9685 Boards

For two boards, leave the first PCA9685 unchanged at its default address:

0x40

On the second PCA9685, bridge the A0 solder pad. This changes its address to:

0x41

The two boards can then share the same SDA and SCL I²C connections while being recognised individually by DCC-EX.

The corresponding HAL configuration could be:

// First PCA9685 - channels 0-15
HAL(PCA9685, 100, 16, 0x40)

// Second PCA9685 - channels 16-31
HAL(PCA9685, 116, 16, 0x41)

This gives the first board vPins 100-115 and the second board vPins 116-131.

After changing an address, power up the CommandStation and check the DCC-EX console to confirm that the expected I²C addresses are detected before configuring your servos.

HAL Configuration for PCA9685

Declare the PCA9685 in myAutomation.h using the HAL(...) macro.

Example PCA9685 HAL entry

// start vPin, count, I2C address
HAL(PCA9685, 100, 16, 0x40)

This creates 16 virtual pins, starting at vPin 100.

vPin to PCA9685 Channel Map

Adding a second PCA9685 board

If you add another PCA9685, it must use a different I2C address and a different vPin range.

HAL(PCA9685, 116, 16, 0x41)

Defining Servo Turnouts in myAutomation.h

After registering the PCA9685 with HAL, define each servo turnout using SERVO_TURNOUT.

Syntax

SERVO_TURNOUT(id, vPin, active_angle, inactive_angle, profile, description)

Basic example

SERVO_TURNOUT(1, 100, 250, 250, Fast, "Turnout 1")
SERVO_TURNOUT(2, 101, 250, 250, Fast, "Turnout 2")

In this example:

• 1 and 2 are the turnout IDs
• 100 and 101 are the vPins assigned through the PCA9685 HAL entry
• The angle values are placeholders for calibration
• Fast is the movement profile
• The final text is the turnout description

Calibrating Servo Angles

Servo angle calibration is usually the most important part of turnout setup. Start gently and avoid forcing the turnout mechanism.

1. Start with both servo angle values the same, for example 250, 250.
2. Use EX-Toolbox or your preferred DCC-EX method to test movement.
3. Adjust the active and inactive values gradually.
4. Upload the updated configuration.
5. Test the turnout in both directions.

Test commands

<T 1 C>
<T 1 T>

These commands test turnout ID 1 in the closed and thrown positions.

Recommended Expansion Path

For a simple DCC-EX layout, the best approach is usually:

1. Start with a working DCC-EX CommandStation
2. Confirm train control is working reliably
3. Add the PCA9685 servo controller
4. Configure servo turnouts in EXRAIL
5. Add more automation later if required

Troubleshooting

Useful Products

The following products may be useful when adding servo turnout control to a DCC-EX layout:

• PCA9685 Servo Controller Board — for controlling multiple servos from DCC-EX.
• PCA9685 Servo Controller with Case — for a cleaner installation.
• CSB1 Ready-to-Run Command Station — complete DCC-EX system for users who want a ready-to-run starting point.

Your DCC-EX CommandStation can now support servo-controlled turnouts through EXRAIL and a PCA9685 servo controller.