Friday, 27 November 2020

A simple model railway DCC accessory controller

A few months ago I converted my N gauge train set to DCC operation. As well as fitting DCC decoders to my locomotives I also converted the points by fitting a tiny decoder in each one.

DCC operation of Kato #4 points

This makes setting up a temporary layout (I don't yet have any sort of permanent layout) nice and easy (as there's no point wiring to worry about) but operating accessories such as points from the NCE PowerCab controller is a bit of a faff. First you press the "accy" button, then select the number, then "enter", then "on" or "off". Half the time you've misremembered the last state of that point so the command has no effect, so you then have to press the "accy" button twice to toggle it.

NCE controllers can be linked together via a system called "Cab Bus". This uses a fairly simple protocol that allows subsidiary controllers to send commands to the main controller (the PowerCab in my case) which then sends the corresponding DCC commands to the track. I decided to make a simple controller to switch my points, using an Arduino. I've never used an Arduino before and I thought this would be a good starter project. I found this useful page that gave me all the information I needed.

I decided to use an Arduino Nano Every as it's quite small, has a serial interface that's separate from its USB port, and has a switching voltage regulator to allow it to run efficiently from the Cab Bus 12 V supply. The other main components are an RS485 to TTL converter, a pair of RJ11 sockets, and push buttons & LEDs.

Arduino CabBus controller

I used two RJ11 sockets to provide "loop through" to another controller (possibly) or an RS485 to USB interface used to monitor the Cab Bus during development. I wrote a simple Python script to show the polling frequency of each polled address and display any non-polling bytes on the bus.

from collections import defaultdict
import sys
import time

import serial

def main():
    with serial.Serial('/dev/ttyUSB0', baudrate=9600,
                       bytesize=serial.EIGHTBITS, parity=serial.PARITY_NONE,
                       stopbits=serial.STOPBITS_TWO) as bus:
        period = 15.0
        last_byte = None
        while True:
            pings = defaultdict(int)
            stop = time.time() + period
            while time.time() < stop:
                data = ord(
                if (data & 0b11000000) == 0b10000000:
                    # ping byte
                    if last_byte is None:
                    pings[data] += 1
                    last_byte = data
                    # data byte
                    if last_byte is not None:
                        print('... {:02x}'.format(last_byte), end='')
                        last_byte = None
                    print(' {:02x}'.format(data), end='')
            for key in sorted(pings.keys()):
                print('{:02x}: {:3.2f} Hz'.format(key, pings[key] / period))
    return 0

if __name__ == '__main__':

The circuit board uses "strip board" or "Vero board". I spent some time planning it (unusually for me) and finished up with a quite compact arrangement. In this diagram the gold coloured lines are tracks on the back of the board and the blue lines are wires above the board.

The Arduino "sketch" is also quite simple. It uses a struct to store the current state of each button. When a button is pressed its LEDs are toggled, and the corresponding accessory command is sent to the cab bus.

// This is a 'type c' controller, so its address should be 8, 9, or 10 when using PowerCab
const int controllerAddress = 8;

// Which serial port is connected to the Cab Bus
#define RS485 Serial1

// Connected to RS485 module's /RE and DE pins
const int txEnablePin = 10;

// Number of push button & LED sets
const int channelCount = 8;

// Configuration of each channel
struct configDetails {
  int buttonPin;  // pin connected to push button
  int ledPin;     // pin connected to "on"/"off" LED pair
  int accyAddr;   // DCC address of the controlled accessory
  bool inverted;  // swap the meanings of "on" and "off"
const struct configDetails configuration[channelCount] = {
  {A4, 5,  1, false},
  {A0, 9,  2,  true},
  {A5, 4,  3, false},
  {A1, 8,  4,  true},
  {A6, 3, 60, false},
  {A2, 7, 61, false},
  {A7, 2, 62, false},
  {A3, 6, 63, false},

// How many ms to wait for button to settle
const unsigned long DEBOUNCE_DELAY = 10;

struct channelState {
  bool on = false;        // "output" value, toggles each button press
  bool down = false;      // button is currently down
  unsigned long wait = 0; // wait until this time for the button to settle
struct channelState stateStore[channelCount];

void setup() {
  const struct configDetails* cnfg;
  for (int channel = 0; channel < channelCount; channel++) {
    cnfg = &configuration[channel];
    pinMode(cnfg->buttonPin, INPUT_PULLUP);
    pinMode(cnfg->ledPin, OUTPUT);
  pinMode(txEnablePin, OUTPUT);
  digitalWrite(txEnablePin, LOW);
  RS485.begin(9600, SERIAL_8N2);

void loop() {
  const struct configDetails* cnfg;
  struct channelState* state;
  for (int channel = 0; channel < channelCount; channel++) {
    cnfg = &configuration[channel];
    state = &stateStore[channel];
    if (pollButton(cnfg->buttonPin, state)) {
      digitalWrite(cnfg->ledPin, state->on?HIGH:LOW);
      switchCabBus(cnfg->accyAddr, state->on != cnfg->inverted);

bool pollButton(const int pinNo, struct channelState* state) {
  // Return true if button "on"/"off" state has just changed
  if (state->wait != 0) {
    // waiting for stability
    if (millis() < state->wait) {
      return false;
  if (digitalRead(pinNo) == HIGH) {
    // button is up, so any waiting is cancelled
    state->down = false;
    state->wait = 0;
    return false;
  if (state->wait != 0) {
    // button has been pressed for long enough
    state->wait = 0;
    state->on = not state->on;
    return true;
  else if (not state->down) {
    // button has just been pressed
    state->down = true;
    state->wait = millis() + DEBOUNCE_DELAY;
  return false;

void switchCabBus(const int accyAddr, const bool on) {
  unsigned char command[5];
  command[0] = 0x50 + (accyAddr >> 7);
  command[1] = accyAddr & 0x7f;
  command[2] = on?4:3;
  command[3] = 0;
  command[4] = ((command[0] ^ command[1]) ^ command[2]) ^ command[3];
  cbSend(command, 5);

void cbSend(unsigned char buf[], int len) {
  unsigned char rxByte;
  unsigned long timeout;
  // Empty input buffer
  while (RS485.available()) {
    rxByte =;
  timeout = millis() + 250;
  rxByte =;
  // Swallow input until our address is polled
  while (rxByte != 0x80 + controllerAddress) {
    if (millis() > timeout) {
      // Cab Bus is probably disconnected
    rxByte =;
  // Respond 100..800 µs after poll
  // Send command
  digitalWrite(txEnablePin, HIGH);
  RS485.write(buf, len);
  digitalWrite(txEnablePin, LOW);

Here is the controller connected up to my PowerCab.

Arduino CabBus controller

Monday, 11 November 2019

Chromatic aberration in digital photographs

In my last post I wrote about my attempts to measure vignetting in the lenses I use with my DSLR camera. Another lens problem that the camera can correct (but only for Canon lenses) is lateral chromatic aberration.

To determine the amount of correction my lenses need I printed out a simple test chart and photographed it at a variety of focal lengths. I also tried different apertures, but the correction required does not vary with aperture. Here is one of my photographs.

Chromatic aberration tests

I tried to write some software to measure any disparity between red, green, and blue images, but failed to achieve anything usable. I settled for judging the error by eye using this simple Pyctools network.

The "reader" component has options to scale the red and blue images to minimise chromatic aberration. By adjusting the scale values and then re-running the graph I was able to find good values for each of my lenses, at several focal lengths for the zoom lenses. The "UVgain" stage makes it easier to see the aberration. Multiplying the U signal by zero and the V signal by 8 or 16 makes it much easier to tune the red scaling, and vice versa to tune the blue.

I've now reached the stage where I can process my "raw" camera images and get better looking results than from the camera's internal processing. I've combined all my chromatic aberration and vignetting measurements into an easy to use Python script. I expect to continue to improve it in future.

Tuesday, 22 October 2019

Digital photograph vignetting revisited

A few years ago I wrote about some experiments I'd done with measuring and correcting vignetting in digital photographs. (Some cameras have such correction built in, called "peripheral illumination correction" or similar.)

I recently purchased a second-hand 10-18 mm wide angle lens for my DSLR. Measuring its vignetting using my previous method is difficult as it has a field of view in excess of 100°. It's hard to provide an evenly illuminated target this large. The answer is to assume the lens has no vignetting at its smallest aperture, and use an image at this aperture as a reference when measuring the vignetting.

I set up the camera on my dining room table with an A4 sheet of translucent Perspex (or similar) an inch or so in front of the lens hood. The camera was facing towards the window, but no other lighting was used.

This is a photo taken at 10mm focal length and maximum aperture, ƒ/4.5.
Vignetting tests

At minimum aperture, ƒ/22, the result doesn't look much different, apart from a bit of fluff on the front of the lens being more nearly in focus.
Vignetting tests

Dividing the ƒ/4.5 image by the ƒ/22 (after converting both to luminance) gives this image.
Vignetting tests

This may not appear to have much vignetting, but my analysis and curve fitting suggests otherwise.
Vignetting tests

The fitted function (shown in orange) is a polynomial in r², r⁴, and r⁶. Using even powers of r ensures the function has zero slope at r=0. The correction required is about half an ƒ-stop (factor of 1.41) at the corners.

Running the process for narrower apertures shows the expected reduction in vignetting.
Vignetting tests

I think the  "measured" curves show some similarity, but a 3rd order polynomial (in r²) is not a good match at smaller apertures. There's also no obvious trend in the polynomial coefficients, so I can't confidently predict the correction required at other apertures. I'd like to find a better fitting function, but I think this is beyond my mathematical abilities.

PS I've just tried fitting a power function 1.0 + (a * (x ** b)) instead of a polynomial and instantly got better looking results.
Vignetting tests