I took the matrixKeyboard example and tested it on the Arduino first. Only the pins had been adjusted. Keypad connected with 1k resistors and it works as expected without problems.

#include <IoAbstraction.h>
#include <KeyboardManager.h>
#include <IoAbstractionWire.h>


//
// We need to make a keyboard layout that the manager can use. choose one of the below.
// The parameter in brackets is the variable name.
//
//MAKE_KEYBOARD_LAYOUT_3X4(keyLayout)
MAKE_KEYBOARD_LAYOUT_4X4(keyLayout)

//
// We need a keyboard manager class too
//
MatrixKeyboardManager keyboard;

// this examples connects the pins directly to an arduino but you could use
// IoExpanders or shift registers instead.
IoAbstractionRef arduinoIo = ioUsingArduino();


//
// We need a class that extends from KeyboardListener. this gets notified when
// there are changes in the keyboard state.
//
class MyKeyboardListener : public KeyboardListener {
public:
    void keyPressed(char key, bool held) override {
        Serial.print("Key ");
        Serial.print(key);
        Serial.print(" is pressed, held = ");
        Serial.println(held);
    }

    void keyReleased(char key) override {
        Serial.print("Released ");
        Serial.println(key);
    }
} myListener;

void setup() {
    while(!Serial);
    Serial.begin(115200);

    keyLayout.setRowPin(0, 24);
    keyLayout.setRowPin(1, 25);
    keyLayout.setRowPin(2, 26);
    keyLayout.setRowPin(3, 27);
    keyLayout.setColPin(0, 28);
    keyLayout.setColPin(1, 29);
    keyLayout.setColPin(2, 30);
    keyLayout.setColPin(3, 31);

    // create the keyboard mapped to arduino pins and with the layout chosen above.
    // it will callback our listener
    keyboard.initialise(arduinoIo, &keyLayout, &myListener);

    // start repeating at 850 millis then repeat every 350ms
    keyboard.setRepeatKeyMillis(850, 350);

    Serial.println("Keyboard is initialised!");
}

void loop() {
    // as this indirectly uses taskmanager, we must include this in loop.
    taskManager.runLoop();
}

Then I connected a MCP23017 to the Arduino via I2C. I did some testing with LEDs and switches around and everything works there as expected.

Then I connected the keypad to the MCP23017 as follows




In the code I changed from the Arduino pins to the MCP23017 and entered the pin numbers accordingly.

/**
 * This example shows how to use the matrix keyboard support that's built into IoAbstraction,
 * it can be used out the box with either a 3x4 or 4x4 keypad, but you can modify it to use
 * any matrix keyboard quite easily.
 * It just sends the characters that are typed on the keyboard to Serial. The keyboard in This
 * example is connected directly to Arduino pins, but could just as easily be connected over
 * a PCF8574, MCP23017 or other IoAbstraction.
 */

#include <IoAbstraction.h>
#include <KeyboardManager.h>
#include <IoAbstractionWire.h>


//
// We need to make a keyboard layout that the manager can use. choose one of the below.
// The parameter in brackets is the variable name.
//
//MAKE_KEYBOARD_LAYOUT_3X4(keyLayout)
MAKE_KEYBOARD_LAYOUT_4X4(keyLayout)

//
// We need a keyboard manager class too
//
MatrixKeyboardManager keyboard;

// this examples connects the pins directly to an arduino but you could use
// IoExpanders or shift registers instead.
//IoAbstractionRef arduinoIo = ioUsingArduino();
IoAbstractionRef mcp = ioFrom23017(0x20);

//
// We need a class that extends from KeyboardListener. this gets notified when
// there are changes in the keyboard state.
//
class MyKeyboardListener : public KeyboardListener {
public:
    void keyPressed(char key, bool held) override {
        Serial.print("Key ");
        Serial.print(key);
        Serial.print(" is pressed, held = ");
        Serial.println(held);
    }

    void keyReleased(char key) override {
        Serial.print("Released ");
        Serial.println(key);
    }
} myListener;

void setup() {
    while(!Serial);
    Serial.begin(115200);

    keyLayout.setRowPin(0, 0);
    keyLayout.setRowPin(1, 1);
    keyLayout.setRowPin(2, 2);
    keyLayout.setRowPin(3, 3);
    keyLayout.setColPin(0, 4);
    keyLayout.setColPin(1, 5);
    keyLayout.setColPin(2, 6);
    keyLayout.setColPin(3, 7);

    // create the keyboard mapped to arduino pins and with the layout chosen above.
    // it will callback our listener
    keyboard.initialise(mcp, &keyLayout, &myListener);

    // start repeating at 850 millis then repeat every 350ms
    keyboard.setRepeatKeyMillis(850, 350);

    Serial.println("Keyboard is initialised!");
}

void loop() {
    // as this indirectly uses taskmanager, we must include this in loop.
    taskManager.runLoop();
}

But unfortunately this does not work that way.

So basically the MCP23017 works. If I simply read or set the ports, I can control LEDs or query buttons.

Only in connection with the KeyboardManager I can't get this to work. I had already worked with an interrupt PIN or set the ports to INPUT or OUTPUT before. But no change.

Not 100% sure it's this, but please try adding Wire.begin() to setup before you perform any actions on i2c.

If this isn't called, Wire will very often freeze.

Thanks,
Dave

Now it works. Unfortunately I had overlooked that, in the LED example it was even written in it.

Thank you very much.

Great news, I glanced over the circuit and all else looks good.

Thanks,
Dave