gb-link.js

var sleep = require('sleep');
var Gpio = require('onoff').Gpio;

SO = new Gpio(6,  'out'),
SI = new Gpio(13, 'in', 'both'),
SD = new Gpio(19, 'in', 'rising'),
SC = new Gpio(26, 'out');

function sendByte(byte) {
  for (var b = 0; b < 8; b++) {
    if ((byte << b) & 0x80) {
      SO.writeSync(1);
    }
    else {
      SO.writeSync(0);
    }

    sleep.usleep(60);
    SC.writeSync(0);
    sleep.usleep(120);
    SC.writeSync(1);
    sleep.usleep(120);
  }
  sleep.usleep(240);
}

function writeString(msg) {
  for (var c = 0; c < msg.length; c++) {
    var charCode = msg.charCodeAt(c);

    sendByte(charCode);
  }
  sendByte(0);
}

// Assuming sending is using external clock
function getByte() {
  var byteReceived = 0x00;
  for (var b = 7; b >= 0; b--) {
    sleep.usleep(60);
    SC.writeSync(0);
    sleep.usleep(120);

    if (SI.readSync()) {
      byteReceived |= 0x01 << b;
    }

    SC.writeSync(1);
    sleep.usleep(120);
  }
  console.log("Got value: " + byteReceived);
}

process.on('SIGINT', function () {
  SO.unexport();
  SI.unexport();
  SD.unexport();
  SC.unexport();
});