docs: private static byte[] createGroupAPDU(int service, DPTXlator t)
  	{
  		// check for group read
  		if (service == 0x00)
  			return new byte[2];
  		// only group response and group write are allowed
  		if (service != 0x40 && service != 0x80)
  			throw new KNXIllegalArgumentException("not an APDU group service");
  		// determine if data starts at byte offset 1 (optimized) or 2 (default)
  		final int offset = (t.getItems() == 1 && t.getTypeSize() == 0) ? 1 : 2;
  		final byte[] buf = new byte[t.getItems() * Math.max(1, t.getTypeSize()) + offset];
  		buf[0] = (byte) (service >> 8);
  		buf[1] = (byte) service;
  		return t.getData(buf, offset);
  	}