isPAC_(b1, b2) {
    // For Preamble Address Codes, the bytes take the following form:
    // b1 -> |0|0|0|1|X|X|X|X|
    // b2 -> |0|1|X|X|X|X|X|X|
    return ((b1 & 0xf0) === 0x10) && ((b2 & 0xc0) === 0x40);
  }
Checks if this is a PAC control code.
@param {number} b1 Byte 1.
@param {number} b2 Byte 2.
@return {boolean}
@private