There are a couple of issues:

1. The first is that Serial won’t work the way you have illustrated in your diagram. HW_SERIAL1 corresponds to pins TX1 and RX1 on an Arduino Board. This is the Serial1 class in Arduino and not all board have these pins. The regular Serial class (pins marked RX and TX – typically pins 0 and 1) on the board cannot currently be used with the Firmata Serial feature. You’ll need to use a board like an Arduino Leonardo, Mega, Due, Zero, 101 or other board that has RX1 and TX1 pins in order to use Hardware serial. With an Arduino Uno, you have to use SoftwareSerial.

2. There was an error in the serialWrite example that was not sending ascii characters so if you would have tried to view the output using the Arduino IDE Monitor you would not see anything printed. This is now fixed: https://github.com/soundanalogous/Breakout/commit/8d7bd0ba5cb50210efb1ce3c4e7702bf04f34d03