Ethernet shield sketch not starting untethered.


I created a sketch that reads some sensors, and uses an Ethernet shield. Everything seems to be working fine, until I disconnect the Arduino from my computer, and try to interact with my device.

My device does the following: setup: serial, setup ethernet, setup i/o pins. loop: read pins, write pins as pwm to leds, write to ethernet if GET requested, write to serial if character 'r' was sent

:question If I plug in my device with no USB, it seems like something keeps the Ethernet from starting. My program is running, as the logic in the loop method is getting run (I can tell because the leds respond to my physical input -- pressure), but the device doesn't respond to ethernet GET requests as it should.

:exclamation I can plug in the USB cable, and still nothing happens. I can start the Arduino IDE, and still nothing happens. As soon as I start the Serial Monitor in the Arduino IDE, the ethernet starts working, and begins responding to GET requests.

I have tried removing all calls to the Serial.* methods, but it still hangs up the same way, not responding to ethernet requests until the serial monitor is connected.

I found this link,, but haven't tried wiring RX to TX yet, since it seems my sketch is running (since the LEDs respond to my physical inputs). Is this a manifestation of the same problem?

Please advise, David

I think you need to reset the Arduino after plugging it in.