OLED won't start up without USB cable plugged in.

I've been using a 4D Systems OLED screen for an arduino project and I've come across a problem with getting the screen to start up when using an external power supply.

I've switched to use the software serial library and found the screen would only start up if the usb cable was plugged in.

I later discovered that the cable only had to be touching the housing of the socket for it to start.

I've tried all sorts of combinations of power and data pins for the screen. I've tried different start up delays to let the screen settle. I've also tried using different power supples with no joy.

The screen /is/ powered and /will/ work without the usb cable connected as once the screen is started the usb cable can be removed and it will continue to work. It's just convincing the screen that it's plugged into somthing I guess.

Any suggestions?

Thanks!

Check that your Ground (0V) wiring is connected properly. Can you tell us the specifications of the external power supply?

The supply is 12V 1.66A.

Which bit of wiring should I check? I've made sure that the screen's grounded. When i was testing last the screen was going right into the arduino so, unless there is somthing wrong my my diecimila's ground and I'm not sure how I would check that.

I've shifted everything over to a proto shield to make sure that the connections are all good and it's seems that the problem was my software serial tx was too high voltage for the device. Perhaps the device couldn't detect the baud rate or something.

Anyway, I'm all good now, thanks for the help!