12V power with arduino shield >> serialport problem

I am using a midi in/out ‘shield’ made for arduino made by Tom Scarff (info here: http://tomscarff.110mb.com/MIDI_IN_OUT_ARDUINO/midi_in_out_arduino.htm). This is basically an enhanced serial port.
I am experiencing the following behavior: when the FEZ Domino is powered via a 12V power adapter SerialPort no longer sends bytes. When powered solely via USB everything works fine. Also, receiving data works in both cases. The midi shield is connected (not stacked as a sheild) to ground, 5V, rx_com1 and tx_com1.
The ground and 5V are shared with a serial LCD 16x2 screen.
Again, everything works fine when FEZ is USB-powered. Any ideas about why I can’t use my midi component when powered with my 12 adapter.
Any feedback appreciated.
Thank you.

Hi Charlie

Does the 12v adaptor output AC or DC ? If DC, does it have good rectification and filtering specs ?

Can you check with another adaptor ? Please use something that provides 7.5volt or 9volt.

If possible please check inserting a 0.1uF and a electrolytic (eg. 500uF 12v) across the + and - of the power supply. This may help clean the input voltage.



A volt meter or better a scope is your best friend in this case

I have made some measurements with a voltmeter.

Across 5V and GND pins…
with 12V adapter: 5.02V
with USB power: 4.48V

Seems correct but the problem persists.