Serial port and external power supply

Hi all,

I am working on a project in which I need external power supply and a RS485 interface.. I had a shield made with a AC/DC transformer and a max485 chip...

The set looks very nice, but I don´t have any serial communication from the arduino board when I power it using the adapter. I have tried the RS485 powering the board with the USB connection and it works fine. Using any kind of external power supply (I have not tried batteries yet) apparently disables the serial output.

Any help? Is there something to be done when powering the Arduino with external power?

Thanks a lot

Did you connect ground from Arduino to gorund on the RS485 board ?

You should!

Hi MikMo,

yes the board is connected to ground, and the MAX485 chip is correctly connected to ground and VCC

Using any kind of external power supply apparently disables the serial output.

No that is not how it works, you can't do that even if you wanted to.

Is the external adaptor actually working?
Are you connecting it to the Vin jack socket or the Vin pin.
Do you see a voltage on the +5V pin?

mm, actually I just realized that it is connected to the +5V pin instead of the Vin pin - design fault :frowning: board leds and so are on though

on the other hand, that does not explain the failure when connecting the external adapter through the power jack right?

that does not explain the failure when connecting the external adapter through the power jack right?

Unless you have blown it up and now it doesn't work at all. :frowning:

well, it still works with the USB power supply so I guess not :slight_smile:

I'm having the same issue.
I have tried searching and this is the only other time I have seen anybody have the same issue.
I have a duemilanove and my issue arrises when I hook my Parallax RFID reader up.
It is actually causing multiple issues. One when powered via an external wall wart everything works but the RFID.
Also, when the TX line of the RFID is hooked up to the Arduino's RX line, when I try to upload a sketch it fails. I need to disconnect the pin every time. I have read somewhere that it is caused by the Arduino trying to upload data from the RX pin. If this is the case I'm hoping there is a way (preferably in software) to disable this action.

But the first issue, no serial when powered externally is more important. If anybody has any suggestions or anything it would be greatly appreciated.

Thanks
-Eric

Edit.
So I tried a different power supply and it works now. I guess the label was lying or something. Whatever external power works.