SoftwareSerial stops working when USB cable removed

Trying to communicate with a Noritake VFD via an Asynchronous serial interface.
A modified SoftwareSerial example communicates as expected with both the hardware
Serial port and SoftwareSerial port when the USB cable is connected to the UNO R3.
External power is supplied via the DC connector. See attached sketch.
When I remove the USB cable and reset the UNO, the external display goes blank.
Reinserting the USB cable and resetting, the code works and displays on both ports
just fine. ::slight_smile:

softserial_test.ino (619 Bytes)

Hi miker

Can you post a diagram (hand drawn will do) of how you have things connected up?

Do you have a GND connection between the Uno and the VFD, as well as the TX and RX connections?



Wiring diagram attached. Pretty simple and clean.

Doc1.pdf (107 KB)

The wiring looks OK.

Please post a link to the datasheet for the VFD you are using. How are you powering the Arduino when the USB is disconnected?

My guess is that it is drawing too much power causing the Arduino to reset. (But just a guess).


PROBLEM SOLVED - Power issue. Although it was powered through the DC input when the USB is disconnected, the external power supply (5Vdc @ 250ma) apparently wasn't sufficient enough to drive things when passed through the ARDUINO voltage regulator. I changed the external power supply to a 6Vdc/300ma supply thru the ADRUINO VIN bypassing the onboard regulator. Display continued to operate and did not skip beat.

Thanks to those who responded. ;D

I think "PROBLEM SOLVED - sort of" might be a better way of putting it. The VIN pin is just an alternative to the power jack and is more used to access the voltage that comes from the latter, so you have not bypassed the regulators, you have simply applied not enough volts to them, but more volts than you did before. The recommended minimum is 7v, so I guess six is at least better than five and might indeed solve the problem - for the moment........

All this is detailed in the power section of the product descriptions, but the role of the VIN pin is rather better described in the part about the 5v pin.

Vin bypasses the reverse polarity protection diode.
6V at the barrel jack would be too low.
6V at Vin almost satisfies regulator input requirement for 5V out:
Vin = 6.5 V to 12 V, Iout = 0 mA to 800 mA
If you measure the 5V pin, you will likely see it is not quite 5v.

NCP1117-D regulator.pdf (222 KB)