Arduino UNO / USB TTL Converter / PC 5V powers Arduino 5 V - but why?

Projects General Guidance
1

Hi, I connect my PC to the Arduino via USB TTL Converter to send some commands to the Arduino.
The Arduino is powered by a 9V power adapter. Sending, receiving data - everything works fine.
I am using this USB TTL Converter: Delock Products 83116 Delock USB 2.0 to Serial TTL Converter with 6 open wires 1.8 m (5 V)

As you can see in my drawing - I connect the 5V from USB to 5V on the Arduino board.
Wrong?

Communication also works without the 5V connection.

If I disconnect the 9V PS the Arduino still get's its power form the PC???
I thought it's only possible to power the Arduino over VIN??

Another question - in this case is RX on 0V and TX on 3V or TX on 5V?

thx

PC2Arduino_Steckplatine.png
PC2Arduino_Steckplatine.png2049×1530 181 KB

2

If I disconnect the 9V PS the Arduino still get's its power form the PC???

Yes.

I thought it's only possible to power the Arduino over VIN??

No you can power it by feeding 5V into the 5V pin.

Another question - in this case is RX on 0V and TX on 3V or TX on 5V

No idea what you mean.
From TTL the normal state of TX will be 5V

3

There are a few odd things here.

One - why are you using the external USB to TTL converter rather than that built into the UNO?

It's not compulsory of course, but it seems more logical to use the built-in one.

I do hope you do not have an LED plugged into the two pins you illustrate, for two reasons. One is that it is unnecessary as the board already provides such an indicator as the "L" LED and does so through a buffer in order not to compromise the use of that port as an input, the other that you should not connect LEDs to the ports without current limiting resistors.

If you want to use the external USB to TTL converter to upload sketches to the board, you should connect the RTS line to "reset" on the board through a 0.1µF capacitor to provide automatic reset for the bootloader.

If on the other hand, you were using the external USB to TTL converter in order to avoid the automatic reset when the port is opened, that is also unnecessary as there is an "RESET EN" (or "RE ET EN") jumper on the board that you can break (or attach a switch) to defeat that function.

4

I do hope you do not have an LED plugged into the two pins you illustrate
I plugged it into Pin 13 because Pin 13 has a resistor - so this should be OK

If you want to use the external USB to TTL converter to upload sketches to the board...
Thx, but I use the USB2TTL cable only for sending and receiving data

5

I plugged it into Pin 13 because Pin 13 has a resistor

No no no no no no no.
Back in 2007 it did but not anymore. This is one of the most persistent zombie myths, because we can't kill it off.

6

Grumpy_Mike:
Back in 2007 it did but not any more. This is one of the most persistent zombie myths, because we can't kill it off.

So - don't tell me - does this mean we still have "Playground" articles with Fritzing diagrams illustrating a LED plugged directly into 13 and the adjacent ground and that is why we keep seeing it appear so in questions?

7

I won't tell you, but that is right.

8

Oh dear.

9

I fell into the same trap. I saw all those "beginner tutorials" with the "hello world" equivalent putting a LED into pin 13.
Just for the fun: http://goo.gl/UDvoPp (Google images with: "arduino hello world led")