Backfeed on TX/RX lines

Forum 2005-2010 (read only) Hardware Interfacing
1

Hi,

I just found an issue with my setup, I am communicating to a PC based system with the TX/RX from the Arduino. The TX, RX, and ground are connected to a TTL to RS-232 adapter that is port powered from the PC side. Ever thing works, but when I disconnect power from the Arduino, there is about 2.5v on the TX/RX line that back feeds the Arduino and the power LED lights very dim.

First thing that comes to mind is a diode, however the voltage drop may affect the signal, ??

Suggestions plz :wink:

2

Well, you can't use diodes at the RS232 side as those signals are AC. You might be able to get away with a diodes which have a .3V drop (and put them between the arduino's pins and the adapter.) I'd imagine that the TTL to RS232 adapter would be tolerant of signals coming in at 0 - 4.7V. I'm not so sure if they'd like a .6V drop though.

3

Thanks for the reply,
Come to think of it, I don't know what the finale cable length will be, so I might need that 0.3v!! :-/

Is there a possiblity of arduino damage? I don't think it is reversed bias, obviously as the LED lights. I guess I could let it sit for a few days and see if it still works :-?

Or see about modifiing the adaptor and power it from the arduino...

4

Don't use a port powered TTL to RS-232 adapter. It is leaving 5V on the TX pin feeding into an un powerd chip. This is not good and is causing the protection diodes to conduct. You might get a latch up condition on power up or you might just burn out the chip.

This is likely to be a high impedance so you could try a pull down resistor (1K) on the arduino's inputs with a series resistor coming from your adapter to the arduino of about 50 to 100R.

5

Don't use a port powered TTL to RS-232 adapter. It is leaving 5V on the TX pin feeding into an un powerd chip. This is not good and is causing the protection diodes to conduct. You might get a latch up condition on power up or you might just burn out the chip.

This is likely to be a high impedance so you could try a pull down resistor (1K) on the arduino's inputs with a series resistor coming from your adapter to the arduino of about 50 to 100R.

I kinda figured that,

So the pull down set up would look something like this?(not my drawing)

But just courious, what is the purpose of the 22K?

EDIT
I make my own adaptors, so would it be just as good to power the ST232 via the 5 volts that I am suppling to the Arduino? If DC voltage is lost, the adaptor and Arduino would be dead.

6

The 22K resistor limits the current flow into the input pin to a safe value when the RS-232 signal voltage goes above 5vdc or negative values. These out of bound voltages cause the input pin's clamping diodes to conduct and would be damaged if the current is not limited by the series resistor.

Lefty

7

Hmm, never seen this mentioned before for the Arduino, but I am still new in useing the Arduino. Is this a required thing when useing a RS-232 to TTL adaptor then, or all the time?

8

It's an all the time thing:-
http://www.thebox.myzen.co.uk/Tutorial/Protection.html
especially when connecting to the outside world.
However, those resistor values are probably too high you don't want to interfere with the normal working of the port. The problem will be pulling the input down to a logic zero if the resistors are too big.

9

Thanks all for the info.

10

Still wondering about this question however, Testing it out, seems to solve the issue.

I make my own adaptors, so would it be just as good to power the ST232 via the 5 volts that I am suppling to the Arduino? If DC voltage is lost, the adaptor and Arduino would be dead.

I may go even further with Opto Isolation.