TTL-RS232 Voltage Causing Issues?

[tripwave]

Hey folks,

Arduino newb here.

I have arduino uno powered via usb. I have 2x TTL-RS232 modules attached. The first one is for a speed radar gun, the second is for my garmin gps module. Everything is functional as far as receiving data. However when the gps is plugged in, the arduino's reset button causes a weird 'brown-out' style glitch where everything goes dim. When I unplug the usb from arduino, the power leds of that, and the ttl converters pulse dimly. I'm assuming the garmin is outputting true TTL-Level voltages?? (tested at +/- 2vdc)... This can't be healthy?? Any ideas of what I'm doing wrong? Could it be that the gps and the laptop/arduino have different 'grounds' ?

Cheers
Tim

[jremington]

If you will take time to post links to the adapters, components and a wiring diagram, following the instructions in the "How to use this forum" post, perhaps someone can help.

Devices should never be plugged into or unplugged from the Arduino while it is powered.

[Grumpy_Mike]

I'm assuming the garmin is outputting true TTL-Level voltages?? (tested at +/- 2vdc)... This can't be healthy?

True.
Are you sure you wired those converters the right way round?

Normal TTL voltages are 0 to 5V but you need a 0 to 3V3 converter on the Zero if they are being fed into normal pins.

[tripwave]

True.
Are you sure you wired those converters the right way round?

Normal TTL voltages are 0 to 5V but you need a 0 to 3V3 converter on the Zero if they are being fed into normal pins.

I believe so as they power on and I can receive rs232 data normally from this speed radar gun we have.

It's a no-name Bi Directional RS232-TTL converter with DB9 connector on one side and 4 pins going to arduino to the respective pins for 5v, gnd, tx, rx. We also have a dfrobot brand rs232-ttl converter and same thing. Speed radar has an actual serial cable connecting to the dfrobot converter, and we have just ground and tx-out from gps going with bare wire into the db9 connector on pin5 and 3. Data receives fine.

No other issues other than when mains power to arduino is off, the power led's have a dim and fast-pulsed look.

Now that I really think of it, not knowing initially that the GPS is outputting 3v ttl level data and connecting it to a converter....do i even need a converter for it if the arduino already accepts 3/5v ttl level signals?...My friend is worried that if for some reason the gps is outputting 12V+ and my meter is too slow to see it?...

So far everything else has been essentially plug and play (solder and go) lol....but even though everything works, this minor 'glitch', if you will, I'm sure will play havoc on the ic's with prolonged exposure to inappropriate voltages.

[Grumpy_Mike]

No other issues other than when mains power to arduino is off, the power led's have a dim and fast-pulsed look.

Never connect an input to an Arduino, or any other piece of electronics with the power off. This causes parasitic powering through the ESD protection diodes and can damage things. This is what you are seeing with the LED.

My friend is worried that if for some reason the gps is outputting 12V+ and my meter is too slow to see it?

If that were the case you would see it because the idle state of a serial TTL line is high and the GPS would not be pumping out data continuously, I believe messages are sent at one per second.

[tripwave]

Never connect an input to an Arduino, or any other piece of electronics with the power off. This causes parasitic powering through the ESD protection diodes and can damage things. This is what you are seeing with the LED.

I certainly don't want to damage anything, and this is why I'm asking for as much advice as possible. If I always ensure the arduino is connected to power does that mean I should be okay as far as protections and not damaging components over time? -or- should I start looking into a 0 to 3v3 converter as you have suggested?

Thanks for your advice, and everyone else's too!!

-Tim

[Grumpy_Mike]

The golden rule is not to exceed 0.5V above the voltage on the power pin of a chip. If the chip has no power this means don't exceed 0.5V. For a chip powered by 3V3 that means don't exceed 3V8.

[tripwave]

The golden rule is not to exceed 0.5V above the voltage on the power pin of a chip. If the chip has no power this means don't exceed 0.5V. For a chip powered by 3V3 that means don't exceed 3V8.

This is great news then. Thank you very much for your time and input on this. I can now put my concerns to rest as I've tested all the voltage rails associated with this and we're well under that tolerance.

Cheers!

Tim

[TomGeorge]

Hi,

we have just ground and tx-out from gps going with bare wire into the db9 connector on pin5 and 3. Data receives fine.

Does the Speed Gun have their own power supply?
If so then only connect gnd, Rx and Tx between them and the UNO like the Garmin.

You do not need the 5V connection on the RS232 side, you will on the UNO side to power the adapter.

Tom....