TX/RX LED

I feel like this is a dumb question, but…
How can I drive a TX and RX LED on a serial line without degrading/impeding the serial signal?

I’m looking for visual confirmation that a device is sending/receiving Data.

Thanks

I just put an led (w/R500) on the Rx line and it works fine. (Haven't tried the Tx line.) I use it to tell me when to press the Reset button when I am uploading a sketch on a breadboarded Arduino.

Thanks florinc - I forget to mention that the resistor goes to Vcc!

Take a look at the schematic of an USB_to_serial board: http://www.sparkfun.com/datasheets/BreakoutBoards/FT232RL-Breakout-Schematic.pdf You will see the LEDs connected to Tx and Rx, going to Vcc through 330 ohm resistors.

Take a look at the schematic of an USB_to_serial board: http://www.sparkfun.com/datasheets/BreakoutBoards/FT232RL-Breakout-Schematic.pdf... You will see the LEDs connected to Tx and Rx, going to Vcc through 330 ohm resistors.

No, that is a Specific LED output of the FTDI chip. Those LEDs aren't on the TX/RX line, those are pin 1 and 5.

Again, what I do for the Rx pin is:

Pin-0 → LED → R500 → Vcc

Works for me.
(Actually I do this on the output side of those little MAX232 boards, but same dif.)

I heard you the first time, and thanks, but you can't tell me if it degrades the signal or may cause issues. I'm sure it will work that way, but is there a proper way to do it?

Are you driving it at TTL or RS232 levels?

Are you driving it at TTL or RS232 levels?

Both ;)

Jassper,

I only repeated it because I left off the Vcc. Your right, I can't tell you its the proper way. On can only add that it's been in circuit for all the sketches I've loaded and all my serial output for the last year or so. For it's worth.

The only thing I am worried about is the extra load on the TX/RX line. This is for a "go between" and I don't know what devices will be connected to the in and out. Useually a serial line can handle a 40ma load, but since I can't be sure, I don't want to over load a device and cause it to burn out the com port.

If you're at TTL levels, then tie the input of a non-inverting buffer to the signal line and drive the LED with the buffer output. That will reduce the load on the signal line. I'm not sure if you can get a logic buffer that can take the RS232 voltage levels. But you could convert it to TTL with another device. That gets to be pretty complicated for a couple LEDs, but if it's worth it for your project...

EDIT: HEY, this was my 100th post! w00t!

I used to just take a bipolar LED and put it in series with the RS-232 line, with a current-limiting resistor to keep it from frying. But these days, with so many gadgets just running inverted TTL instead of true RS-232, that's not reliable.

I'd suggest getting an old 1489 quad RS-232 receiver (you might need to find it surplus: I saw in the google search that National has discontinued it), and using it to drive the indicator LEDs. Note that it doesn't have much drive capability, but its few mA should be enough to give you a good indication on high-brightness LEDs.

It's cheap, simple, and well-protected from the full range of RS-232 voltages.

Ran

Thanks, I'll take a look at that.