TX -RX indicator

vpan · Joined: 31 Oct 2008 Posts: 14

Is it possible to connect at TX and RX pin
one led and resistor ?
I want to see if led blinking when I send
or receive data.

Guest · Posted: Thu Jan 15, 2009 3:42 pm

One LED and two resistors + two ORing diodes. If one line is at space and one line is mark then the two voltages work against each other - hence you will need an ORing diode in each line.

Use the largest resistor possible and a high efficiency LED to keep loading down.

On a fast / low activity link you won't see much in the way of 'blinking' as the data will zip by really fast. Usually a pulse stretcher is used to drive the LED to full intensity. A simple pulse stretcher can be built from a 74HC00 NAND gate.

HTH - Steve H.

Ken Johnson · Joined: 23 Mar 2006 Posts: 197 Location: Lewisburg, WV

We use one led for each (Rx/Tx) driven through a small-footprint buffer.

Even at 115,200 baud, activity is visible without a pulse-stretcher.

Good luck !

Ken

Guest · Posted: Mon Jan 19, 2009 11:20 am

Your eyes must be better than mine - one character at 115k is 86 uSec. That's a pretty short pulse...

Not everything 'streams' some things only send one or a few chars now and then.

;-)

Steve H.

Ken Johnson · Joined: 23 Mar 2006 Posts: 197 Location: Lewisburg, WV

Hi Steve

Well, maybe your eyes are better than you realize Smile

Yes, I'm typically sending more than 1 byte, but I just did an experiment: put a breakpoint in the Tx ISR, so I could send only 1 byte (at 115200 baud).

Yep, the led blink is plainly visible. It's quick, of course, but not difficult to see. That short burst of photons hits the eyeball, and the eyball does its thing. You won't perceive multiple pulses at that rate, but that's not what you want anyway (is it?).

Try it !

Ken

Guest · Posted: Tue Jan 20, 2009 10:28 am

Oh I know that you can see it if you are looking at it - it does not appear to be full brightness. I have had numerous complaints from QA and customers about 'apparent brightness' it seems less technologically oriented folks don't like the apparent brightness change with pulse rep rate. They also appear perturbed that one has to stare at the indicator to see activity instead of it being on for .25 - .5 seconds or so which is easy to spot at a glance. This is probably due to them not understanding the physics behind the circuit / indicator. So I listen to them and just put in a pulse stretcher - if you have a buffer anyway, why not just add a few R's and C's and make the customer happy? This has been my experience, your mileage may vary.

:-)

Steve H.