LED's on data buses

[Silenus]

Hey guys. I am looking to put a bit of visibility on a pcb of mine and am considering putting LED's in places that have data transmission. I am unsure however, whether they will adversely affect the data signal if I put them in series with the data bus. For instance, if I have an rx/tx bus with 5V logic and LEDs with a forward bias voltage of ~0.9V then the logic high at the other end would be 4.1V correct? Would this data bus still work? Would I need to put a current limiting resistor for the LED as well?

[David_AVD]

Put the LED (and it's series resistor) from the data line to 0V (or Vcc if it's active low).

Use a high efficiency LED and a fairly high value resistor (say 4K7) to keep the loading on the data line low.

[SL4P]

Don't put the LEDs in series.

You may get away with LED and series resistor in parallel with the target input, but you are far better off by buffering the LED from the buss.
That could be a small transistor, or a hex/octal buffer in a chip.

If the simple LED in parallel works, the two things you need to be aware of are maintaining the 0/1 voltage swing, and stray capacitance that could slow down the edges (if they are fast).

Good luck.  Good question.
EDIT: (Sorry David AVD, not trying to counter your simultaneous posting!)

[Silenus]

Looking at the design for an arduino uno they have put the uart leds on two separate pins that I assume they just turn on in sequence with the uart data?

[SL4P]

that's correct.
the USB coprocessor or FTDI chip manages the Tx/Rx LEDs independently of the data stream pints.
Fine if you have the spare pins, and cpu time between the serial bits to toggle the LED States.

[SL4P]

Yes... in 99% of applications.
Keep the LED wiring and resistor leads as short and tidy as possible if the data speeds are getting up higher.

[Silenus]

Would something like this work?

[Silenus]

Yes... in 99% of applications.
Keep the LED wiring and resistor leads as short and tidy as possible if the data speeds are getting up higher.

Thanks   Sorry just realised I had my LEDs around the wrong way (used to the top power port being vcc) hence the deleted comment.

[Muxr]

LEDs are not high impedance, usually data goes to high impedance components. This means data pins on an IC are not meant to provide current for driving components like a LED.

Will it work? It might. It will potentially degrade your signal, and put strain on the driving circuit. Your data may become corrupted because of degradation imposed by LEDs drawing current.

As SL4P has suggested you want to buffer the signal from data lines to LEDs. Use transistors as switches from your bus lines. This will limit the current drain imposed on the data lines since transistors will provide the current for the LEDs.

[David_AVD]

Muxr, I'm not sure what ICs you normally use but ...

Your typical microcontroller output pin can source 10 - 20mA so putting an LED on there that draws 1 - 2mA is not going to be an issue.

Similarly, a lot of 74xx logic outputs will also be able to source plenty of current.

There are plenty of LEDs out there that will light brightly with only 0.5mA of current.

[Muxr]

Muxr, I'm not sure what ICs you normally use but ...

Your typical microcontroller output pin can source 10 - 20mA so putting an LED on there that draws 1 - 2mA is not going to be an issue.

Similarly, a lot of 74xx logic outputs will also be able to source plenty of current.

There are plenty of LEDs out there that will light brightly with only 0.5mA of current.

I've ran into issues with SPI data signal quality on the esp8266 due to high load. Also usually IC's overall source and sink budget is much less than the sum of all its IO pins. So it would be prudent to buffer something as critical as the data bus. I agree though for 1 - 2 mA there shouldn't be too much issue.