Given the following circuit , electrically speaking makes any difference the position of leds?

enter image description here

I know R8 and R9 are used to allow the override of the RX/TX pins if any other device is connected, does placing R7,R8 and the Leds after or before these resistors affect this behaviour?

  • \$\begingroup\$ You lose some LED functionality and the RXD level might be reduced below an acceptable level. Why bother moving them is my question to you. \$\endgroup\$
    – Andy aka
    Dec 7 '18 at 9:05
  • \$\begingroup\$ I have a design that includes and RS485 serial port attached to the RX/TX portion of the arduino , I use a jumper to connect the RS485 transcriber or leave the USB serial port, I want to have independent activity leds for USB and RS485 \$\endgroup\$
    – Marc
    Dec 7 '18 at 9:07
  • \$\begingroup\$ This non-Arduino but approximately compatible board doesn't seem very well thought through, unless they are intentionally making the current through the LEDs differ to balance apparent brightness. Actual Arduino products use distinct signals to drive the LEDs, rather than the actual TX/RX lines. \$\endgroup\$ Dec 8 '18 at 3:21

The RX LED (TXD pin) is going to blink in either location but will reduce the signal level to the CH340 if moved west of R9. this may render the CH340 unable to receive. It may be possible to use the TX-enable input to the RS485 to control which RX led illuminates.

the TX LED (RXD pin) in it's present location is reducing the signal level to the CPU as it forms a voltage divider with R8. It will probably blink brighter if moved west of R8, and the CPU will see a stronger serial signal. probably other signals on the RXD pin will not cause it to blink.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.