First off, I searched for this question and while I found other similar questions, none had an answer for this.

I've got a serial cable from a (Linux) PC supplying data to a PCB (AVR XMEGA). I'm trying to monitor the data sent across this serial cable using a Bitscope.

I've got CHA connected to pin 3 of the serial port DB9 (FWIW, this is a UART-backed serial port, not a USB "serial port")and GND connected to pin 5 of the DB9.

No matter how I change the Bitscope settings, I can't get anything resembling a square wave, whether data is being transmitted across the serial port or not. The attached image is all I can see.

Sorry to be such a n00b, but if anyone can tell me what I'm doing wrong, I'd appreciate it.

enter image description here

  • 2
    \$\begingroup\$ Does the application actually work without and with the scope connected? Welcome to EE.SE but please capitalise your sentences, personal pronoun ("I") and acronyms properly for legibility. Your question will be taken more seriously (leading to better answers) and you will appear more intelligent. \$\endgroup\$
    – Transistor
    Sep 19, 2016 at 17:06
  • \$\begingroup\$ Thank you for your feedback.The application is not working, but I am certain the serial data is being sent across the I/O port. \$\endgroup\$
    – TwistOneUp
    Sep 19, 2016 at 17:41
  • \$\begingroup\$ You need to explain because your scope (and the application) is telling you otherwise. \$\endgroup\$
    – Transistor
    Sep 19, 2016 at 17:44
  • \$\begingroup\$ Are you expecting to be sending data from the PC or to the PC? \$\endgroup\$
    – brhans
    Sep 19, 2016 at 17:59
  • 1
    \$\begingroup\$ Might want to try pin #2 instead of #3. \$\endgroup\$
    – JimmyB
    Sep 19, 2016 at 18:11

1 Answer 1


enter image description here enter image description here

Figure 1. A simple RS232 monitor can be built with some back-to-back LEDs and a couple of resistors. For basic monitoring only TX Data and RX Data need monitoring. Source: Zonker's RS-232 Passive Signal Tracer Page.

The real minimalist circuit is just one pair of back-to-back LEDs to monitor the TX line. In normal operation one LED should stay on when the line is quiet and both should blink (at the baud rate - depending on the data pattern) when the line is active.

Try this and report back.

[OP's comment:] I built a small one-line (TX) circuit ... on a breadboard. One light dimly flashes during data transmission but the other is always off, so I'm experimenting with different ohmages and baud rates.

Right, now we know that there is activity on the line. Next tackle why the scope isn't working. Hook the scope probe to the LED TX Data line and the ground to ground. What do you see? If you see nothing while the LED blinks then you know you have test equipment trouble - faulty probe, connection or software.

  • \$\begingroup\$ I built a small one-line (TX) circuit, as you have graciously shown above, on a breadboard. One light dimly flashes during data transmission but the other is always off, so I'm experimenting with different ohmages and baud rates. At the least, I can now trace problems in the circuit. I've also ordered one of the RS232 monitors you have in the picture above so i can check all the serial lines. Thanks! \$\endgroup\$
    – TwistOneUp
    Sep 19, 2016 at 18:51
  • \$\begingroup\$ See the update. \$\endgroup\$
    – Transistor
    Sep 19, 2016 at 18:58
  • \$\begingroup\$ Bingo! I now have square waves on my bitscope. Turns out that the original serial card in the linux PC was bad. I swapped cards and that fixed it. Thanks again! \$\endgroup\$
    – TwistOneUp
    Sep 19, 2016 at 19:08

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