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Topic: Can't capture UART signals (Read 3155 times) previous topic - next topic

Can't capture UART signals

Hi all,
I just received my Open Bench Logic Sniffer and Bus Pirate and managed to get them working with each other. The Bus Pirate was generating UART and the Sniffer was capturing it (using OLS) (I followed a guide since I'm pretty new to this stuff).

I'm reading through a tutorial which aims to get the reader familiarized with a logic sniffer and the UART protocol. The author suggests a Saleae Logic or Open Bench Logic Sniffer coupled with a FT232RL USB To Serial Adapter. I went with the Open Bench and I'm having trouble seeing any signals from the adapter. I also tried the same thing with the Bus Pirate and that couldn't detect anything either. I've tried all sorts of baud rates for the adapter and the logic sniffer in both the OLS GUI, the terminal I use to send signals to the adapter and the Windows Device Driver entry for each COM port and nothing works.

The strange thing is, when I connect to the UART adapter and send characters to it, the TX light flashes so I assume that side of things is working fine. I have attached the GND pin to the GND of the logic sniffer and the TX pin of the adapter to pin 0 of the logic sniffer. I've spent all night on this thing and completely exhausted all ideas. I would usually put this down to faulty hardware but I doubt that's the case. It must be something stupid I'm doing so I'm bound to see this problem with other devices. The only other thing I can try is to modify the solder jumper pads to switch this adapter from 5V to 3.3V.

Does anyone have any ideas?


Cheers.

Re: Can't capture UART signals

Reply #1 –
Hey,

Make sure you trigger on 0, not on 1, since the line is high in idle, so you want to start recording when it gets low (when data gets transmitted).

After that's done, start sniffing and the client will say that it's waiting for a trigger. At this point you can start sending data over UART and it would be picked up by the sniffer.

Re: Can't capture UART signals

Reply #2 –
Thanks jadew, that got it capturing something. Unfortunately now it's just spitting out a couple 0xFF values no matter what I type into the terminal. Must be something else now, I'll keep at it.

Re: Can't capture UART signals

Reply #3 –
Hi Lamanna,

The problem you are experiencing can be due to multiple factors.

This is what I would check:

1. If there is any DC voltage on the tx pin or even rx pin relative to the ground (the tx should have about 5v when idle for TTL levels). If you have a multimeter you can check this to see if anything is driving any of those pins, sometimes the rx and tx are labelled the other way around. It would be wise to have this check before connecting any unconfirmed tx and rx serial pins between devices as if both devices end up driving the same pin at the same time bad things can happen ...
 
2. Are you setting any hardware/software handshake in your program/terminal? (it should not be enabled for the purpose of this test)
3. (advanced) Are any pins set to be inverted in the FT232RL eeprom. The FT_Prog utility from the FTDI site could tell you this but please only use this program AFTER reading the documentation and datasheet!
4. Are you setting a common baudrate say 9600 and using the OLS to capture at about 4x that?
5. (advanced) Are you using any custom baudrate. The FTDI drivers at least on windows allow remapping of baudrates in registry or driver inf files. You probably did not do this but the adapter manufacturer could  (even though very unlikely) have done it if you got the drivers from them instead of FTDI.
6. For full duplex devices (and without setting any handshaking) you could connect the rx and tx in a loopback and receive what you sent in the same terminal (I tried this on a FT232RL some time ago but did not worked for me, although it worked with a PIC uC so the terminal/workflow I was using was apparently ok). If this would  work with the FT232RL you could check that it accepts its own output, but again it did not work for me, and I could not find any settings to turn full duplex on so maybe, just maybe it cannot do it.

Thus plenty of things to go wrong but I would start with item 1.

Regards,
tkg

Re: Can't capture UART signals

Reply #4 –
Hi lamanna;
  If you are using a serial adapter, you might be generating +-12V signals.
A Max232 or similar chip does this.

Also by default the OLS captures at 200Mhz to start with.
It has a small buffer, so the first serial bit will fill it at once.

As tkg said, lower the capture speed to 4x the baud rate, or say 100KHz to start with.
You should be able to short pins 2 and 3 on the DB9 serial connector to get it to echo back to itself.
(RX to TX).

Try a capture, and invert the trigger if it does not work.

Mick M