I'm using a known good FTDI USB -> UART bridge to talk to a 3rd part module on my board. I've just assembled the board, and it's not impossible that I fried a component in the process (it's what I'm trying to understand right now)

When I send data through the UART, I get this:

enter image description here

The yellow is the incoming TX (RX from the module's perspective) line and the blue is the RX. As you can see, data is coming in fine, but it's causing this weird "echo" effect on the module's TX pin, with no response from the module (I'd expect something back).

The module is a Bluetooth module I've worked with before, connected like this:

enter image description here

(the TX and RX lines are flipped appropriately on the FTDI connection)

Any ideas what this is? Is it just a fried module?

  • \$\begingroup\$ That's an incredible amount of crosstalk... \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 5 '15 at 0:15
  • \$\begingroup\$ it's possible - the UART is just for debug purposes, and I've got a 4" long ribbon cable taking it off-board. If that were the case, would the cross-talk prevent the module from responding? \$\endgroup\$ – kolosy Mar 5 '15 at 0:16
  • 1
    \$\begingroup\$ No, the response signal would be superimposed on the crosstalk noise. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 5 '15 at 0:20
  • \$\begingroup\$ Is the part marked incorrectly or have you pulled the active-high reset high? \$\endgroup\$ – Samuel Mar 5 '15 at 0:29
  • \$\begingroup\$ Nice scope capture, often they're pretty unreadable. \$\endgroup\$ – tcrosley Mar 5 '15 at 0:31

The serial BT module can be checked easily without any tools. Short TX to RX then pair the module with a phone, PC, etc., open a terminal and type something. If module is working you'll get echo back. Make sure 'local echo' is turned off in the terminal. If you have known good module practice on it first.

FTDI USB to serial can be tested using the same technique.

  • \$\begingroup\$ That's a fair point. Even having it show up in a pairing list would mean it's not completely dead. \$\endgroup\$ – kolosy Mar 5 '15 at 3:59

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