Simple serial loopback with USB-FTDI and TXB

Thread Starter


Joined Jun 18, 2021
I have a bunch of ESP8266 chips and I'm trying to build my own DYI programming board for those.
After running into some issues, I started experimenting with simpler circuits to try to narrow down the what I'm doing wrong.

I first plugged the FT232RL based 5V USB-FTDI cable to my laptop and ran a simple serial loopback test just by shorting the RX and TX pins.
This works well and I can see anything I type getting echoed in the terminal.

Then I moved on to try working with a TXS0108E level shifter (as the ESP is a 3.3V device).
The level-shifter chip I have comes on a simple module with two small capacitors (I think 0.1uF each, to smooth the power supply?).
a) Connected a 9V battery to a simple power supply to provide 3.3V.
b) Connected together the ground pins of the 3.3V supply and FTDI cable to create a common ground reference.
c) Connected the TXS0108E's Vcc-A and OE (output enable) pins to the 3.3V supply.
d) Connected the Vcc-B and GND pins to the FTDI cable (5V).
e) Plugged the FTDI's TX to TXS0108E's B1 and RX to B2.
f) On the other side of the TXS0108E chip, I simply connected A1 to A2.

I expected this simple setup to work too, but when I try the loopback test again I'm getting all kind of noise in response to whatever I type in the terminal.

What am I missing here? What might be the source of the problem?
Any help would be appreciated!