SparkFun Simultaneous RFID Reader - M6E Nano failure to connect to Raspberry Pi Zero W

To start off, I do believe the issue is with the hardware, and not the software. I have also confirmed that the Pi’s UART (GPIO 14 and 15) are working (tested with an oscilloscope). I have also been using an external antenna (which I confirmed has been setup correctly). I have the following setup (both supplied with sufficient power at 5V):

Pi Reader

Tx → Rx

Rx ← Tx

Gnd ↔ Gnd

The reader outputs 5V (have not found a way to bypass the onboard level shifter), but the Pi is not 5V tolerant. Therefore, I setup a voltage divider to step down the reader voltage from 5V to 3.3V for the Rx pin on the Pi. Everything seemed to work after confirming the voltages with a multimeter. Fast forward to the following day, the exact same setup is configured, but now, the 5V from the Tx on the reader dropped down to around 4V and the Rx on the Pi (after voltage divider), dropped down to around 2.11V. Nothing was changed from the previous setup (including the software), but the values changed somehow. The correct voltages are read if no connections are made between the two devices as well. I am led to believe that the logic level shifter needs a certain amount of current to be drawn to receive the desired voltages and that somehow, enough current is not being drawn by the Pi. I don’t know how that would change when the setup is the same, but that is the only thing I can think of. We have done more testing, but this has been the most recent findings. Any help would greatly be appreciated.

I have noticed weird behavior before in different setups.

The Onboard level shifter (TXB0104) does not play well when connected directly to another level shifter (e.g. an external to reduce the 5V from the Reader TX to 3V3). Also, it does not like when a resistor (>10K) is placed in serial, as is done in some FTDI cables from 5v to 3v3. Look at the post on https://github.com/sparkfun/Simultaneou … /issues/10.

The datasheet of the TXB0104 mentions a number of aspects around resistor and minimum current e.g. that for any pull-up or down resistor it is advised to be larger than 50K. ??