Why can't I talk to Notecard over UART from my custom board?

I created my own board to integrate the Notecard into my hardware design.

When I plug the Notecard into my board, I cannot communicate with it any longer.

I can send a message over UART to the Notecard, but it doesn’t respond.

I have been using UART (RX/TX pins), and I’ve tried using the AUX UART (AUX RX/TX).

If I take the same Notecard and plug it into a Notecarrier A, then I can communicate with it just fine.

What is going on?

Things to Check

Double check the wires are set correctly.

For UART, the TX on your processor needs to be connected to the RX on the Notecard. And the RX on your processor needs to be connected to the TX on the Notecard. This is a cross-over connection, and differs from using I2C, which is not a cross-over connection.

Ensure AUX EN is High for AUX RX/TX

The AUX EN (aux enable pin) must be set to logic high for the AUX RX/TX pins to function.
See What is AUX RX/TX and how do I use it?.

Check logic voltage levels

The Notecard logic (and peripherals) can operate at 3.3V or 1.8V. If you are using a low voltage chip as the host processor, be sure that its logic voltage levels are compatible with the Notecard configuration.

Check UART configuration on the host processor

Be sure the UART on the host processor is configured to communicate with Notecard

RX/TX AUX RX/TX (default)
Baud Rate 9600 (bits/second) 115200 (bits/second)
Data 8 bit 8 bit
Parity none none
Stop bits 1 bit 1 bit
Flow control none none
Shorthand 9600 8N1 115200 8N1

Is Notecard Reset Active?

If the NRST pin on Notecard is being pulled low, then it will initiate Notecard resets. Pulling it high could also hinder operation, so if it’s not being used, then it’s best to leave it unconnected.

Check the “Reset” section of the datasheet for a few more details.

Notecarriers may invert NRST
Some Notecarriers invert the NRST line so they are active high. If you develop using a Notecarrier, and then switch to a custom board, be sure that polarity of the NRST is set correctly, as it might be inverted from what you expect from working with the Notecarrier.

Review the Host System Design Guide

If you haven’t already, it’s worth reviewing the Host System Design Guide to see if there is something else that may tip you off to the source of the problem.