I have a UART connection from an ESP32 (esp datasheet to a Quectel BG95 that I cannot get to work. At first I thought it was code related but after reviewing the AT manual several times. I am thinking it must be circuit related...

Here is a schematic of the connections first we go from the esp through a level shifter datasheet (3.3 to 1.8 V) then to the BG95 datasheet.

esp connection

level shifter

quectel connection

Below is the code I used, which sends an AT command with return carriage, according to the manual I should be getting an OK back.

#include <HardwareSerial.h>
int greenpower = 32;
int led = 33;
int pwr = 5;

void setup() {
  pinMode(pwr, OUTPUT); //for verifying power to the board
  pinMode(led, OUTPUT); //for verifying power to the board
  pinMode(greenpower, OUTPUT); //for allowing power to auxillary processes
  digitalWrite(greenpower, HIGH); //allow power to aux devices
  digitalWrite(led, HIGH);  //apply power to verification circuit
  digitalWrite(pwr, LOW);  //begin modem start-up
  digitalWrite(pwr, HIGH);  //start-up signal
  //digitalWrite(pwr, LOW);  //end modem start up sequence
  delay(5000);  //wait for stabilization

  Serial.begin(115200);  // Open serial communications and wait for port to open:


void loop() {
  while (!Serial2.available()) continue;
  while (Serial2.available()) {
    char g = Serial2.read();

PCB footprint

Note that the level shifter is upside down. But does anyone have a clue as to what I am doing wrong here?

  • \$\begingroup\$ From the scope picture of the Rx connected to the Tx indicates a broken or open connection. Since they are connected they should be the same. I use an inexpensive china logic analyzer, it makes this a lot easier to troubleshoot. Then can be gotten to your door for under $10.oo in the USA. \$\endgroup\$
    – Gil
    Aug 7, 2021 at 3:23

1 Answer 1


Probably, you are pulling the last hair out.
Assuming everything correct you did, looking at the third scope picture..., very random thoughts:

  1. Check if that broken signal pin requires pull-up... Why would it?
  2. Ohm (continuity check) that signal pin/trace with adjacent signal traces/pins.
  3. Remove and re-solder suspected parts.
  4. Ask the assembly house or PCB manufacture to X-ray the assembled board.
  • \$\begingroup\$ Thanks for these suggestions I am updating the question with some recent findings. \$\endgroup\$
    – Feynman137
    Aug 10, 2021 at 2:25
  • 1
    \$\begingroup\$ Way to go @Feynman137 ! It looks good, \$\endgroup\$
    – jay
    Aug 10, 2021 at 12:59
  • \$\begingroup\$ Thanks man.. I am confused though I am seeing the signal on the scope but getting blanks in the serial monitor. \$\endgroup\$
    – Feynman137
    Aug 11, 2021 at 4:23

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