SerialNINAPassthrough sketch in working condition for MKR Vidor 4000 (https://forum.arduino.cc/index.php?topic=675947.0) (based on PRs 112, 111, 110, 109) https://github.com/arduino-libraries/WiFiNINA/pulls

SerialNINAPassthrough.ino

/*
  SerialNINAPassthrough - Use esptool to flash the u-blox NINA (ESP32) module
  Arduino MKR WiFi 1010, Arduino MKR Vidor 4000, and Arduino UNO WiFi Rev.2.

  Copyright (c) 2018 Arduino SA. All rights reserved.

  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public
  License along with this library; if not, write to the Free Software
  Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/

#ifdef ARDUINO_SAMD_MKRVIDOR4000
#include <VidorPeripherals.h>
#endif

unsigned long baud = 115200;

#define BUFFER_SIZE 512
static unsigned int buffer_length;
static char buffer[BUFFER_SIZE];

int rts = -1;
int dtr = -1;

void setup() {
  Serial.begin(baud);

#ifdef ARDUINO_SAMD_MKRVIDOR4000
  FPGA.begin();
#endif

  SerialNina.begin(baud);

#ifdef ARDUINO_SAMD_MKRVIDOR4000
  FPGA.pinMode(FPGA_NINA_GPIO0, OUTPUT);
  FPGA.pinMode(FPGA_SPIWIFI_RESET, OUTPUT);
#else
  pinMode(NINA_GPIO0, OUTPUT);
  pinMode(NINA_RESETN, OUTPUT);
#endif

#ifdef ARDUINO_AVR_UNO_WIFI_REV2
  // manually put the NINA in upload mode
  digitalWrite(NINA_GPIO0, LOW);

  digitalWrite(NINA_RESETN, LOW);
  delay(100);
  digitalWrite(NINA_RESETN, HIGH);
  delay(100);
  digitalWrite(NINA_RESETN, LOW);
#endif
}

void loop() {
#ifndef ARDUINO_AVR_UNO_WIFI_REV2
  if (rts != Serial.rts()) {
#ifdef ARDUINO_SAMD_MKRVIDOR4000
    FPGA.digitalWrite(FPGA_SPIWIFI_RESET, (Serial.rts() == 1) ? LOW : HIGH);
#elif defined(ARDUINO_SAMD_NANO_33_IOT)
    digitalWrite(NINA_RESETN, Serial.rts() ? LOW : HIGH);
#else
    digitalWrite(NINA_RESETN, Serial.rts());
#endif
    rts = Serial.rts();
  }

  if (dtr != Serial.dtr()) {
#ifdef ARDUINO_SAMD_MKRVIDOR4000
    FPGA.digitalWrite(FPGA_NINA_GPIO0, (Serial.dtr() == 0) ? HIGH : LOW);
#else
    digitalWrite(NINA_GPIO0, (Serial.dtr() == 0) ? HIGH : LOW);
#endif
    dtr = Serial.dtr();
  }
#endif

  if (Serial.available() > 0) {
    buffer_length = min(Serial.available(), BUFFER_SIZE);
    Serial.readBytes(buffer, buffer_length);
    SerialNina.write(buffer, buffer_length);
  }

  if (SerialNina.available() > 0) {
    buffer_length = min(SerialNina.available(), BUFFER_SIZE);
    SerialNina.readBytes(buffer, buffer_length);
    Serial.write(buffer, buffer_length);
  }

#ifndef ARDUINO_AVR_UNO_WIFI_REV2
  // check if the USB virtual serial wants a new baud rate
  if (Serial.baud() != baud) {
    rts = -1;
    dtr = -1;

    baud = Serial.baud();
    SerialNina.begin(baud);
  }
#endif
}