Uncorrect modbuss reception packet

[Rocketct]

Trying the modbus library with the following sketch:

#include <ArduinoModbus.h>

float temperature, humidity;

void setup() {
  Serial.begin(9600);
  while (!Serial);

  Serial.println("Modbus Temperature Humidity Sensor");

  if (!ModbusRTUClient.begin(9600)) {
    Serial.println("Failed to start Modbus RTU Client!");
    while (1);
  }
}
 int i=0;
void loop() {
  if (!ModbusRTUClient.requestFrom(1, HOLDING_REGISTERS, 0x00, 2)) {
    Serial.print("failed to read registers! ");
    Serial.println(ModbusRTUClient.lastError());
  } else {
    short rawtemperature = ModbusRTUClient.read();
    short rawhumidity = ModbusRTUClient.read();
    temperature = rawtemperature / 10.0;
    humidity = rawhumidity / 10.0;
    Serial.println(temperature);
    Serial.println(humidity);
  }
 Serial.println(i);
  delay(1000);
  i++;
  if(i>2){
    while(1);
  }
}

rise an issue related to reception of the packet from the RS485 channels, it alternate a good reception with a discard, as show in the in the image.
I have sniffed the channel with the Saleae analyzer, and the packet are the same, the capture are in the following .zip.

modbus.zip

The problem could be located in the library libmodusb/modbus.c in modbus_read_registers(modbus_t *ctx, int addr, int nb, uint16_t *dest) or in the integrity check function of the class modbus_backend_type_t.
In particular must be checked how the CRC is calculated in two consecutive reading from the RS485 .
Following, also, a stamp of serial monitor output:

[sandeepmistry]

From the trace I see:

0x01 0x03 0x04 0x01 0x1d 0x02 0x4f 0x2b 0x5d
0x01 0x03 0x04 0x01 0x1d 0x02 0x4f 0x2b 0x5d
0x01 0x03 0x04 0x01 0x1d 0x02 0x4f 0x2b 0x5d

which are 3 identical responses sent, so there is no CRC error on the line.

The data values 0x11d and 0x24f correspond to the 28.5 and 59.10 values seen in the second request printed.

Why aren't we seeing the request on the line too? It would be good to print out what the SAMD thinks it's receiving on the UART ...

[Rocketct]

Update: new capture with Uart Rx and tx channel, and the Z channel of the RS485
The digital analyzer have as ground the ground of the mkr board, not the ISO GND of the RS485 board.
the capture shows that the UART RX channel receive a copy of the request packet UART TX:
0x01 0x03 0x00 0x00 0x00 0x02 0xC4 0x0B

the request is also transmitted by the RS485. in the receiving step, instead, the host receive one time the correct packet, and after packet that is a copy of request but without the last 3 bytes as follow:

1  <- first byte of the request Packet
3
0
0
0
-1 <- return receiving function show bad reception
failed to read registers! Invalid CRC
1 <- first byte of the response Packet from the RTU
3
4
1
19
2
59
EB <- first byte of the CRC response Packet
52
2   <- return of the function that, show that all are gone good
28.10
60.10

the capture file is the following:

ModbussIssue.zip

and here a stamp of the capture:

[Rocketct]

Update: adding the noReceive() and receive() in _modbus_rtu_send the function, as follow:

  RS485.noReceive();
  RS485.beginTransmission();
   size = RS485.write(req, req_length);
   RS485.endTransmission();
   RS485.receive();

the repeated packet problem disappear, because in half duplex communication the uart rx pin mast be in high impedance.
however the modbus issue follow to rise up , as next step could be useful check the delay between the transaction of the pin before mentioned, printout the packet transmited in the uart rx , try to manage the delay int modbus_set_byte_timeout(modbus_t *ctx, uint32_t to_sec, uint32_t to_usec)

[sandeepmistry]

This has been fixed via a core change arduino/ArduinoCore-samd#349.