Python: convert array of uint16_t to string (from Arduino over RS-485)

Question

I'm trying to send json over modbus rtu (I know, it is very bad use of modbus.)

My schema is similar to:

I have connected Arduino USB to PC as COM5 and RS485 converter connected to USB-RS485 to PC as COM4.

If I read data from Arduino using QModBus application, I will see a lot of bytes (COM4).

For control: after I send from QModBus "read holding registers" Arduino serial monitor on "Arduino usb port" (COM5) print valid string. So I mean that modbus bytes are ok:

There is my Arduino code:

#include <ArduinoJson.h>
#include <ModbusRtu.h>
#define ID   1

// assign the Arduino pin that must be connected to RE-DE RS485 transceiver
#define TXEN  2 

Modbus slave(ID, 0, TXEN); // this is slave ID and RS-232 or USB-FTDI

// data array for modbus network sharing
uint16_t au16data[100];

boolean state;
String json = "{\"idx\":1430,\"nvalue\":0,\"svalue\":\"-58.00\"}";

void setup() {
  slave.begin( 9600 );
  StaticJsonBuffer<200> jsonBuffer;  
  JsonObject& root = jsonBuffer.parseObject(json); 
}

void loop() {
  // https://github.com/smarmengol/Modbus-Master-Slave-for-Arduino/blob/master/ModbusRtu.h#L1391
  byte x = 0;
  for (byte i = 0; i < json.length(); i += 2) {
    uint16_t temp;
    temp = word(
      json[i],
      json[i+1]);  
    au16data[x] = temp;
    x++;
  }

  state = slave.poll( au16data, 100 );  
} 

But I don't know how to convert these bytes back to json string in python. My code:

import serial
import minimalmodbus

MODBUS_3  = 3  # Read holding registers

dev1 = minimalmodbus.Instrument('COM4', 1)  # port name, slave address (in decimal)
dev1.serial.baudrate = 9600
dev1.serial.bytesize = 8
dev1.serial.stopbits = 1
dev1.serial.parity = serial.PARITY_NONE
dev1.debug = False

data = dev1.read_registers(0, 20, MODBUS_3)
print(data)

Code print my the same values as QModBus:

[31522, 26980, 30754, 14897, 13363, 12332, 8814, 30305, 27765, 25890, 14896, 11298, 29558, 24940, 30053, 8762, 8749, 13624, 11824, 12322]

Can you please help, how can I convert these numbers to json string as you can see in arduino serial monitor?

And how to convert python string to "uint_16t" for sending over modbus.

Thank you!

Answer 1

This should work:

import struct
dataStr = b''
for uint_16t in data:
    dataStr += struct.pack('>I', uint_16t)
print dataStr

Output based on your supplied list:

  {"idx":1430,"nvalue":0,"svalue":"-58.00"

Not sure why it is missing the closing } though...

Edit: To remove that weird whitespace you can do:

for i in dataStr:
    if ord(i) != 0:
        newDataStr += i
print newDataStr