Unable to communicate with Modbus device

[diablodude64]

Hello everyone,

I have been scouring the forums trying to get my setup functioning and so far no luck. I am a bit of a novice so, that's isn't exactly helpful either.

I have an Arduino Mega 2560 and a piece of hardware that communicates using the Modbus standard via RS485. My RS485 converter is a MAX485.

The modbus hardware info:
-maximum polling interval of 10 ms
-32-bit Floating point or signed integer Little Endian [CDAB]
-the manual says all data is stored in 2 16-bit registers with starting addresses listed in the register map. The address below is the starting address for example.
-I am interested in measuring temperature which according to the register map verbatim is Hex address 0x41 which is type 32-bit integer.

I think part of my problem is that the library is expecting a 16-bit chunk of data and I am not exactly sure how to break this up into 2 pieces and then recombine them.

I have the MAX485 to Arduino, respectively, wired as follows:

Vcc - 5v
GND - GND
A - A on modbus device
B - B on modbus device
R0 - RX0
RE/DE - tied together to Digital pin 3
DI - TX0

I am using the ModbusMaster.h library from the Arduino IDE. There is a line near the bottom of my code, 'node.getResponseBuffer(0x00/10.00)' that I am not exactly sure what should go in place of '0x00'. The 10 is scaling I believe from other examples that I can find.

I would appreciate any resources or help that anyone can provide to get this functioning.

Thank you!

My code:

#include <ModbusMaster.h>


#define MAX485_DE      3

// instantiate ModbusMaster object
ModbusMaster node;

void preTransmission()
{
  digitalWrite(MAX485_DE, 1);
}

void postTransmission()
{
  digitalWrite(MAX485_DE, 0);
}

void setup()
{

  pinMode(MAX485_DE, OUTPUT);

  digitalWrite(MAX485_DE, 0);

  // Modbus communication runs at 115200 baud
  Serial.begin(38400);

//Baudrate of modbus device is 38400
  Serial1.begin(38400);
  
  // Modbus slave ID is 9
  node.begin(9, Serial1);
  // Callbacks allow us to configure the RS485 transceiver correctly
  node.preTransmission(preTransmission);
  node.postTransmission(postTransmission);
}

void loop()
{
  uint8_t result;
  //Need data from starting address 0x41 with length 2
   result = node.readHoldingRegisters(0x41, 2);

    //digitalWrite(MAX485_DE, 0);
     
    Serial.println(node.getResponseBuffer(0x00)/10.00);

  delay(500);
}

[pylon]

@diablodude64

R0 - RX0
RE/DE - tied together to Digital pin 3
DI - TX0

but

@diablodude64

  node.begin(9, Serial1);

I guess you connected to RX1 and TX1 but you should check that!

@diablodude64

result = node.readHoldingRegisters(0x41, 2);

What values do you get in result?

Why do you know you cannot communicate with the device?

Where is the link to the other device's manual?

@diablodude64

  Serial1.begin(38400);

Modbus defines to use the parity bit (although quite a few devices ignore that). Did you check that detail?

[diablodude64]

I changed to Serial instead of Serial1 and defined SERIAL_8N1 as well.

I see a bunch of jumbled stuff in the serial monitor.

The link to the manual for the modbus device is: https://www.vpinstruments.com/download/119/manual-vpflowscope-in-line/1qrQXZZ8SbpXxw88MMzQbSTv6LlPBS-7I/Manual%20VPFlowScope%20In-line%20-%20EN

Again, I am pretty new to this so I do appreciate the help!

Thank you

[diablodude64]

I downloaded a modbus traffic monitor to see what the arduino is broadcasting. Here is what I get:

The initial broadcast is wrong of course as I am trying to read a holding register which should be function code 03 not 02.

Could this be timing related or am I missing something completely?

[pylon]

@diablodude64

I changed to Serial instead of Serial1 and defined SERIAL_8N1 as well.

Bad idea as you currently use that Serial for debugging.
You should keep Serial1 and connect to RX1 and TX1!

@diablodude64

Could this be timing related or am I missing something completely?

Probably just the debugging output you put on the same serial interface.

[diablodude64]

Turns out the manual for the flowmeter has A/B reversed. I swapped them and if I use modpoll and a rs485 converter I can get what I need. I am doing that just to verify that I have data coming from the flowmeter.

I get the same response whether from Serial or Serial1. I have changed to Serial1 as a precaution noting what you said regarding debugging.

When I use Serial Port Monitor now, I see the following:

Any suggestions? Also, thank you for your patience. I imagine that answering some of these questions is rather trivial and monotonous after a while.

[diablodude64]

Okay so further update... I changed the baudrate to 9600 on the flowmeter as well as in the code and I get exactly what I need now. It works...