WattNode and ModBus

Projects Programming
1

Hi All,

I'm trying to get my Arduino w/ an I/O expansion shield to communicate with a WattNode electrical meter via modbus. I'm able to send a packet and the WattNode appears to be accepting it as the light flashes green (this is what the manual says it should do when communications are correct). However, I am not really sure how to read the response. I have tried doing Serial.read() but this just results in a -1.

Any help would be great. And if I need to provide more info, just let me know.

Thanks!

James

int EN = 2;
int incomingByte = 0;   // for incoming serial data

void setup() {
  pinMode(EN, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  digitalWrite(EN,HIGH);
  Serial.write((byte)0x01);
  Serial.write((byte)0x03);
  Serial.write((byte)0x04);
  Serial.write((byte)0xC5);
  Serial.write((byte)0x00);
  Serial.write((byte)0x01);
  Serial.write((byte)0x95);
  Serial.write((byte)0x07);
  digitalWrite(EN,LOW);
  incomingByte = Serial.read();
  Serial.println(incomingByte, DEC);
  delay(1000);
}
2

I don't see a Serial.read there.

3

Oops, I changed it. With it setup like that, I now get a blinking yellow like from the WattNode. The manual says that "If the WattNode meter sees packets on the bus addressed to other devices, it will light the LED yellow for 200 milliseconds or longer if the packet duration is longer than 200 milliseconds." This doesn't make sense since I didn't change the address.

4

Right. You need to read up on how serial works. Almost certainly you would get -1 there.

  incomingByte = Serial.read();
  Serial.println(incomingByte, DEC);

Hint: You need to test Serial.available. In a loop, because it won't become available instantly.

5

I kind of thought it was something along those lines. I'll do some reading on it. Thanks for the link and pointing me in the right direction!

James

6

So I did a bit more reading, and tried a few more things to read the response from the WattNode, but not luck. I'm checking with Serial.available(), but still am not seeing anything.

int EN = 2;
int incomingByte = 0;   // for incoming serial data

void setup() {
  pinMode(EN, OUTPUT);
  Serial.begin(9600);
  digitalWrite(EN,HIGH);
}

void loop() {
  
  for (int i=0; i<5; i++) {
    digitalWrite(EN,HIGH);
    Serial.write((byte)0x01);
    Serial.write((byte)0x03);
    Serial.write((byte)0x05);
    Serial.write((byte)0x14);
    Serial.write((byte)0x00);
    Serial.write((byte)0x16);
    Serial.write((byte)0x84);
    Serial.write((byte)0xCC);
    delay(100);
    digitalWrite(EN,LOW);
  }
  delay(2000);
  digitalWrite(EN,LOW);
  // send data only when you receive data:
  if (Serial.available() > 0) {
    // read the incoming byte:
    incomingByte = Serial.read();
  
    // say what you got:
    Serial.print("I received: ");
    Serial.println(incomingByte, DEC);
  }
  digitalWrite(EN,HIGH);
  delay(1000);
}
7

Are you using the serial port to talk to the WattNode or to talk to the Serial Monitor? You can't use it to talk to both.

8

Honestly, I guess I'm not sure. I believe I am communicating with the WattNode over serial with an I/O Expansion Shield.

9

Honestly, I guess I'm not sure.

I think you need to connect your device to two other pins, and use NewSoftSerial (0023 or earlier) or SoftwareSerial (1.0+) to talk to it, and use Serial to talk to the PC/Serial Monitor.

10

Hmm, I feel like I'm still missing some important link here. I was under the impression that using the I/O expansion shield for talking to the WattNode was what I needed. So far it seems to be taking my communications, but I'm not able to read its response.

11

Can you post a link to your expansion shield?

12

I'm using for RS485 communication with the WattNode.