Modbus RTU on Industruino IND.I/O

Wind speed meter with Modbus over RS485


RS485 on Industruino IND.I/O

The IND.I/O has a standard RS485 port that can be accessed from the hardware serial, Serial1, which does not affect the USB serial connection. This is an advantage of the 32u4 and 1286 chip of the Industruino over the standard 328 on the Arduino Uno.

The wind speed sensor (anemometer) that i have, uses the Modbus protocol over an RS485 connection, aka Modbus RTU. The sensor is the Modbus Slave and the Industruino is the Modbus Master.

The Industruino RS485 is half-duplex, aka 'two wire' = A and B (plus GND). My sensor had 4 wires, for Vcc, GND, A and B, which i connected straight to the Industruino as in above picture. I also had to connect an external power supply on the Industruino as the 5V USB from the laptop is not sufficient to power the sensor (min 7V). I supplied 9V from my Industruino PROTO power supply, described in another blog post.

Modbus RTU library

First i had a look at this ModbusMaster library but it did not allow sufficient configuration, such as the RS485 Transmit Enable pin. The SimpleModbusMaster library does allow more configuration. Latest version available here, including a useful Manual.

The library uses 3 simple commands:

  • modbus_construct: to initialise a packet (= an instruction) in SETUP
  • modbus_configure: to set baud rate, timeout, scan rate, TxEnablePin, register variable in SETUP
  • modbus_update: to execute the instruction in LOOP
In my case, i only had 1 instruction: READ_HOLDING_REGISTERS of the Slave ID 2, and the wind speed is stored as an integer (scaled x10) in the register with index 0. So my Master request has to retrieve only 1 register.
I created a simple display using the U8G library which allows for different fonts.
Below the code as running on my IND.I/O with 1286 topboard. 

  wind speed sensor, anemometer on INDUSTRUINO IND.I/O topboard 1286
  RS485 connection with wind sensor, Modbus protocol
  Industruino = Modbus Master
  Wind sensor = Modbus Slave ID: 2
  read holding register 0
  divide by 10 for wind speed in m/s
  more info on the SimpleModbusMaster library: 
#include <SimpleModbusMaster.h>

#include "U8glib.h"
U8GLIB_MINI12864 u8g(21, 20, 19, 22);    // SPI Com: SCK = 21, MOSI = 20, CS = 19, A0 = 22

//////////////////// Port information ///////////////////
#define baud 9600                                                                                  // SENSOR SPEC
#define timeout 1000
#define polling 200 // the scan rate
#define retry_count 10

// used to toggle the receive/transmit pin on the driver
#define TxEnablePin 9                                                                           // INDUSTRUINO RS485

// The total amount of available memory on the master to store data
#define TOTAL_NO_OF_REGISTERS 1                                                // SENSOR SPEC
// sensor sends integer of wind speed * 10 (m/s)

// This is the easiest way to create new packets
// Add as many as you want. TOTAL_NO_OF_PACKETS
// is automatically updated.
  PACKET1,                                          // only need 1 type of operation: read wind sensor
  TOTAL_NO_OF_PACKETS // leave this last entry

// Create an array of Packets to be configured
Packet packets[TOTAL_NO_OF_PACKETS];

// Masters register array
unsigned int regs[TOTAL_NO_OF_REGISTERS];

void setup()
  // Initialize each packet: packet, slave-id, function, start of slave index, number of regs, start of master index
  // READ_HOLDING_REGISTERS (no info in input_registers)                        // SENSOR SPEC
  // slave-id: 2                                                                                               // SENSOR SPEC
  // number of registers to read: only 1 = speed                                       // SENSOR SPEC
  modbus_construct(&packets[PACKET1], 2, READ_HOLDING_REGISTERS, 0, 1, 0);
  // Initialize the Modbus Finite State Machine
  // default SERIAL_8N2 -- wind sensor brochure mentions n,8,1 = 8N1??
  modbus_configure(&Serial1, baud, SERIAL_8N2, timeout, polling, retry_count, TxEnablePin, packets, TOTAL_NO_OF_PACKETS, regs);
                                                                                // Serial1 = INDUSTRUINO RS485
  pinMode(26, OUTPUT);
  analogWrite(26, 100);

void loop()
  modbus_update();                                                             // send 1 simple Master request to Slave, as defined above
  float wind_speed = regs[0] / 10.0;
  Serial.print("wind speed (m/s): ");
  do {
     u8g.print(" m/s ");
     u8g.print("Modbus RTU");
  } while( u8g.nextPage() );