Modbus Slave example

How to implement it

1. Open Arduino 1.0.5 or higher and create a new sketch.
2. Create a data array that shall be linked to the Modbus network. This data array should contain any data to be exchanged (for example, digital inputs, analog inputs, analog outputs or pulses to an stepper).
3. Add "Modbus.ino" file with Sketch / add file. Until I do not creat the library, there is no other choice. :-(
4. Push Ctrl+Shft+N to add another TAB to the project.
5. Type this:

Modbus slave(1,0,0);

void setup() {
  slave.begin( 19200 );
}

void loop() {
  slave.poll( au16data, 16 );
}

This shall create an object called "slave" at node number 1 and shall start its serial at 19200 baud. Arduino defaults are 8N1. At the end of the loop() section, there must be a poll request and the data array linked to the poll.

How to debug it

Before connecting the Arduino to the real world, a good choice is to debug its application with a simulator. There are good simulators for Modbus like:

• QModbus
• QModMaster
• Multiway

Though Modbus library does most of the Serial work, it's a serial port and serial ports feature several settings. These settings need to be looked carefully before any attempt.

For those not proficient with these things, I strongly recommend to read something like this.

The serial port should have the next settings:

• baudrate (common baudrates are 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200);
• bits per byte (7 or 8);
• bits at the end of the byte (1, 1.5 or 2);
• parity (none, even or odd).

Apart from these, there are other questions such as: Which port is being used for this USB-to-serial adapter? Depending on the PC OS,

With QModbus, my example looks like this: