Modbus programming

I have a Controllino Mega running a code that rotates a stepper one revolution CW, and one revoluction CCW. I need the program to run, just when a signal is received from an HMI that I have connected to the Controllino via ethernet. This is my first time using modbus. I have seen some examples using the modbus library, and I know I have to declare the variables in void setup, but don't know how many variables I need and if they have to be analog or digital. I don't know either how to read those variable within void loop.

    #include <SPI.h>
    #include <Controllino.h>
    #include <Ethernet.h>
    #include "MgsModbus.h"

    MgsModbus Mb;

    const int pul = 46;  // pin 46 to PUL -
    const int dir = 47;  // pin 47 to DIR -


    void setup () 
     {

        byte mac[] = {0x90, 0xA2, 0xDA, 0x0E, 0x94, 0xB5};
        IPAddress ip(10, 7, 4, 17);
        IPAddress gateway(10, 7, 4, 17);
        IPAddress subnet(255, 255, 255, 0);
        Ethernet.begin(mac, ip, gateway, subnet);
        delay(5000); // Time to open the terminal
        Serial.begin(9600);

        pinMode (pul, OUTPUT); 
        pinMode (dir, OUTPUT); 
     }

     void loop()
     {

 Mb.MbsRun();

   digitalWrite(dir, HIGH); 
  /*Turns the motor 400 steps*/
  for (int i = 0; i < 400; i++)
  {
    digitalWrite(pul, HIGH);
    delayMicroseconds(1000);
    digitalWrite(pul, LOW);
    delayMicroseconds(1000);
  }

 delay(1000); 

  digitalWrite(dir, LOW); 
/*Turns the motor 400 steps*/
  for (int i = 0; i < 400; i++)
  {
    digitalWrite(pul, HIGH);
    delayMicroseconds(1000);
    digitalWrite(pul, LOW);
    delayMicroseconds(1000);
  }

delay(1000);

 }