How to communicate M-Duino PLC and ESP32 PLC with Modbus RTU - Part 1

/*
    Copyright (c) 2018 Boot&Work Corp., S.L. All rights reserved
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU Lesser General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
    GNU Lesser General Public License for more details.
    You should have received a copy of the GNU Lesser General Public License
    along with this program. If not, see <http://www.gnu.org/licenses/>.
*/

#include <ModbusRTUMaster.h>
#include <RS485.h>
ModbusRTUMaster master(RS485);
uint32_t lastSentTime = 0UL;
const uint32_t baudrate = 38400UL;
//////////////////////////////////////////////////////////////////////////
void setup() {
    Serial.begin(9600);
    RS485.begin(baudrate, HALFDUPLEX, SERIAL_8E1);
    
    master.begin(baudrate);
}
//////////////////////////////////////////////////////////////////////////
void loop() {
    if (millis() - lastSentTime > 1500) {
        if (random() & 0x01) {
            bool values[5];
            for (int i = 0; i < 5; i++) {
                values[i] = random() & 0x01;
            }
            if (!master.writeMultipleCoils(31, 0, values, 5)) {
                // Failure treatment
                Serial.println("Request failed");
            }
        }
        else {
            if (!master.writeSingleCoil(31, 0, random() & 0x01)) {
                // Failure treatment
                Serial.println("Request failed");
            }
        }
        lastSentTime = millis();
    }
    // Check available responses often
    if (master.isWaitingResponse()) {
        ModbusResponse response = master.available();
        if (response) {
            if (response.hasError()) {
                // Response failure treatment. You can use response.getErrorCode()
                // to get the error code.
                Serial.print("Error ");
                Serial.println(response.getErrorCode());
            } else {
                // Get the discrete inputs values from the response
                if (response.hasError()) {
                    // Response failure treatment. You can use response.getErrorCode()
                    // to get the error code.
                    Serial.print("Error ");    
                    Serial.println(response.getErrorCode());
                } else {
                    Serial.println("Done");
                }
            }
        }
    }
}

/* 
    Copyright (c) 2018 Boot&Work Corp., S.L. All rights reserved
    This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU Lesser General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
    GNU Lesser General Public License for more details.
    You should have received a copy of the GNU Lesser General Public License
    along with this program. If not, see <http://www.gnu.org/licenses/>.
*/

#include <RS485.h>
const int slave_address = 31;
int relayOutputsPins[] = {
    R0_4, R0_5, R0_6, R0_7, R0_8,
};
//////////////////////////////////////////////////////////////////////////
void setup() {
    Serial.begin(9600);
    RS485.begin(38400);
}
//////////////////////////////////////////////////////////////////////////
void doWriteMultipleCoils(uint8_t total_pins, uint8_t values) {
    // turn on relays
    for (int i = 0; i < total_pins; i++) {
        (values & (1<<i)) ? digitalWrite(relayOutputsPins[i], HIGH) : digitalWrite(relayOutputsPins[i], LOW);
    }
}
//////////////////////////////////////////////////////////////////////////
void doWriteSingleCoil(uint8_t value) {
    // turn on digital output
    if (value == 255) {
        digitalWrite(Q0_0, HIGH);
        digitalWrite(Q0_1, LOW);
    }
    else {
        digitalWrite(Q0_0, LOW);
        digitalWrite(Q0_1, HIGH);
    }
}
//////////////////////////////////////////////////////////////////////////
void readModbus(uint8_t aux[]) {
    int b = 0;
    int max_bits;
    while (1) {
        if (RS485.available()) {
            byte rx = RS485.read();
            aux[b] = rx;
            if (b == 1) max_bits = (rx == 0x05) ? 8 : 10;
            if (++b == max_bits) break;
        }
    }
}
//////////////////////////////////////////////////////////////////////////
void loop() {
    uint8_t aux[10];
    readModbus(aux);
    if (aux[0] == slave_address) {
        if (aux[1] == 0x05) doWriteSingleCoil(aux[4]);
        else if (aux[1] == 0x0F) doWriteMultipleCoils(aux[5], aux[7]);
    }
    else Serial.println("Bad slave address");
}