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I'm trying to read values stored in a bool* pointer from a modbus coil call.

I'm using modbus-esp8266 library. it's all ok with Input Registers and Holding Registers but i cannot read the result of a coil read.

#include <ModbusRTU.h>
#include <SoftwareSerial.h>

SoftwareSerial S(D2, D1);
ModbusRTU mb;

bool cb(Modbus::ResultCode event, uint16_t transactionId, void* data) { // Callback to monitor errors
  if (event != Modbus::EX_SUCCESS) {
    Serial.print("Error result: 0x");
    Serial.print(event, HEX);
  }
  return true;
}

void setup() {
  Serial.begin(115200);
  S.begin(9600, SWSERIAL_8N1);
  mb.begin(&S);
  mb.master();
}

void loop() {
bool* coils;
  if (!mb.slave()) {    // Check if no transaction in progress
    mb.readCoil(1, 1, coils, 1, cb); // Send Read Hreg from Modbus Server
    while(mb.slave()) { // Check if transaction is active
      mb.task();
      delay(10);
    }
    Serial.println(*coils);

  }
  delay(1000);
}

I get this kind of error when trying to print *coils

Exception 28: LoadProhibited: A load referenced a page mapped with an attribute that does not permit loads

How to get the boolean result value from it?

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3
  • 1
    please add the full error message text Commented Mar 4, 2022 at 17:03
  • 1
    The coils pointer points to some random place as you didn't bother to define any value for it (depends on what was in the memory where that pointer gets space). Commented Mar 4, 2022 at 19:57
  • bool coils; and mb.readCoil(1, 1, &coils, 1, cb);. this way the function can the value of the variable coil Commented Mar 5, 2022 at 16:18

1 Answer 1

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0

When programming an Arduino you are actually writing in the language C. As such you can search the internet for C programming answers of which there is likely 50 years of answers.

You have defined "coils" as a pointer. This is done by preceding the name with an asterisk where it is defined. In this line of code:

bool* coils;

This means you are using coils to store the memory location of where the actual value is to be stored. However, in the case of the code in the posted question, the memory location of where the actual value is to be stored is never defined.

This is like going on a bad clue hut. You receive instructions to obtain the next clue at a specific location. But when you get there you find no further instructions.

Beginning programmers often find pointers confusing. But later find pointers very useful tools that simplify many otherwise complex programs.

Consider dropping the asterisk in front of "coils" on the two lines of code where is exists and try running the program again.

Added later:

Guessing, it may be that the call to readCoil function assumed to be defined in ModbusRTU.h requires a pointer to be passed to in as the 3rd formal variable parameter. This is more advanced C programming. If true, then keep your original code but add a line of code setting the value of "coils" to the address of a new bool variable. Perhaps something along these lines:

bool* coils;
bool coils_value;
coils = &coils_value;
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