Help modbus_write_register - Not working - Raspberry pi

[Sathya Narayanan]

HI All,

Can someone please help me using the modbus_write_register function. I am writing a c program that would be executed in my Raspberry pi  running Raspbian Jessie.

I am trying to update the value of the register of a pic microcontroller. I am able to do this with Medford.exe. So I suppose the pic microcontroller is in working order.

Note: I only have 1 input register and 1 holding register in PIC microcontroller the slave ID of my PIC Microcontroller is 0x02

 int16 hold_regs[] = {0x1100};  

 int16 input_regs[] = {0x1100};

Please help me with the code below. where I am trying to update the input register of the pic microcontroller using Raspberry pi. I am using a USB to RS485 converter and I have verified that the device is in /dev/ttyUSB0.  

#include <stdio.h>

#include <modbus.h>

#define MB_BITRATE 9600

#define MB_DATABITS 8

#define MB_STOPBITS 1

#define MB_PARITY 'N'

#define MB_SLAVE_ADDRESS 2

#define MB_REG_BASE 0

#define MB_REG_COUNT 1

int main ()

{

 modbus_t *ctx;

 uint16_t tab_reg[MB_REG_BASE+MB_REG_COUNT];

 int n, i;

 int errno = 0;

  ctx = modbus_new_rtu("/dev/ttyUSB0",

                MB_BITRATE,

                    MB_PARITY,

                     MB_DATABITS,

                   MB_STOPBITS);

 if (ctx == NULL) {

   fprintf(stderr,

        "Unable to create the libmodbus context on serial port %s\n",

          "/dev/ttyUSB0");

   return -1;

 }

 printf("connection created \n");

  if ( modbus_set_slave(ctx, MB_SLAVE_ADDRESS) )

               {

                fprintf(stderr, "Failed to set modbus slave address\n");

               errno = -1;

            goto fail;

           }  

    printf("slave set \n");

 

 if ( modbus_connect(ctx))

   {

     fprintf(stderr,

       "Unable to connect to modbus server");

     errno = -1;

     goto fail;

   }

     printf("connted to slave \n");

 printf("Starting write \n");

 n = modbus_write_register(ctx, 0, 255); //I suppose 2nd parameter is the memory address.

 printf("Value of N is %d \n", n);

 if ( n <= 0 )

   {

     fprintf(stderr, "Unable to read modbus registers\n");

     errno = -1;

     goto fail;

   }

 

 for ( i=MB_REG_BASE; i<MB_REG_BASE+n; i++ )

   {

     printf ( "[%d]: %d\n", i, tab_reg[i]);

   }

 fail:

 modbus_close(ctx);

 modbus_free(ctx);

  return errno;

}

[Sathya Narayanan]

Nobody? wow...

[Stéphane Raimbault]

 //I suppose 2nd parameter is the memory address.

Don't suppose anything, just read the documentation:

http://libmodbus.org/docs/v3.1.3/modbus_write_register.html

so it's the Modbus address of the remote host.

Your comment says: Unable to read whereas you try to write...

BTW it's useless to loop over tab_reg after a write

 

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