I am trying to develop Modbus RTU function 1 , i.e read coil register.
Below peice of code written for Modbus RTu with function code 3 .i.e holding register.
.
function 1- uses bit values
function 3 uses byte value to store.

Now i am having 50 digital io status to be read. the status of digital io are 0 or 1.

my request goes like this
Serial_Request()
Is function i am calling in interrupt. I am checking first 8 byte of messages 01 01 00 00 00 31 fd de
rxbuf[0]= 01 //device Id always
rxbuf[1]= 01 //Functional code
rxbuf[2]= 00 //higher byte of start address
rxbuf[3]= 00 //Lower byte of start address
rxbuf[4]= 00 //higher byte of end address
rxbuf[5]= 31 //Lower byte of end address
rxbuf[6]= FD //CRC1
rxbuf[7]= DE //CRC2
First i will check CRC1 & CRC2 is Valid or not if valid
Get rxbuf[0]= 01 match with device ID then
rxbuf[1]= 01 match with function code
Then calculate length assign start address & length
Store the values & check CRC again.

1) how can i store bit value of digital io in byte and call for legth
2)how can i pack data & send for CRC check
3)how can i calculate length of bits

i need simple modified code below.Here receiving data no issue. CRC check no issue. only need modofiation in length,store bit value & pack the data

length= first byte =0000in hex
sec byte =0031 in hex
length = 31/8 = 6 in hex

6 byte of data where first 8 bit packed in first byte i.e 8 digital io status in first byte and 2nd 8 bit data in sec byte & so on...

Then pack data & send for CRC check








CRC check function
Code:
unsigned int crc_fn(unsigned char *dpacket,unsigned int len) // CRC Function(Error calcualtion)
{
     unsigned int crc = 0xffff,poly = 0xa001;
     unsigned int i=0; 
                
     for(i=0;i>= 1;
                   crc ^= poly;
              }
              else
                     crc >>= 1;
        }
     }
     return (crc); 
}
below function designed for function3
Code:
void Serial_Request()
{
     unsigned int address,crc1,crc2;
     unsigned char length,i,j=0;
    
     
    // Serial_1_Send_byte(rxbuf[0]);
    crc2=crc_fn(&rxbuf[0],6); //crc function for received protocol from request 
     __delay_ms(10); // Changed on 20.01.2017
    if((rxbuf[6]==(unsigned char)(crc2))&&(rxbuf[7]==((unsigned char)(crc2>>8))))
    {
                
           if(rxbuf[0]==device_ID)
           {
            
            //Serial_1_Send_byte(rxbuf[0]);
            
                if(rxbuf[1]==READ_REG)
                {
                     address=(((unsigned int)(rxbuf[2]<<8))+((unsigned int)(rxbuf[3]))); 
                     if(rxbuf[5]>=1)
                     {
                           // changes i made  for function1, if i used i get an error.
                           length=(rxbuf[5]/8);// function code 1
                           address=0X01;// function code 1
                          // original function 3 code, if i keep below code it works fine for function 3
                          // length=(rxbuf[5]*2);// function 3
                          // address=(address*2);// function 3

                           ser_data[0]=device_ID;
                           ser_data[1]=rxbuf[1];
                           ser_data[2]=length;
                           
                           crc_data[0]=device_ID;
                           crc_data[1]=rxbuf[1];
                           crc_data[2]=length;
                           j=3;
                           for(i=address;i<((address+length));i++)
                          {
                                crc_data[j++]=ser_data[i+3];
                           }
                           crc1 =crc_fn(&crc_data[0],(length+3)); //crc function for response protocol from the device

                           Serial_1_Send_byte(ser_data[0]);
                           Serial_1_Send_byte(rxbuf[1]);
                           Serial_1_Send_byte(ser_data[2]);
                    
                           for(i=address;i<((address+length));i++)
                           {
                                Serial_1_Send_byte(ser_data[i+3]);
                        
                           }
                           Serial_1_Send_byte((unsigned char)crc1);
                           Serial_1_Send_byte((unsigned char)(crc1>>8));
                  
                     } 
                } 
            __delay_ms(5); 
       }
           
     }
    index=0;
     rec_flag = 0;
   
     
}