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Writing multiple registers function in modbus

 
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paddy



Joined: 04 Nov 2012
Posts: 15

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Writing multiple registers function in modbus
PostPosted: Fri Jan 11, 2013 4:34 am     Reply with quote
Hello All,

I am using modbus_slave.c example file in which i am using FUNC_WRITE_MULTIPLE_REGISTERS.

It works fine when i am writing data from starting address 0001. But when i change it with some another address, it will not work. I am not able to write data on other address except 0001.

Please have a look and suggest what i need to do.

Thanks

Code:


case FUNC_WRITE_MULTIPLE_REGISTERS:
                  if(modbus_rx.data[0] || modbus_rx.data[2] ||
                     modbus_rx.data[1] >= 122 || modbus_rx.data[3]+modbus_rx.data[1] > 122)
                     modbus_exception_rsp(MODBUS_ADDRESS,modbus_rx.func,ILLEGAL_DATA_ADDRESS);
                  else
                  {
                     int i,j;
                     int8 k;
                     
                     k=modbus_rx.data[1];
   
                     //for(i=0,j=5; i < modbus_rx.data[4]/2; ++i,j+=2)
                     for(i=k,j=5; i < modbus_rx.data[4]/2; ++i,j+=2)
                        hold_regs[i] = make16(modbus_rx.data[j+1],modbus_rx.data[j]);
   
                     modbus_write_multiple_registers_rsp(MODBUS_ADDRESS,
                                    make16(modbus_rx.data[0],modbus_rx.data[1]),
                                    make16(modbus_rx.data[2],modbus_rx.data[3]));
                 
                     event_count++;
                  }
                  break;
RF_Developer



Joined: 07 Feb 2011
Posts: 839

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PostPosted: Fri Jan 11, 2013 7:03 am     Reply with quote
There's a problem in your register writing loop:

Code:

                     k=modbus_rx.data[1];
   
                     //for(i=0,j=5; i < modbus_rx.data[4]/2; ++i,j+=2)
                     for(i=k,j=5; i < modbus_rx.data[4]/2; ++i,j+=2)
                        hold_regs[i] = make16(modbus_rx.data[j+1],modbus_rx.data[j]);


As modbus_rx.data[4] is the count of data bytes in the message, i.e. twice the number of registers, you can't use it in that way as the end condition of the loop. Instead do:

Code:

                     k=modbus_rx.data[1];
   
                     for(i=0,j=5; i < modbus_rx.data[4]/2; ++i,j+=2)

                        hold_regs[k + i] = make16(modbus_rx.data[j+1],modbus_rx.data[j]);


This is limited to 128 registers and they mus all fall inside the 0-255 register address range.

In my implementation of this function I don't have all registers as "real" registers in the hold_regs array. For read, I have sixteen scratch, i.e. temporary, registers and send them to an virtualisation/abstraction routine that collects the data to the correct parts of my code. For write I simply send each register value in turn to my virtualisation routine.

Code:

                  // Data is big endian.
                  for(i=0,j=5; i < modbus_rx.data[4]/2; ++i,j+=2)
                     Write_Register(k + i, make16(modbus_rx.data[j],modbus_rx.data[j+1]));


This limits the number of registers that can be read in one message to the number of scratch registers.

RF Developer
paddy



Joined: 04 Nov 2012
Posts: 15

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PostPosted: Sat Jan 12, 2013 4:34 am     Reply with quote
Yes!!! It worked!!!!

Thanks for your kind support.
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