Need Information about Implementing of Modbus protocol in fpga ( mostly spartan 6)

There are basically two ways to do Modbus. One is Modbus RTU, using a UART with RS-485. On the hardware side you need a UART with a transmit enable control line. Everything else is in software.

The other is Modbus TCP/IP over Ethernet. On the hardware side, you need Ethernet. Everything else is in software.

So "Modbus on an FPGA" means having a processor (soft or hard) and software on the FPGA or the board, and a UART or Ethernet. Trying to implement Modbus directly in FPGA hardware without a processor is insanity.

Thanks David for information. I just wanted to know that are you suggesting to use external processor for implementation.

and also i would like to know while writing vhdl program what steps should i follow.

I am suggesting you use a processor for handling the protocol. It is entirely up to you if this is external to the FPGA, a hard core in the FPGA, or a soft core in the FPGA.

If you have to ask this, I would recommend that you drop the idea of an FPGA altogether. A $2 microcontroller will let you build a Modbus RTU slave on RS-485 - it will have a UART and all the processing power you need. If you are thinking of Modbus over Ethernet, your microcontroller will be around $5.