"16 bit crc is required for modbus."

As Dan has pointed out, that is almost meaningless!
All you have told us is the size of the CRC - the important thing is what actually goes into those 16 bits!
For that, you need the Generator Polynomial. Surely, the Modbus specs must tell you what that is?

Read the Dallas App Note 27: Understanding and Using Cyclic Redundancy Checks with Dallas Semiconductor iButton Products
http://pdfserv.maxim-ic.com/arpdf/AppNotes/app27.pdf
This tells you all about CRCs, and how to do CRCs in hardware and in software ('C' and assembler) by both calculation and lookup table.
All the principles will apply to your application - the only difference may be the Generator Polynomial and, once you've read & understood the App Note, you'll know how to cope with that!