Modbus Communication Software – Any Modbus enabled ESD product has been tested using the following utility software. There is 10 minute trail version of this software at the following link.
You can try it out for yourself.

http://www.modbustools.com/download.html

Real Term is also a good software which can used for analyzing and finding problem with Communication on Serial Port. Its an Open Source software and can be downloaded at the following link.

http://sourceforge.net/projects/realterm