There is a nice Java piece of software that we can use to create multiple slaves.
It can be downloaded in:
I've attached it as well.
It doesn't work for RTU mode unless you install supporting library. For this purpose we need to download RXTX library. You can find this lib at: http://www.jcontrol.org/download/rxtx_en.html.
I'v attached it as well.
Next thing is to put required lib and bin files. Do it as said in the README within rxtx-2.1-7bins-r2.zip.
For windows it's essentially two steps:
RXTXcomm.jar goes in Java\jreXX\lib\ext (under java)
rxtxSerial.dll goes in Java\jreXX\bin
when XX is the version of Java installed on your computer finding in the Program Files folder.
There is a good help file for that you may use it; I've attached it.