I refrained from answering your previous question because really where to start. The cost of the vendor software is probably going to be insignificant compared to the time you will need to spend here.
I can give you a generic approach that I would employ in this situation.
1. Obtain a technical manual from the vendor - you will need the command set for the device. There may be an industry standard command set for these devices, if so that is what you need. If you don't have this information do not proceed.
2. Download a serial console such as PuTTY:
http://en.wikipedia.org/wiki/PuTTY[
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3. Hook up your device to your computer using the proper cable. If a laptop you will need a USB serial adaptor:
http://www.instructables.com/id/The-serial-port---software-setup/[
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4. Become comfortable with sending and receiving data from the device using PuTTY. This is where you learn the instructions for your device and confirm that everything is working.
5. Read this article or others like it to learn how to use your own software in place of PuTTY.
http://csharp.simpleserial.com/[
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This link should get you started but there is a wealth of material on the internet on this subject particularly right here on CP. I am assuming you will use C# as you have not stated you have an alternative in mind.
Return here as and when necessary with particular questions.