That document looks similar style to those that I have from Texecom - I have a private repo on github at the moment with an HG interface module, HG interface test application, and then a texecom Interface test applicaiton and finally a texecom interface..
I would suggest these could be used as some sort of starting point but I cant share everything as I am under NDA
I could share the code I am using to process the raw messages, and I support using either Ethernet or serial.
I can certainly try and assist you, however I'm no dev!! and I'm learning slowly..
First things first I would suggest getting connected via a serial port to a desktop with a copy of visual studio and start writing an interface to communicate.. I wouldn't however advise developing in homegenie to start with due to the lack of debugging available.
I would then start by getting a checksum routine sorted as a starting point and prove that you can send a command correctly..
IE create something like (not real c#!)
bool SendCommand(string message){
message = CreateMessageWithChecksum(message)
resp = Serial.Send(message);
if resp == ACK
return true;
if resp == NAK
return false;
}
Eitherway you have everything you need to get started