Awesome work. Basically same way an AlarmDecoder(*) work. I’ve integrated with a Honeywell Vista20p through the rs232 directly connected to the Pi, so I didnt quite get why you needed the Moxa Nport 5110? You can also easily replace that with a tiny $2 ESP01 (WiFi) and you’ll have a serial-wifi interface, there’s also a lot of other ethernet (wired) alternatives (arduino, pi, c.h.i.p., or simple diy AVRs etc)
Nice job. Very detailed. I would suggest you extend all these how-to into an actual binding 
and host your code in github so people can contribute more efficiently.
- I went with AlarmDecoder anyway since it’s a more mature product than my DIY *