I currently work at ISV, which mainly works with the other of the three major sales systems.
First of all, get the Micros environment in a virtual machine.
Further, most of these back-office systems work either in Windows 2000 or in Windows XP (depending on the age of the system), so be very careful with the version of .Net that you choose.
Now, with regard to architecture, start the service in the back office, which regularly contacts the centralized web service (the website the client is ordering for) after the order is available, after which you can insert it into the <insert pos type here>.
You will also find in most cases, especially with Micros, the largest selling point on the market for restaurants, they will not be very useful with documentation and will help you integrate into your software.
In addition, I think My Micros (their enterprise application) already supports this type of function.
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