What is the fastest and most efficient way to add multiple Modbus TCP device connections in the Gateway for Ignition using a CSV file in the latest version Ignition 8.3

Have you tried my Spreadsheet Import Tool yet? Found in the Exchange, and also here:

I haven't thoroughly tested in 8.3, but I'd love to have feedback on that.

(It can create both the devices and the corresponding UDT instances for you.)