Disclaimer: You are solely responsible for anything you may muck up in the PLC. By going online with the PLC, you are assuming full responsibility for anything that happens to your PLC or associated equipment or persons. Do not come back into this forum stating that I/me/we/them/they broke your PLC or caused any other type of calamity. Going into this without any knowledge of PLCs is a tall task, frankly scary if your PLCs are used for mission critical applications.
There really is no "best method", just the one that you are comfortable with and will work in your situation.
Before you go much further down this road, check these:
Do you have the Mitsubishi PLC software, I believe it is GxWorks?
Do you have copies of the PLC programs on your PC?
Can you get training on Mitsubishi PLCs, or is there someone you know who can help?
If you answered no to either of those questions, you need to solve that first.
If yes to all, then read below.
Not very familiar with Mitsubishi PLCs, but there is a node you can download to node-red called
This claims to be able to read/write data over ethernet to/from your PLC.
That's great if your PLCs have ethernet, or rather that is one box checked. The next thing is if there are variables in the PLC that are granted access to be read/wrote from ethernet. Generally, PLCs don't have all of their data set to be read/wrote from external sources by default. The user typically has to go in and tell the PLC which variables it should grant external access. Again, I am not familiar with Mitsubishi, so you will need to check for yourself.
Its possible that your PLCs have ModbusTCP support, there is a node for that as well. Again, it will require you to setup which variables to grant access.