I've read that recent Roomba 6XX from iRobot got a serial connector, but I removed the cover on my Roomba 681, and I haven't found any. From what I've found, 681 hasn't the same cover than other 6XX robots. Does this model have an accesible serial port?
The 68x series has a Mini-DIN connector on the top surface, toward the robot's right side, but it's now under the translucent diffuser surrounding the buttons. It is held in by snaps. Lift the handle, stick your fingers under, and pull the plastic piece up and out.
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I have the Roomba 671 from 2018 Prime Day, I had to pop the plastic top off with a screwdriver with some force. The ROI aerial port is on the front left of my model.