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FlexTrack IRT501 connected to IRB1520

Hi All.
I have a new IRB1520, with the new controllers.
I also have an old FlexTrack IRT501 with the old TLseries motors.
With the Flextrack I have an old slave computer (with LAN port etc)
How can I connect the FlexTrack to the new controller, such that the controller can control the FlexTrack.