The IBM 3270s did not ground the signals in the terminal, just for that reason. The coax was only tied to frame ground in the controller. The terminal end had the front end of the signal receiver floating off of the EGC.
When we did have ground shift problems we found bonding was a lot more effective than IG. It was exactly the opposite methodology. We ran a fat "drain" wire between machines creating that dreaded ground loop. It was always black, never green and we never used the "G" word, so as to avoid article 250 questions. It simply bonded the frame ground of the machines together. It was how we fixed the lightning problems in a lot of Florida installations.
We never saw any signal related problems and our line driver/receiver cards stopped expelling the magic smoke every afternoon.
