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While you will have to load software, there still isn't communication registering on the pendant screen.
Do the red lights on the computer board blink at any time? With the software lost, the controller should do a self-test with the results displayed on the pendant.
I don't see in the thread that you have tried another known good pendant or tried this pendant on another working controller to verify it is good.
If that doesn't help, the pendant communication comes from the DSQC373 Robot Computer Bd, up the backplane, DB15-to-DB15 cable to the Panel Unit, out the Panel Unit X7 to XS20 on the front of the controller and out XS20 to the teach pendant. Make sure connectors on the backplane and panel unit are all properly seated.
Since it looks like the software will have to be reloaded, try reseating the 3 computer boards. While you have them pulled out, look carefully and make sure none of the pins in the connectors are bent.
If that doesn't work, disconnect the CAN connections X9 on Panel Unit, and X10 & X16 on the backplane in case one of them is loading the CAN bus.
After that, I would try another Power Supply, Panel Unit and then DSQC373 Robot CPU, Main CPU, Extended Memory board and finally the backplane.
One last question; is that a 6400R in the picture?
Yes this 6400R