I forgot about the difference that Chris Rake mentioned - the cRIO setup for FRC (and, I believe, all of the new "integrated" cRIO setups) does NOT include provisions for a second (redundant) power supply. We learned about that diff during development but decided to use the "complete" pinout that all of the "non-integrated" cRIOs utilize (which is, for our purposes, also compatible with the FRC cRIO).
Unfortunately, this means that we can't exploit redundant power supplies in FRC. Don't be too sad, though - designing fault tolerant systems is what I do for a living and they're generally not cheap (and they need to be completely and fully architected from the ground up to be truly fault tolerant). You really have to NEED such performance to justify the development effort and cost (eg military, medical and aerospace apps).
Datasheet for the cRIO-FRC
http://decibel.ni.com/content/servle...structions.pdf
Note the "NC" (Not Connected) pin in the middle of the 4-pin power connector on page 1. That "NC" pin is used as the redundant power supply input on all "non-integrated" cRIO systems (ie the versions that include separate controller and backplane modules).
It's because of that "middle" NC pin that the FIRST Power Diagram stipulates that the outer two pins be used to power your cRIO. Since we wanted to use the same connector on the PD and copper traces on PCBs don't really cost anything, we connected the "NC" pin on the PD to the same 24V output from the PD (hence the "+ - + -" arrangement on the PD instead of "+ - NC -").
Regarding that backup battery - eliminating the need for a backup battery was a highly desirable feature for the PD. The use of switching power supplies with wide input voltage range and solid transient response obviates the need for that extra storage device. I don't think that anyone's going to miss that battery...
Sorry for the long-winded response. I do like to "talk".
Russ