OK, I found it:
Origin2000 and Onyx2 Deskside and Rackmount Installation Instructions
Document Number 108-0155-002
12/96
pg. 6-36
"Caution: You cannot install CrayLink interconnect cabling on a
standalone deskside system because of power grounding
requirements. The power differential between two interconnected
modules should not exceed 500 millivolts; otherwise severe damage
can result to boards and other components inside the chassis. The
power distribution unit (PDU) inside the rack provides a common
ground source for the modules. In addition, the groundstraps are
installed on multirack configurations to help provide common
grounding across the racks as described in Section 6.16"
That seems about as blunt as you can get.
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