BC11A UNIBUS cable
The BC11A UNIBUS cable is used to connect together UNIBUS elements (such as CPUs, system units, and free-standing devices such as the RP11 disk controller). It carries all 56 UNIBUS signals, and 64 ground lines (alternating with the signals, to prevent crosstalk).
Note that correct DEC nomenclature does not include a hyphen before the 'A'; an added hyphen, followed by a two-digit number, is used to specify the length. It comes in standard pre-manufactured lengths, from 2 feet up to 50 feet.
It consists of a pair of 60-conductor Flexprint cables (not flat cables - they contain flat traces, not wires) separated by a foam layer (probably to prevent crosstalk, or maintain a constant impedance). On each end is a short dual PCB (M919 and M929), which plug into the 'UNIBUS in' or 'UNIBUS out' slot of the element's backplane.