Many embedded controllers have a micro-GPIB connector instead of a full-sized connector to save space. As mentioned in the GPIB section of the
PXIe-8840 datasheet, the part number for the adapter that goes to a full-sized GPIB connector is 183285-0R2. The part number for the microGPIB to GPIB cable is 183285-01 (for the 1m cable) or 183285-02 (for the 2m cable).
Additional Information
The easiest way to find these parts is by looking at the
"Accessories" of the PXIe-8840 and check under "Cables and Adapters"