[TUHS] PDP 11/34a Serial Communications Problem

Hello all, This is my first time emailing the list, so please let me know if this doesn't belong here of if I'm breaking any rules. A few months ago, I rescued a PDP 11/34a with 2 RL01 drives from the scrap heap. The unit appears to work fine based on my limited front-panel testing. I haven't gotten the drives running yet since someone cut the power cords when the cabinet was being removed. There is a DL11-W serial line unit/realtime clock (M7856) installed in the 11/34 that I want to use for serial input/output. I have configured the card for 9600 baud, 8N1. Using some jumper wires, I carefully connected the card to a serial cable and a computer running a terminal and I was able to send some characters back and forth successfully. For a more permanent solution, I designed a simple adapter board that connects to the BERG 40 connector on the DL11-W and converts it to a DB9 serial port (In restrospect, this product was already available at https://oshpark.com/shared_projects/uTMf3v08 but I didn't know about that at the time). I also ordered a 40-pin (non-IDE) ribbon cable to connect the DL11-W to my adapter. When I connected everything, the 11/34 would start but no lights would appear on the front panel. I tried disconnecting the adapter but leaving the ribbon cable plugged into the BERG connector, but the problem persisted. When I removed the ribbon cable entirely, the unit powered on with no problems. Since this is a straight-through ribbon cable, I don't see what could be causing this problem. I have checked the continuity of each wire in the cable, and there doesn't appear to be a problem. I'd appreciate any advice that anyone has to offer. Yours, Mark D. Hare markhare(a)buffalo.edu University at Buffalo B. S. Civil Engineering '16 M. S. Structural/Earthquake Engineering Student