Hi Josh,
Thanks for looking into this.
All the wiring seems in order. There is negligible resistance in our wires/connectors. The BNC-screw terminal adaptors I use are these:
https://www.parts-express.com/bnc-male-to-screw-terminal-connector--090-106?utm_source=google&utm_medium=cpc&utm_campaign=pla
I tried something that may be informative. With the oscilloscope reading the TTL high voltage between the NI box and the pulsepal it reads only 1.8V, but 5V when I remove the PulsePal. Adding a 10Kohm resistor in series with the PulsePal's trigger-in recovers the 5V TTL, but I still don't see the PulsePal trigger.
I suspect all this may relate to the somewhat more-complicated-than-anticipated way these little NI boards can produce digital signals. Figure 15 here:
http://www.ni.com/pdf/manuals/371303n.pdf
suggests that I need a pull-up resistor to get the full 8.5mA if my digital output is push-pull. I'm a bit novice here and don't completely understand what digital out mode I'm using, or how to change it.
I'm afraid I'm going to end up swapping out this little USB NI board for a big proper one, but, for several reasons, I'd love to get things working with these little 6009 boards.
Matt