Ok, updated from my original post, correcting a few problems (it’s good to have smart friends).
I had to dig quite a bit to be sure of the right way of constructing the cables, so I might as well document it for others. First, buy good DB-9 connectors. No, not at Radio Shack. Buy aviation/military grade crimp connectors, the kind that use a ratcheting crimp tool and have four points of contact. Yes, they’re shockingly expensive compared to Radio Shack. So is the crimper. Your life depends on it; think of it that way. Remember not to put the pins into the connector on one end before running the cable, lest you end up in the situation where the DB-9 won’t fit through your wire holes. Then you have to disassemble everything and put it back together again.
As far as pinouts, the network cables are straight-through. Pin 1 to Pin 1. Pin 2 to Pin 2. Etc.
Last, but certainly not least, two pairs of the nine total wires need to be twisted. Dynon (and the RS-485 spec) recommends eight to ten twists per foot. Yes, this is annoying and tedious. Yes, your cables will work fine on your workbench (or even in your plane sitting in the hanger) if you don’t twist the wires. But cable runs without the twists are considerably more susceptible to interference than cables with the twists. And 5000 feet in the air isn’t the time to be getting network errors in your avionics. Take the time and twist the wires properly. Make sure you use the strain relief inside of the connector hood. Also, make sure to put heat shrink around each end (inside the hood), and zip tie everything every six or eight inches (it’s easy to see how often you need ties once you get your cables put together).
Twist wires 1 and 6, and 4 and 8. The other wires just go straight through, with no twists. Make four of the wires (1, 4, 6, and 8) just a little bit longer than the others. They’ll be twisted, and if you don’t allow for this, they’ll be shorter than the other five wires. An extra inch or so for every six feet seems to be about right. For the servo cables, you only need to wire pins 1, 4, 6, and 8 (leave the other five holes empty). The servos are powered directly, rather than through the network cables like everything else. Use at least 20 gauge wire for the servo power, rather than the 22 gauge you use for normal network cables.
Finally, buy Dynon’s new hub instead of making/buying Y-cables. It’s much cleaner (and they’ve very cheap). One hub has five ports, which is enough for a moderately complex single-screen system. If you need more ports, buy two hubs (again, they’re cheap).
Unless you need to make cables, just buy them. It really is much easier. Making (and of course, testing) cables is not the most fun aspect of building an airplane.