So I attempted to build the parallel link as shown on ticalc.org. I soldered everything and tested it and it appeared to work. When I ran linktest, it gave this:
send 00 - read 10 - should be 00 send 01 - read 30 - should be 20 send 02 - read 30 - should be 10 send 03 - read 30 - should be 30
I double-checked everything and found that some wires were touching. After insulating everything (which involved re-soldering the ground) I tried again and linktest gave 30,30,30,30 - which is the same as when nothing is plugged in. I also tried the "debug" method with a voltmeter and only detected low voltage on the calculator end.
The only thing that seems like it might be wrong is that the diodes appear to have a resistance of 3K ohms (which seems way too high). They aren't backwards unless the schematic is wrong or they were defective. Even with 4K resistance from input to output pins, there should be some change in the voltage, right?
At this point I'm considering just buying one if no one can help me - my "$5 link cable" cost me $13 so far, but the TI one is $20 dollars! Does anyone sell the homemade kind anymore?