It's standard null-modem. For a 9-pin to 9-pin, thats 2-3, 3-2, 5-5. For a 9-to-25, it's 2-2, 3-3, 5-7.
Usually I just use ribbon cable (since I know its pinout) and a Radio Shack null modem adapter, with the appropriate gender & 9-25 converters.
This cable kit will connect to 90%+ of all PLCs & HMIs that allow RS-232 communication. If null modem doesn't work, straight through usually does. I've added a cheap breakout box to the kit, for those times where I need a custom cable, but have yet to use it.