Well... if you look at the Cygnal Development Boards, the Cygnal Devices are interfaced directly to a Scipex 3223ECY chip. However, this Scipex part may be a 3V RS232 tranceiver... I'm not sure.

I would suggest to experiment. Use a Pull-up resistor that is a low enough value that it will pull enough current to pull up the voltage quickly, but not so low that the Pull-down transistor in the Cygnal part cannot make a fast fall time. The Cygnal part I/O is spec'd to 8.5mA on the low side, so you should be able to easily calculte an adequate pull-up value.

For 56kbps, I wouldn't imagine that you need very fast I/O's?! Someone else correct me if I'm wrong (analog is more my forte... and I've not done to much with RX/TX on these devices).

-Rob