We use a UVW1200 deck with Decklink Extreme Card on our G5.
Instead of using a RS422 to RS- 232 adapter (which never worked) we did what others recommended:
1. bought a Keyspan USB TWin Serial Adapter (got ours from Aamazon, about $60) to hook into the G5
2. Bought a serial to RS-232 cable from Digitalpipeline.com, which runs from the Keyspan adapter to the UVW1200 deck. Make sure you get the right cable, it has a round serial connector on one end, and the RS-232 connector on the other. DigitalPipeline support can help on this.
3. Then in FCP set the capture control settings to match. Ours works great
John