What OS do you have installed on your G5?
Which version of catalyst are you using?
What is the RS232 device that you are using?

With an old G5 you probably cant use the most recent version of catalyst...

There used to be issues with the startup order - particularly with USB drivers if catalyst is in start-up items. It is possible that catalyst is starting before the USB drivers are loaded for RS232 device... Same used to happen with the USB dongle, it wasnt recognised if catalyst was in startup items in some cases.

However - this depends on what OS you are using - it was never the same on each Mac...

Try writing a simple apple script that delays catalyst startup for 5 sec

This is one i wrote for G5 with Catalyst v3.3g21 (long time ago)

-- Catalyst delay loader - SNP Productions Ltd
-- Silent version 3 seconds delay

on run
activate

delay 3

-- hide applications
tell application "System Events"
set visible of every application process to false
end tell

-- launch applications
tell application "Catalyst 3.3_g21_Pro.app"
activate
end tell

end run
Also see this post for script details:
samsc.com/showthread.php?t=1033&highlight=script

samsc.com/showthread.php?t=856


If not USB - then maybe a script to change the settings might work...

Simon