Hideki,
Could be some progress here.

On the trigger Rigol gives the following options:
Edge, Pulse, Runt, Slope, Video, Pattern, Setup/Hold, RS232, I2C, and SPI
I think RS232 does inform the scope to look for the +3/+15 and -3/-15 volts, but my sense is that the trigger is not that sensitive and it can determine a HI from a LOW pretty easily. (So maybe I can leave it on RS232 and just adjust the polarity?)
On the decoder the Rigol gives the following options:
Parallel, RS232, I2C, and SPI.
I'm still playing with it but any suggestions on the combination of trigger and decoder options are welcome.
Thanks! EF
PS, before we go too far down any wrong paths (and one of the reasons I thought we were getting close with the analog pins) I just wanted to restate that the eventual goal is to be able to at a minimum generate bits that make strings of bytes (render words on the display, etc.) - not because I like seeing words on the display but to gain control and understand the patterns including the control characters. Ideally I'd like to be able to adjust single bits so as to adjust bytes from within into new bytes (rather than just assign values like 65). What I'd really like to be able to do is to adjust HIs and LOWs to toggle start and stop bits as well as data bits - but maybe we can save some of this for "Phase 2"

Thanks again