PS2 controller emulation for a 3D controller project

[realtea]

I'm embarking on a project to build a 3D controller for the Sony PS2.  It is ambitious and involves getting down and dirty with low level code. Don't you love it when things get dirty?

Having scoured the web for PS2 interface protocol, I've found the following useful links:

http://www.gamesx.com/controldata/psxcont/psxcont.htm
http://www.geocities.com/digitan000/Hardware/22/e22_page.html
http://pinouts.ru/Game/playstation_9_pinout.shtml
http://sophiateam.undrgnd.free.fr/psx/index.html

All fine and dandy in theory, except that the my PS2 console don't accept the bit patterns that I am spitting out in Analog Red mode (need the analog stuff).  Incidentally, I'm using a SPI interface to do all the hard work.

I've read in a post that it may be due to some kind of timing/bit violation, but details are sketchy.  Any bright sparks out there who can throw some light on this?