On Friday, November 2, 2012 2:05:34 PM UTC-4, Craig Funston wrote:

Dave,
I'm interested in more information regarding the rendering "invisible"
potential of Mode A/C. I have a Terra Mode C available to me for the
coming season & the research I did indicated to me that it is visible
to TCAS equipped aircraft as long as the AD has been complied with.
Is there something I missed in my research?
Thanks,
Craig

Yes, read especially the technical notes at the bottom of this page:
http://www.gliderpilot.org/FLARM-Transponders

For any new installation, get a Mode S unit !

Hope that helps,
Best Regards, Dave