RAs will be given also for targets with Mode C only, only will they not be coordinated.
If you have TCAS II, you will have mode S yourself.
See also
http://www.pprune.org/forums/showthr...S&pagenumber=2 :
TCAS I: Uses a directional antenna to view Mode A, C, or S
transponders on other aircraft to generate a situation
display and "Traffic Advisory" (TA) for nearby targets.
This TA is used to help pilots visually locate nearby
co-altitude traffic (Mode C) or unknown altitude traffic
(Mode A).
TCAS II: Uses a directional antenna to view Mode A, C, or S
transponders on other aircraft to generate a situation
display and a TA for nearby targets.
For target aircraft with Mode C or S transponders, the TCAS
display can generate a "Resolution Advisory" (RA), which
commanded vertical manuever (climb/descent) to avoid nearby
co-altitude traffic.
For target aircraft with Mode S transponders *AND* TCAS II
equipment, RAs will be coordinated between aircraft
(e.g. the two TCAS processors will cooperatively agree to
send one aircraft in a climb and the other in a descent.)
Note: aircraft equipped with TCAS II must have Mode S
transponders installed.