iPhone app does not run on old device (3G, 3GS, ...)

Possible Duplicate:
Is it possible to target older iOS versions when using Xcode 4.2 and iOS 5 SDK?

I have developed an app which works on iPhone 4 (iOS 4.3 & 5) (which used during developing). Now I have tried to test on 3GS (iOS 4.3.3) and 3G(iOS 4.2). But app does not load into these devices. I can see following messages on consol when try to deploy to device.

Sat Jan  1 17:27:38 unknown lockdownd[16] <Error>: 2ff68000 handle_connection: 
            Could not receive USB message #6 from Xcode. Killing connection 
Sat Jan  1 17:27:38 unknown com.apple.mobile.lockdown[16] <Notice>: Could not 
            receive size of message

I have tested on Xcode 4.0.2 as well as 4.2 beta. Restarted devices and MAC but still same. Can anybody know about this issue? Thanks

Solution

This can come for various reasons. Until now this is are the cases pople have encountered:

bad cable -> try another one, or hold it really still when programming
unsuported features mentioned in info.plist -> check plist
unsuported architecture (by default Xcode4.2 uses standard arch= armv7 ) -> add armv6