3com network cards

Hello,

In my network we have always used 3com 10/100 3c905b-tx cards however, on occasion I have tried other 10/100 cards and although I can connect to my servers and print I have never been able to get the generic cards to get out on the internet. Just recently I purchased a new workstation with a 3com 10/100 3c905 no B and the same thing happened. I took out that card and replaced it with a 3com 3c905b and it worked fine internally and externally. This has always bothered me. Anyone have any idea why this happens.

thanks