COM problem

Hi everybody, i m newbie in hardware and i have a strange problem i write a
program for my device mini2440 for send and get data in text with RS232
communication,i write another similar program in XP so when i connect my
device to my computer i can send and get data with this programs, but when
i connect a Magnetic Card reader to my device i cant get any data from
that,so i connect card reader to my computer and test with my program and
its work and i get data from card reader, whats the problem here?

when i m connect GND and VCC of card reader to pin 2,3 CON0 of mini2440, my
device reset automatically,when i can use this 2 pins for getting power?

thanks a lot