About DP83849ID,cannot link with exchange

Other Parts Discussed in Thread: DP83849IDMy PCB board has a DP83849ID chip, now there is a strange problem bothers me. When it can communicate with a computer properly, but sometimes it can not establish a link with the switch. I use a multimeter to measure the impedance of differential transmission line between the TX± and found the resistance only 254Ω. I measured the resistance of normal ports up to thousands of ohms. However,some days ago the PCB board mounted inside a device and EMC tested . I'm not sure if the PHY chip pin damaged in the testing process. there is a transformer in my circuit, but any ESD or TVS is not added. I want to know why? I want to learn the internal structure of I/O of the chip,if there are some technical documents can help me?because I want to further analyze the problem.Another question is that what's the deference between the voltage driven and current-driven about UTP port.Thank you!