why the power is off

[realfenglin]

HI Guys
 
    Right now, i connect SD card(16G), network cable and mini usb with the Sheevaplug. System boot from SD card. everything is fine till i unplug and plug back the Normal usb cable into the sheevaplug. USB cable is used to connect my Serial-USB device and sheevaplug. when i plug the usb into the sheevaplug, i can see the ttyUSB0 in /dev. unplug the USB cable, it will show you the USB device is disconnected. however, sometime, if i plug back the USB cable, the whole systm is power off. blue light is off. i tried 10 times, only twice, it works fine. when i plug back the usb deive, the system shows the device is connected. the rest just power off.

    i think the power is not enough for all the connections are used?

any ideas? thank you.

[realfenglin]

confirming this USB-SERIAL board does not seem to upset 5v supply from usb port of sheeva plug .  maximimum dip of voltage noted was 0.5V- down to 4.5V
 
Also current draw was between 35 and 50mA (LED ON)