I should buy a "Usb serial relay/shutter" for my Nikon D90, for exposures > 30 seconds.
I searched in the forum but I have not understood well which type is working or not.
Can I ask which adapter you are using for your Nikon DSLRs ?
I use a KMtronics USB One Relay. You can find it googling. You need the connector for the camera (it is connected to the GPS/remote) .
There is also DSUSB (google that) which also sells the cables for the different cameras but it is a bit expensive in comparison to KMtronics. Anyway it can be the more convenient if you live in USA (no problem with import offices).