rhkramer writes: > Hmm, that surprises me -- I guess I would have expected that you want > the data pins connected first (while no power is applied), and > disconnected last (again, while no power is applied).
Data pins can supply current. They have to in order to charge and discharge load and transmission line capacitance and drive load resistance. If you connect a data pin that has voltage on it to an input of an integrated circuit that's part of an unpowered device current will flow into the input, through the IC, and out the power pin to the rest of the device. This current may exceed the maximum that the input can handle. There has to be a return path, of course. This could be the common, another data pin, or, worst of all, chassis ground in which case you will be subjecting both systems to whatever hum and/or ground loop current is present. All external connections to a device should incorporate protection, but this not only adds cost: it slows data. -- John Hasler [email protected] Elmwood, WI USA
