Re: Swapping Drives - Sanity Check
- Date: Sat, 23 Feb 2019 10:55:00 -0600
- From: John Hasler <jhasler@xxxxxxxxxxx>
- Subject: Re: Swapping Drives - Sanity Check
> 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.
Elmwood, WI USA