30 Apr
2012
30 Apr
'12
1:34 p.m.
On Apr 30, 2012 1:09 PM, "Richard" <legalize at xmission.com> wrote:
None of this is making sense to me.
Please clarify what you mean by these terms in this context:
"polled nature"
"asymmetric"
"half-duplex"
The data transfer schedule is completely controlled by the host. A device
can transfer data at a lower rate than given opportunities to do so by
NAKing transactions in the bus, but a device cannot transfer data faster
than given opportunities to do so by the host, or force the host to
initiate a data transfer.
In USB 1.0 and 2.0 there is a single D+/D- pair between host and device so
it is half-duplex in the sense that data can only be transferred either IN
or OUT at any given time.
It is asymmetric in that you can only connect a host to a device. You can't
connect two hosts together without a device in the middle, nor can you
connect two devices together.
For most common host and device usage scenarios these aren't real
restrictions.