1 May
2012
1 May
'12
1:59 p.m.
On May 1, 2012, at 2:52 PM, Tony Duell wrote:
I suppose it depends on what you mean. The USB host is constantly
polling the devices; when the devices have a message for the host,
it gets sent then. There's no out-of-band signaling like an IRQ,
if that's what you mean. Devices just have to wait their turn.
It's like Token Ring in a lot of ways.
- Dave
"Asymmetric" refers to adherence to a master-slave topology; a member
on the bus is either a controller or a slave. What ties this with
"polled nature" is that there is no way for a slave to initiate a
transfer--all conversations are initiated by the controller. In
other words, conversation between slave and controller is "speak only
when spoken to". Thus, to determine the status of an operation, the
host must periodically poll the slave asking "Is it soup yet?".
Is there no equivalent of a 'service request' in USB? (I;ve not read the
stnadard seriously)?