Hi Rick,
To go into a little more detail, a selective suspend and a global suspend look the same to a USB device, and are handled exactly the same by NI hardware. All USB devices support both. Selective suspend is a feature of hubs, not of devices.
Here are some details about how it works, from the USB 2.0 Spec:
From section 11.9, "Global suspend/resume is implemented through the root port(s) at the host. Selective suspend/resume is implemented via requests to a hub."
From section 7.1.7.6.1, "Global suspend is used when no communication is desired anywhere on the bus and the entire bus is placed in
the Suspend state. The host signals the start of global suspend by ceasing all its transmissions (including the SOF token). As each device on the bus recognizes that the bus is in the Idle state for the appropriate length of time, it goes into the Suspend state."
From section 7.1.7.6.2, "Segments of the bus can be selectively suspended by sending the command SetPortFeature(PORT_SUSPEND) to the hub port to which that segment is attached. The suspended port will block activity to the suspended bus segment, and devices on that segment will go into the Suspend state after the appropriate delay as described above."
(The "delay as described above" is from section 7.1.7.6, "All devices must support the Suspend state. Devices can go into the Suspend state from any powered state. They begin the transition to the Suspend state after they see a constant Idle state on their upstream facing bus lines for more than 3.0 ms. The device must actually be suspended, drawing only suspend current from the bus after no more than 10 ms of bus inactivity on all its ports.")
Hope that helps!
William Earle
Staff Software Engineer
DAQ Software
National Instruments R&D
