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SERVSWITCH™ USB AND USB PLUS
3.1 Setting the Enumeration Guard Time (Positions 5 & 6)
When devices are connected to or disconnected from a USB bus, the devices on
that bus then need a certain amount of time for uninterrupted communication in
order for the currently selected CPU to determine the new makeup of the bus—
which USB devices are connected, what capabilities those devices have, etc. This
process is called “enumeration.” The USB devices attached to the ServSwitch USB
begin to perform enumeration each time that a new CPU channel remains
selected for more than one second.
If enumeration is interrupted before it’s complete, some or all of the devices on
the bus can crash. So, starting one second after a new channel has been selected,
the ServSwitch USB enforces a “guard time” during which no further switching is
possible. The front-panel 7-segment display will begin blinking to show you that
enumeration is underway and the guard time is in effect, and it will keep blinking
faster and faster until the guard time expires.
(You can still quickly cycle through the channels for as long as you like, because
there is an initial one-second window after each channel change during which the
ServSwitch USB is busy resynchronizing the video and doesn’t even attempt to
communicate with the computer’s USB port. Also note that enumeration is not
performed during autoscanning; video from each CPU is displayed, but no USB
communication occurs until you stop the scan at a particular port.)
The factory-default enumeration guard time is 2 seconds; the other possible
settings are 4, 8, and 16 seconds. The bus will probably only need two seconds if
you only have a USB keyboard and mouse attached to it, but as you add more
peripherals (especially hubs and Zip
®
drives), enumeration will require more time
to finish. We suggest that you time the process—wait until the CPU’s on-screen
hourglass or clock stops running and all of your USB devices return to normal
operation—then set the ServSwitch USB to use the shortest guard time that’s still
longer than the actual enumeration time.
To set the enumeration guard time, move positions 5 and 6 of the DIP switch on
the bottom of the ServSwitch USB to your desired setting:
Guard Time Position 5 Position 6
Two seconds (default) OFF (down) OFF (down)
Four seconds ON (up) OFF (down)
Eight seconds OFF (down) ON (up)
Sixteen seconds ON (up) ON (up)