Black Box KV822A Setting the Enumeration Guard Time Positions 5, Servswitch Usb And Usb Plus

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)

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