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CHAPTER 4: Operation
If the Extender seems to be operating properly, turn off the Extender’s power
switch and connect the other IEEE devices to each of the Extender’s IEEE ports.
Connect the local Extender to the remote Extender via the serial or fiberoptic
cable. Apply power to all devices in the system. If the Extender’s ERROR LED
continues to blink after the power-on sequence, this indicates one of two possible
conditions: either the system controller has not yet asserted the Attention line, or
communication with the mating Extender has not yet been established. Be sure
both Extenders are powered on, the cabling has been installed, and the controller
has asserted Attention before assuming there is a problem.
Once all IEEE devices have been connected and powered on, the Extender will
allow the system controller to command up to 13 IEEE devices on its local bus (in
addition to the controller itself and the local Extender), and up to 14 IEEE devices
on the remote bus (in addition to the remote Extender). The Extenders have
no address of their own, and therefore will operate completely transparent to the
system (with the exception of parallel poll—see below). Be careful not to have
two IEEE devices with the same address connected to either remote or local buses.
If you do, the bus will “lock up” when one of the devices is accessed.
4.2 Parallel Poll Response Times
The Parallel Poll function is the only extended bus transaction which is not
completely transparent to your system. This is because the speed at which data is
transferred serially is slower than the IEEE specification for Parallel Poll response.
Consequently, it is necessary for the controller to perform two Parallel Polls.
When the local Extender detects the parallel poll, it outputs the most recent
Parallel Poll information to the controller on the local bus. This data should be
discarded by the controller because it is not current. After receiving a Parallel Poll,
the local Extender commands the remote Extender to perform a Parallel Poll, and
returns the remote Parallel Poll data byte to the local Extender. The bus controller
should now perform another Parallel Poll and use this data as the valid Parallel
Poll byte. The table on the next page describes the minimum times the controller
should wait between consecutive Parallel Polls in order to assure valid data on the
second poll.
