18
PowerLab Owner’s Guide
A hub is simply a device that lets you extend the USB tree. It connects
to a USB device such as the computer, and multiple USB devices
(including other hubs) can connect to it in turn. It provides power for
those attached devices that need it. Up to five hubs can be connected
between the computer and the furthest USB device. As many as 127
devices can be connected on one tree, counting the computer. The
cable between any USB devices (including hubs) must allow a signal
to be transmitted in a certain time; in practical terms this means it
must be no more than 5 metres (16 feet) in length. Thus, no device can
be more than 30 metres (98 feet) from the computer.
USB is more tolerant than SCSI. It lets one plug in and remove
devices while the computer is on. It re-enumerates (provides
addresses for) devices as they are connected and disconnected, rather
than requiring fixed ID numbers. It has only one pair of connector
types, unlike SCSI, and does not need termination as SCSI does.
USB still has some restrictions. A USB connection is a bit slower than
SCSI for data transfer, and so may limit sampling rates (in Chart) or
increase delay times between sweeps (in Scope). Since all the devices
on a USB tree have a common communication pathway, if you have
many devices with a lot of information to transfer, they compete for
time. Using a video camera and a scanner at the same time as a
PowerLab may limit the sampling rates considerably.
USB has two sorts of cables, which it should not be possible to
interchange. Low-speed cables are cheaper, and suitable for mice and
so on. They are either captive (one end is permanently fixed to the
device) or use a custom connection, not a standard USB one. All
detachable USB cables are high speed, with fully shielded twisted-
pair cables, and standard USB connections: a narrow rectangular A
plug at one end and a squarer B plug with a bevelled top at the other.
All devices that can act as hubs have high-speed connections. The
PowerLab needs a high-speed connection, and is provided with a
suitable cable.
Connecting the PowerLab Using USB
Use the USB cable supplied with your PowerLab to connect the USB
port on its back panel to the USB port on the computer, or to an active
USB hub connected to the computer. USB ports and cables should be
marked with a trident-like icon (some non-standard ones might have
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