A passive device is a device
that "passes" a signal through or possibly alters the impedance of the signal
but "passes" it through. A general rule is if it doesn't require power to work,
then it is passive. Baluns are passive devices that change a balanced signal to
an unbalanced signal (see Balanced & Unbalanced
Signals for an explanation of Balanced and Unbalanced). Passive hubs
typically contain only resistors between the ports. Almost all cables and cable
adapters are considered "passive", typically all they're made of is wire and
plugs. A user will typically look for a "passive" device to suit their needs,
because passive devices are cheaper. But in some cases, we may have no choice
but to go "active" to suit their needs.
An active device
translates or strengthens the signal. As a rule, these need power to work. The
Parallel Extender may appear to be passive because it
runs without a power supply. But it is really an active device, pulling the
power it needs from the handshake signals on the printer. In some cases, where
the printer can't give enough power to it, the Parallel Extender will need an
external power supply. Converting the PS/2 style keyboard to USB requires an
active device, the USB to keyboard (and mouse)
connection. Connecting an old printer with Serial or Parallel port to a USB
port requires an active device, ADP3141 is USB to serial
and ADP3140A is USB to parallel. USB devices get their
power from the USB port.
With SCSI Terminators, a passive
terminator contains only resistors. The passive terminator works with low to
medium speed SCSI signals, but there are some SCSI circuits that the Passive
terminator will not work on. An Active terminator contains a regulator for
better precision, which is needed for higher speed SCSI signals. The LVD
terminator is an Active terminator made for the LVD interface. |
