Anon said:
hi, I know that on some computers, long 50m video cables work and on
others they don't. Similarly with parallel cables.
And it depends on the card, how much oomf it's got.
The question is, what is the 'oomf' is it current?
Could I measure with a multimeter and see if a card would support a
50m cable?
I'm interested in both parallel and video
For cables shorter than a quarter wavelength or so, the cable looks
essentially like a capacitor to be charged to each signal voltage
level. So the limit in length is essentially the current available to
charge the capacitance and the time available to wait for the charging
process. Most parallel port processes rely on hand shaking (provide a
new set of levels to the data bits, send a new data sent status bit,
and wait for the device on the far end of the cable to return a
confirmation signal that the data has been latched). This handshaking
process just slows the data rate as cables get longer, till the skew
between different signals in the multi conductor cable gets so large
that the new data available status bit arrives before all the various
data bits have settled to their correct states.
Video is an open loop process that just streams signals into the
cable. But the driver is usually matched in impedance to some
standard cable impedance (usually 75 ohm) so the signal is degraded
only by cable loss as length increases, assuming the cable impedance
is correct, and is terminated with a matching impedance to minimize
reflections.
