[Gb-ccb] forwarded message from Martin Shepherd

[Martin Shepherd]

On Wed, 20 Aug 2003, Richard Lacasse wrote:
> > were needed for your cable. Smaller resistors at the driving end will
> > then be used to meet the current limits of the diodes and provide
> > short-circuit protection. I have drawn this up, and worked out the
> > numbers, so I think that this is settled.
> >
> You can use the resistors at the driving end to help match the line driver to
> the line as well.

In principle I could, but the problem is that if I were to add
sufficient resistance at the transmitting end to match the cable there
as well, the voltage at the input to the opto-isolator wouldn't be
high enough to turn on the opto-isolator LED. Thus, since an impedance
mismatch at the transmitting end is irrelevant if there are no
reflections from a mismatch at the receiving, I decided to only match
the receiving end. The job of the resistors at the transmitting end is
thus solely to add a bit of resistance, both to satisfy the
opto-isolator LED requirements, and to give better short-circuit
protection than the miserly 1 second of protection built into the
differential driver chip.

Martin