[Gb-ccb] forwarded message from Martin Shepherd

[Martin Shepherd]

On Thu, 21 Aug 2003, John Ford wrote:
> Remember that the purpose of the opto-isolated lines is to drive
> switching signals, and so Rich's pulse shaping method is probably more
> appropriate than creating a perfect match.

I believe that the cable is too short for cable skew to be a
problem. The filters are more of a concern. I'm aiming at 100ns rise
times, so frequencies much higher than 10MHz aren't significant. The
32MHz RFI filters have insignificant attenuation up to about 20MHz, so
these filters alone presumably aren't a problem either. However, what
I don't know is the frequency response of the 2 RFI filters loaded by
a section of cable and two impedance matching circuits, especially if
the latter include pulse-shaping capacitors. I don't have enough
details of either the cable or the filters to figure this out. As
such, I am thinking of simply specifying a circuit that ignores pulse
distortion concerns, along with a caveat that the PCB make allowance
for the possible addition of ceramic capacitors in parallel with the
matching resistors, and that the resistors used for matching be of a
form that can be easily desoldered and replaced.

Martin