On 04/07/2011 12:55 AM, Don Otknow wrote:
Well actually there's another level here which is that some of the
lines are bidirectional data lines - each device at the end of the
lines will occasionally drive these. However one of the devices (lets
call it device A) can request a wait from the center device via a
separate line so it should be possible to mediate issues for that
device by just telling the center device to wait until the signals are
stable.
However, the other device (lets call it device B), with no wait
request line, is probably not going to be as happy. But I think that
(assuming that the data pins of the center device are high impedance
while device B is driving the line and device B has a 50 ohm source
impedance), then device B is going to see a 100 ohm transmission line
in parallel with a 100 ohm termination, which should be fine, and at
the other end of the transmission line (device A) a 100 ohm
termination, which is also fine. Assuming the center device is high
impedance, then there shouldn't be any weird reflections.
OK, now you have a bus.
This should work, as long as the transmission lines are allowed to ring
out such that it doesn't cause interference.
Another solution is to have open-ended sides and then use Zener-diodes
to clip out the reflections.
See PCI for instance.
Cheers,
Magnus
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