[time-nuts] Transmission Line Question

Don Otknow donald.otknow at gmail.com
Wed Apr 6 22:55:56 UTC 2011


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.

Don

> Hi Don excuse me I was thinking RF :-))  That should be OK  Though it
> depends what you are trying to do bus termination is usually slightly
> different to RF. At least you should not get nasty reflections from the ends
> like that. There may come a problem if you send a pulse from a 100
> termination into the network. There will be a little diggle and reflection
> as it sees the 50 ohm pin as a small mis-match. But if driving out only from
> the middle I see no problem. (but then I might just be wrong :-))  )

> Alan


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