roy@alanine.phri.nyu.edu (Roy Smith) (04/06/91)
Today, I was trying to boot a new FastPath (for those not familiar,
a FastPath is an AppleTalk-Ethernet bridge/router/protocol-converter/etc).
Anyway, it appeared that being connected to the ether while being
configured via the AppleTalk side was confusing it, so I decided to unplug
it from the (thin) ethernet segment it was on until I had it set up
properly.
Making sure I didn't disrupt the ether segment was easy, I just
took off the base of the T-connector. But then I was left with an
unterminated BNC on the FastPath. The "right" way to terminate it, I
suppose, would have been to get another T and put a thin terminator on each
leg, but I didn't have any more terminators handy. What I did instead was
to just take a 1/8th Watt 51-ohm resistor and connect it across the center
and shield contacts right on the BNC jack. Shouldn't that have been all I
needed? The box kept complaining about ethernet errors, so it would appear
it wasn't, but I don't understand why. Can somebody enlighten me?
--
Roy Smith, Public Health Research Institute
455 First Avenue, New York, NY 10016
roy@alanine.phri.nyu.edu -OR- {att,cmcl2,rutgers,hombre}!phri!roy
"Arcane? Did you say arcane? It wouldn't be Unix if it wasn't arcane!"henry@zoo.toronto.edu (Henry Spencer) (04/06/91)
In article <1991Apr5.191653.28927@phri.nyu.edu> roy@alanine.phri.nyu.edu (Roy Smith) writes: >...but I didn't have any more terminators handy. What I did instead was >to just take a 1/8th Watt 51-ohm resistor and connect it across the center >and shield contacts right on the BNC jack. Shouldn't that have been all I >needed? The box kept complaining about ethernet errors... Plausible idea, but wrong resistance. The cable is 50 ohms in *each direction*, so the impedance seen at the BNC is 25, not 50. 1/8 watt also sounds a bit small, although perhaps it is okay. If I recall correctly, the standard recommends 1W terminator resistors, but that is undoubtedly sized for a severe worst case. -- "The stories one hears about putting up | Henry Spencer @ U of Toronto Zoology SunOS 4.1.1 are all true." -D. Harrison| henry@zoo.toronto.edu utzoo!henry
jrd@cc.usu.edu (04/07/91)
In article <1991Apr5.191653.28927@phri.nyu.edu>, roy@alanine.phri.nyu.edu (Roy Smith) writes: > Today, I was trying to boot a new FastPath (for those not familiar, > a FastPath is an AppleTalk-Ethernet bridge/router/protocol-converter/etc). > Anyway, it appeared that being connected to the ether while being > configured via the AppleTalk side was confusing it, so I decided to unplug > it from the (thin) ethernet segment it was on until I had it set up > properly. > > Making sure I didn't disrupt the ether segment was easy, I just > took off the base of the T-connector. But then I was left with an > unterminated BNC on the FastPath. The "right" way to terminate it, I > suppose, would have been to get another T and put a thin terminator on each > leg, but I didn't have any more terminators handy. What I did instead was > to just take a 1/8th Watt 51-ohm resistor and connect it across the center > and shield contacts right on the BNC jack. Shouldn't that have been all I > needed? The box kept complaining about ethernet errors, so it would appear > it wasn't, but I don't understand why. Can somebody enlighten me? > -- > Roy Smith, Public Health Research Institute > 455 First Avenue, New York, NY 10016 > roy@alanine.phri.nyu.edu -OR- {att,cmcl2,rutgers,hombre}!phri!roy > "Arcane? Did you say arcane? It wouldn't be Unix if it wasn't arcane!" ---------------- Roy, I know this will sound simple but... A transmitter attached to a Tee connector on the coax sees the impedance of the coax divided by two: two pieces of coax in parallel. So you need a 25 Ohm load directly on the BNC connector of the FastPath box. Joe D.
roy@phri.nyu.edu (Roy Smith) (04/07/91)
I wrote: > What I did instead was to just take a 1/8th Watt 51-ohm resistor and > connect it across the center and shield contacts right on the BNC jack. > Shouldn't that have been all I needed? Several people have already mailed me on this. What I should have done was used a 25-ohm resistor (2 50's in parallel; one for each "end" of the cable). Assuming a ethernet transmitter is a current source (as was suggested), having double the resistance put double the voltage on the cable, which is exactly what you would see in a collision. I'm embarrassed that this didn't occur to me when I was doing it. -- Roy Smith, Public Health Research Institute 455 First Avenue, New York, NY 10016 roy@alanine.phri.nyu.edu -OR- {att,cmcl2,rutgers,hombre}!phri!roy "Arcane? Did you say arcane? It wouldn't be Unix if it wasn't arcane!"