25 Aug
2016
25 Aug
'16
10:26 p.m.
On Thu, Aug 25, 2016 at 11:16 PM, tony duell <ard at p850ug1.demon.co.uk> wrote:
47 and 56 were resistors of choice back in the day for make-shift
termination on thin-net networks in a pinch.
10base5 also had rules for minimum bend radius as well as tap locations
to be at the maxima of the reflection point. For early gear, failure to
put it at a vibration node would often result in unreliable behavior, though
I can't recall if that included collisions or not.
Warner
You will need to terminate the coax.
(terminator)---------------<tap>------. . .
----<tap>----------------(terminator).
He said he has connected a 47 Ohm resistor at each end of the
coax. That's close enough to the correct 50Ohm terminal to
work. Anyway, you
need to terminate the line or your going to have so many issues
you may not even get a packet to make it from one end of the line to the
other.
Correct. You won't. No matter how short the coax is. You will get collisions.
The reason is that the transmitter in a coax MAU is a current source which
effectively develops a voltage across the terminators. The receiver is a
voltage detector. A collision is sensed by the MAU if it sees more voltage
across the coax than it should. This (on a correctly terminated cable) means
that 2 transmitters are putting current into the cable/terminators at the
same time.
But if the termination resistors are too high or missing (even if only one
is missing) then a single transmitter's current will develop enough voltage
across the coax to be detected as a collision.