My first 10BASE5 network segment

[tony duell]

> 
> 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.

-tony