On Thu, Aug 25, 2016 at 11:16 PM, tony duell <[email protected]> wrote: >> >> 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.
47 and 56 were resistors of choice back in the day for make-shift termination on thin-net networks in a pinch. >> 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. 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
