bnc network-last 2 computers in error

[pc81]

hi there i m from bosnia
now i m working on network for one school.
they have 13 computers and i have connected first five in bnc network.
when i connect 4 computers it works ok(i use 3com isa cards).
but when i add one more computer then 5 th computer see all computers exept computer no. 4.
and computer no. 4 dont see computer five.
they see eah other but they cant enter to each other(ping does not work).
first 3 computers can enter in all computers and 4 th and 5 th can enter into them but
cant enter into each other.i check cables terminators and all and its ok.
but when i add one more computer (now 6 computers in network) then last 3 computers cant enter to each other and
can enter into firs 3 computers and first 3 can enter into them.
i have changed computers,cards.
i think that is something about ohms in cables or because of not (good) grounded eletrical instalations.
on network with 5 computers when i have 26 ohms in cable i have error with that last 2 computers but
when i have 18 ohms in network i have error with 3 last computers.i know that in network must be 25 ohms.
i have terminators at the end (50 ohms) and cabel(50 ohms) i have trying with grounded terminator but...
please help me if you can

 

[wybnormal]

What is the total length of the coax? Do you have any crimps or sharp bends in the cable? I seen this with a bad crimp on the BNC connector. I even found a bad T once.. that took some doing.

This is where a cable tester is worth it's weight in gold.. Check the drop cables with the ohm meter for shorts.. there should not be ANY measurement between the center pin and ground.. nothing.. if there is, that is the problem.

MikeS
Find me at

http://www.packetattack.com

"The trouble with giving up civil rights is that you never get them back"

 

[pc81]

thanks for response
all that is ok because last two computers working when i just connect those two.
cabel and terminators are ok.

 

[Guest_imported]

If possible, have someone that knows electrical wiring check the outlets. If it is actually three conductor wiring, check to see if there is a ground loop current between the computer and the ground connection in the outlet. If there is, the polarity of the outlet is reversed and it's inducing a ground loop current into the BNC. Also check the total length of the BNC leg. If this it thinnet coax you can't exceed 200 meters on a leg without a repeater. Always use a grounding terminator at ONE end of the leg, not both.

If your 3com cards are combination: ethernet/BNC, you'd save yourself much troubleshooting by using an ethernet hub and UTP Cat5 connections. If one node goes down with thinnet, all are down. With ethernet, if one node goes down the rest of the network is ok. (99% of the time) I realize these components may be difficult to obtain.

Dennis