Dear friends,
I read some simple test in the article http://webdevelopersjournal.com/columns/greg3.html. It goes like this...
"What do you do when something goes wrong? Get out your little multimeter and test the cable. From any T-connector, you should read 27 +/- 5 ohms; from any end of the cable, remove one of the terminators, and you should read 55 +/- 10 ohms. Remove BOTH the terminators, and you should read open circuit—infinity ohms! If you get these readings, your cable is probably A-OK. If not, track it down".
I did this test with my setup. It seems ok. But I cannot find an explanation to why we should read 27 +/- 5 ohms at each T connector.
I would really appreciate any hint from any one.

Thanks in advance,
irastha

 

Because the two 50Ω terminations are parallel, and will be half their normal resistance. The extra ohms mentioned are the cable wires resistance. I still have my old 10Base2 working, along side my 10BaseT and my wireless. My network evolved, it wasn't planned per se.

 

Thanks. I also guessed so but I am confused at some points.
I have this kind of setup for example.
50(t)-T--------------T---------------T-50(t)
l l l
220ohm 220ohm 220ohm

where --------coax cable (50 ohm)
T=T connector
50(t)= 50 ohm terminator.

I want to electrically see the behaviour of this setup. I did the measurement by giving input at the first node and measured voltage at second and third node. But I am not able to understand the behaviour of the circuit.
Can anybody think of the equivalent circuit of this setup.
i mean, if we want to draw the equivalent circuit of this setup. Should we replace the cable by 50 ohm resistance in series or parallel or something else?
I know its different task than a regular 10base2 ethernet, but I need to find this out for my work.

Thanks in advance.

 

Ethernet acts like a form of RF, and the 50Ω terminations absorb the signal, which prevents it from reflecting back and overwriting the signal, making it unreadable. This also prevents standing waves, another RF phenomenon concerning improperly terminated lines. DC may show 25+Ω, but the RF sees the local resistance.

Tee's are pure parallel, electrically. The center pins stays center, and the jacket is the shield, same as the coax cable.