need help for networking loop back, Thanks

John Lin - TAO ([email protected])
Wed, 28 May 1997 11:41:08 +0800

For testing network system on PC, we plan to design a device -- "loop
back". With this loop_back, we can run burn-in program on PC and test
the stability of the network system. Currently, we need to use
100 meter long cable to work as the loop_back.

My question is how to design the loop_back without using 100 meter long
cable and still have the effect of 100 meter cable?

My idea is that using two short cables, 50cm, with resistors to
simulation attenuation of long cable shown below:

R1 : about
8.55 ohms to works as DC resistor for 100 m cable.
L1 (50 cm )
>--R1-------------------+ Because of
Characteristic imped. Z0 for cable is 100 ohms
| R2 // R3 =
100 ohms (to avoid reflection).
<---------------R2 +---+
L2 (50 cm) | R2/(R2+R3) =
attenuation of 100 m cable.
/ / / (gnd)

Could somebody tell me if my idea is correct or not?
Thanks in advance.

John Lin
CAE of EDA dept, PC Engr.
Digital Equip. Corp. Taiwan
Email: [email protected]
TEL: 886-3-3900000 ext. 2565