[SI-LIST] Re: problems about 1-to-2 FE connection

If I understand correctly, by "1-to-2 transformer" you don't mean the 
turn ratio, but that the diff-pair routing on the switch-side is 
paralleled from the transformer to the two switches.

In that case, and IF (and only IF) 
1) you always have two switch boards plugged in
2) the length between the transformer (on the processor board)and the 
PHYs(actually termination) on both switch boards is equal (meaning 
the switch boards are in symmetrical slots relative to the processor)

then consider using 1:sqroot(2) transformer ratio to bring back the 
impedance from 50 ohm (100||100) to the required 100 Ohm.

Neven



--- In si-list@xxxxxxxxxxxxxxx, "peter zhu" <peter.zhu@xxxx> wrote:
> Hi,experts:
> 
> Very tough problem for me in Ethernet redundancy design.
> 
> In our backplane, we use an illegal Ethernet switch board 
redundancy scheme. our processor board has one FE(Fast Ethernet), and 
two switch boards are resided in backplane for redundancy. the 
processor board FE connect to the two switch board at the same time, 
but one switch board is in standby state(The PHY on this board is 
tristated), so the processor board FE will communication with the 
active switch board. But you know, FE must have a transformer for 
isolation, so the processor board FE has 1-to-2 transformer 
connection because of the two switch board on this FE. This will 
cause to Ethernt singal distortion and this is not compliant with 
standard, and some processor boards can not transmit data with very 
high speed.
> 
> If we use AC104QF from Broadcom, maybe 2 boards FE will fail among 
12 boards, other 10 boards FE are very normal.
> 
> But if we use AM79C875KI from AMD, maybe 5 boards FE will fail 
among 12 boards.
> 
> Our redundancy scheme is not changeable, so how to let the FE work 
nomal in this situation?
> Does anyone encounter similar problems?
> 
> Urgent, I very appreciate your help and information.
> 
> 
> Thanks!
> 
> Peter
> 
> UTStarocm
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