[clue-tech] making ethernet autonegotiation not work

[Jim Ockers]

Hi,

How can we break autonegotiation for ethernet link speed & duplex
with a minimum of expense & effort?

So we have some nice ethernet equipment (NICs, switches) which
supports 10/100 BaseT half and full duplex, and will happily
autonegotiate to the best mode that both link partners support.

http://en.wikipedia.org/wiki/Autonegotiation

For operational reasons we need to make sure that 100BaseT never
gets negotiated.  We have configured all link partners (software
device driver config) to negotiate only 10BaseT and tried to
disable autonegotiation in software, but since the (unmanaged) 
ethernet switch supports 100BaseT (and will autonegotiate it if 
asked) occasionally the link goes to 100BaseT.

Don't ask why, just trust me on this, we HAVE TO GUARANTEE THAT
IT WILL ONLY EVER BE 10BASET.  It has to do with the fact that the
cabling and connectors are NOT CAT5 nor are they intended for
ethernet.  Plus they're long.  It all seems to work OK at 10BaseT 
but not 100BaseT.

The unmanaged switch is $50 and the managed switch is $500.  Of
course we can make the managed switch set its ports to only 10BaseT.
Not so easy with the unmanaged switch.

I was hoping we could put a "high pass filter" or something made
out of cheap analog components in line with the cable which would
make the autonegotiation pulses not work.  A hardware solution seems
like the best idea.

However this might not work because ethernet is baseband and does
not use a modulated carrier at some frequency, say 10MHz.  Maybe
this means that a "high pass" filter might be ineffective.

Anyway does anyone out there have some ideas how we can break the
autonegotiation in hardware so everything falls back to the lowest 
common denominator of 10BaseT half-duplex?

Thanks,
Jim

-- 
Jim Ockers, P.Eng. (ockers at ockers.net)
Contact info: please see http://www.ockers.net/