Re: Network Wiring

On Mon, Jun 20, 2011 at 01:23:23PM -0500, John Bundy wrote:
> I think it would be safer to keep the high AC voltages associated with
> ringing on landline phones away from ethernet:
> 
> from http://www.derose.net/steve/guides/wiring/
> *Warning:* When the phone rings, the voltage generates an electromagnetic
> field, which in turn generates voltage in the adjacent Ethernet wires. Since
> Ethernet runs at about 2-5 volts rather than this 50-90 volts, conceivably

I'll never argue against keeping the small signal wires away from the
large-signal ones, especially not since the time Eric and I spent
several days trying to repair a Data General CPU board (back when
minicomputers were built from SSI TTL chips).  They hadn't warned us
about this syndrome, caused by the on/off/lock switch which switched,
on theoretically isolated poles, the AC and a logic line that ran right
into the middle of the fifteen inch square CPU board...

But the idea that 90V of very low frequency sine wave will produce any
significant cross-talk in an adjacent twisted pair without a short
circuit is silly.  Just sayin'


(Why did DG do that?  Well, this was a classic TTL and core machine,
and the "lock" (I may misremember what they called that mode) was where
the machine would (1) stash the CPU state into some reserved memory
locations on power fail - it had this HUGE capacitor bank that provided
enough reserve to do that - and (2) when power came back it would pick
that saved state out of core and resume instead of doing a normal boot. 
So having the AC and that logic line controlled by the same switch
wasn't frivolous.  But, yeah, shouldn't have been quite so easy to fail
that way.  I want to say this was their "Eclipse" model, but that may
have been the similar but smaller mini that had the then-new dynamic
RAM memory: 32K words on a single board instead of 4!)

-- 
The most common implementation of SMTP is contained in sendmail.
This program is included free in most UNIX software distributions,
but you get less than you pay for.  -- Cheswick, Bellovin & Rubin

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