[146818] in North American Network Operators' Group
Re: OT: Traffic Light Control
daemon@ATHENA.MIT.EDU (Robert Bonomi)
Tue Nov 22 16:35:52 2011
Date: Tue, 22 Nov 2011 15:36:06 -0600 (CST)
From: Robert Bonomi <bonomi@mail.r-bonomi.com>
To: nanog@nanog.org
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
On Tue, Nov 22, 2011 at 02:26:34PM -0500, Jay Ashworth wrote:
>
> > Some other things to consider.
> >
> > Relays are more likely to fail. Yes, the relay architecture was
> > carefully designed such that the most failures would not result in
> > conflicting greens,
>
> My understanding was that it was completely impossible. You could
> fail dark, but you *could not* fail crossing-green.
Just to put one point to rest.
I, personally, have witnessed traffic lights showing 'green both directions'.
*TWICE*. One was in the mid-1960s, with what was undoubtedly relay-based
control logic; the second was in the late 1990s, *probably* with solid-state
'management' controls , but I don't know for certain. (The 'relatively
recent' unit's I've seen the insides of have solid-state logic driving final
'output' relays that provide power to the actual signal head.)
In the first case, the pedestal-mounted control unit had been subjected to
excessive impact forces, and some of the 'output' wires had shorted together.
