[Elecraft] 2 radios, 1 antenna... finally a prototype!

[Don Wilhelm]

Vic,

What you say is certainly true - it does take two redundant signals to
provide a fail-safe mechanism that includes one of the signals being
disconnected.  We all must examine the exposures and weigh them against the
risks of using simpler, but less protected solutions.  There is a compromise
in there somewhere, and each implementer must make his own decisions based
on his own risk assessments.

As we used to say in my engineering group - 'You can endeavor to make it
foolproof, but then some 'fool' will still break it'.  Your level of risk
acceptance is uniquely yours - do whatever is needed to produce a risk level
that is acceptable to you.

73,
Don W3FPR

> -----Original Message-----

>
> Don Wilhelm wrote:
>
> > Such fail-safe mechanisisms can certainly be devised if deemed
> necessary.
> > The 8R signal mentioned is actually the inverse of the Amp-Key
> line, so that
> > signal can be produced with an inverter and no additional
> connections to the
> > internals of the K2.
>
> Amp-key is derived from 8r and is in some sense the inverse of it, but
> neither 8r not amp-key are high/low outputs.  They both have one state
> which is indistinguishable from 'disconnected'. If you use amp-key by
> itself you can't distinguish between these situations:
>
> 1) Amp-key output is open and the K2 is receiving; and
> 2) Amp-key output is accidentally disconnected and the K2 is transmitting.
>
> 8r by itself can't distinguish between
>
> 1) 8r is open and the K2 is transmitting; and
> 2) 8r appears open (accidentally disconnected) and the K2 is receiving.
>
> So I suggest adding logic *inside* each rig to assert a signal (called
> 'safe') only when 8r is on or amp-key is not grounded.  Then you would
> connect safe and amp-key to the interface.  The interface would connect
> rig 1 to the antenna only when rig 2 is asserting safe and not grounding
> amp-key, and vice-versa.
>
> Now let's see what happens when things get disconnected.  If you
> disconnect rig 2's safe line, rig 1 will always be disconnected from the
> antenna.  If you disconnect rig 2's amp-key then safe will still go low
> when rig 2 transmits (because of the logic inside rig 2), disconnecting
> rig 1 from the antenna.  And vice versa.
>
> Of course, then you would have to turn on both K2s to listen to either
> one of them. I am still trying to figure out a way to avoid this without
> losing the fail-safe nature of the system!
>
> > One of the ways to accomplish such 'fail-safe' operation is to inhibit
> > keying (or PTT) until all other conditions are properly satisfied
>
> I think the complications/limitations that this introduces are worse
> than turning on both radios!
>
> --
> 73,
> Vic, K2VCO
> Fresno CA
> http://www.qsl.net/k2vco