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

[Vic K2VCO]

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