[Elecraft] TX_INH input (pin 7 on ACC) behavior

[Bob Wilson, N6TV]

Don is correct.

FYI, after looking over the K3 schematics with N6KR at Visalia many years
ago, we came up with a safe and simple "SO2R" lockout circuit (given two
K3s) that is documented here:

https://bit.ly/K3lockout

The 470 ohm resistor is a safety feature designed to prevent a potential
short of 5V to ground if TX INH is set back to the default position (OFF,
which resets Pin 7 as a "K3 ON" signal with +5V), and then another K3
shorts that PIN to ground.

This simple lockout circuit is implemented in the Y-BOX
<https://bit.ly/Y-BOX> as a convenience option (two Y-BOXs required).  The
"CW and Phone" version uses the FSK line (Pin 1) to get the +5V, so no
external power supply is required, but that precludes the use of that pin
for FSK RTTY.

If you want to both FSK RTTY keying and a lockout, the Y-BOX also makes it
easy to connect an external +5V source, giving you an "all mode" lockout
circuit.

See the following for wiring examples.

   - Simple SO2R Lockout Circuit for CW and Phone
   <https://www.kkn.net/~n6tv/Y-BOX/Features.html#SimpleLockout>
   - Simple SO2R Lockout Circuit for All Modes Including FSK
   <https://www.kkn.net/~n6tv/Y-BOX/Features.html#FSKLockout>

The 5V *regulated* linear power supply that I highly recommend for the
latter implementation is Jameco P/N 1953612
<https://www.jameco.com/webapp/wcs/stores/servlet/ProductDisplay?catalogId=10001&freeText=1953612&langId=-1&productId=1953612&storeId=10001&>
.

Be advised that these are not sophisticated "first one wins" or "last one
wins" lockout circuits.  If both radios try to transmit at the same time,
*neither* will be able to transmit.  That's because the KEY OUT line will
still close, even when a radio is inhibited from transmitting any RF, and
that locks out the other radio.

In way, that's a good thing, because both operators hear what is going on.
It is also makes it impossible for both rigs to transmit at the same time,
for even 1 ms, so you can safely power two K3s with a single 25A power
supply.  This lockout system works safely with all potential keying methods
including internal and external keying, K3 memories, KY commands, VOX
tripping, etc.  Other lockout systems using SO2R boxes or software commands
cannot provide the same safety.

But, attempting to give one radio "priority" by opening its TX INH line
does not work very well.  If a K3 is currently transmitting when its
Inhibit pin goes low, it takes approximately 10-500 ms before the RF
actually stops, so using a single power supply is no longer safe (it can
overload briefly), and there is technically a contest rules violation
because it's possible for both radios to transmit at the same time (though
briefly).

73,
Bob, N6TV

On Fri, Oct 19, 2018 at 12:46 PM Don Wilhelm <donwilh at embarqmail.com> wrote:

> Brian,
>
> That is not how you implement a pull-up resistor.
>
> Connect one end of the resistor to a +5 volt source.  Connect the other
> end to the TX INH pin.
> Then connect the control line between the TX INH pin and ground.
> If the control line is an open circuit when it should be inhibiting, set
> the TX INH menu to inhibit on high.
> OTOH, if the control line is closed circuit while inhibiting, set the TX
> INH to inhibit on low.
>
> You don't have to put the pullup resistor right at the ACC connector, it
> can be (and should be) near the contacts which control the open/short
> contacts.
>
> 73,
> Don W3FPR
>
> On 10/19/2018 12:47 PM, Brian Moran via Elecraft wrote:
> > With my early K3, I'm attempting to control the TX_INH line on the ACC
> Connector, and am finding this odd (to me) behavior.
> > Starting with the radio off, I'm driving the pin with either 0 or 5v
> through a 6.6K resistor per the Elecraft manual. I set the TX_INH mode to
> HI in the config screen, and even with the line NOT driven, TX_INH is
> indicated by the flashing TX in the display.
> > If I get out my Fluke VOM, I see that there's around 0.2v measured at
> pin 7... BUT THEN THE TX INHIBIT goes off (that is, it starts working as
> described in the manual)?  Is the TX_INH line floated in the radio?
> >   Do I need to also "pull down" this line to ground through a suitable
> high-value resistor?
> >
> > Adding a 470K ohm to ground seems to make everything happy. If this is
> expected behavior (requiring pull down as well as pull up), perhaps this
> could be mentioned in the manual in the future (maybe it only matters to my
> era of K3 radio -- #512)
> >
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