[Elecraft] Transmit Inhibit

[Bob Wilson, N6TV]

What amp. are you using and why do you need to use TX INH?  Perhaps it has
KEY IN / KEY OUT, and you can use that instead, to activate PTT on the
radio only when the amp. is "ready".  PTT signal or footswitch keys amp.,
amp. keys K3 PTT IN, instead of PTT/footswitch keying radio, and radio
keying amp.  Or you could set CONFIG:TX DLY to a large value.

If you must use the TX inhibit pin, 5V is available on K3 ACC Pin 1 (FSK
keying).  I use that plus a single resistor on my Simple K3 Lockout Circuit
(see https://bit.ly/K3lockout which downloads a PDF schematic).  The Y-BOX
<https://bit.ly/Y-BOX> has this lockout circuit built in, as well as a 10K
pull-up resistor for a separate 5V source if the FSK pin is already in use
for RTTY.  But, the circuit uses TX INH LO and you need TX INH HIGH?

You could use the 12V from the amp. to key the base of an NPN transistor
(through a 1K resistor).  Ground the emitter and and connect a 470 ohm
resistor to the collector.  Connect other side of the resittor to the TX
INH pin and +5V (Pin 1), as in my lockout schematic.  When your amp outputs
12V, the transistor closes the circuit, and the INH pin goes low.   When
your amp. outputs 0V, that opens the circuit, and the INH line goes high
(+5V).

73,
Bob, N6TV

On Fri, Jun 8, 2018 at 8:12 PM, Clay Autery <KY5G at montac.com> wrote:

> I'm going to use ACC Pin #7 as Transmit Inhibit (High), and the manual
> says this:
>
> "TX INH (Transmit Inhibit Signal)
> Pin 7 of the ACC connector can be configured as a
> transmit inhibit input by setting CONFIG:TX INH
> to LO=Inh (or HI=Inh). Holding pin 7 low (or
> high) will then prevent transmit. An external 2.2 to
> 10 K pull-up resistor (to 5 VDC) is required.
> If TX INH is set to OFF, pin 7 reverts to its
> default output function, K3S ON (see above)."
>
> This has confused me a bit...  I'm not an electronic expert. The input
> signal is 12 vdc, present all the time EXCEPT when the K3s keys up... the
> amp gets that message and drops the signal to ) vdc...  Not sure how to
> pull off reducing the input from 12 to 5vdc with a single external resistor.
> I DO know how to do it with a voltage divider....  Vout = Vin x
> R2/(R1+R2)  Calculate resistor values, tie them together at one end and
> that end connects to Pin #7 at K3s end.  R1's other end ties to the (+)
> signal in from amp, and R2's other end ties to the ground (conductor on Pin
> #5).
>
> Is there a simpler way to do this with a single resistor?  Am I missing
> something?
>
> And am I in fact reading the manual correctly?  The K3 expects a TTL logic
> level + 5 vdc on Pin #7 to activate inhibit with TX Inh set to "high"?  A
> straight 12vdc exceeds the spec for that pin as an input?  (When it's used
> as the K3s ON output to XVERTER, it OUTPUTS +5 VDC)
>
> I'm trying to get this custom cable built for this amp this weekend....
> I'd appreciate a little help here.  I don't want to let the smoke out of
> anything....
>
> 73,
>
> --
> ______________________
> Clay Autery, KY5G
> (318) 518-1389
>
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