[Elecraft] Transmit Inhibit

[Don Wilhelm]

Clay,

The answer lies in your driving device and not the K3.  Yes, a 12 volt 
source may damage the K3.
A simple voltage divider will not do the whole job.  That output must 
swing from 5 volts to zero volts., so at the the time the output should 
go to zero volts, the voltage divider will still be producing voltage.

Look at the schematic for the driving device.  What is the device 
providing the output?  A relay, or a solid state device?
If a relay, is it a SPDT relay with the output taken from the common? 
Is the NO contact connected to ground?  Is the NC contact connected to 
+12v - if so, remove the source of 12 volts and connect a 2.2 to 10k 
resistor to that contact with its other end connected to a source of 5 
volts.

If instead of a relay, the output is a solid state device (transistor or 
FET) disconnect the collector (or drain) resistor and replace it with a 
2.2k to 10k resistor with the other end connected to  source of 5 volts 
instead of 12 volts.

In other words, you will have to modify the driving device - 
alternately, you could add a device to the circuit (internally or 
externally) which takes the 12 volts as an input and uses a 2.2k to 10k 
collector (or drain) resistor terminated in 5 volts - a 2N7000 FET would 
work fine.
That output would go from 0 volts when the input is 12 volts and go to 
+5v when its input is 0 volts.

73,
Don W3FPR

On 6/8/2018 11:12 PM, Clay Autery 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)
>