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

[Don Wilhelm]

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)
>