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

[Edward R Cole]

Brian,

Page 19  of K3 manual near bottom of second column tells what you 
need to know.  Using TX INH set to HI=inh you apply about 5v to keep 
radio inhibited from transmitting.  To active TX you take line 7 = LO 
(ground).  Elecraft shows how you can wire +12V thru a 2.2k to 10K 
resistor to provide 5v to pin 7 (6.6K probably will be fine to do 
this).  You need to ground pin 7 to enable RF OUT IN TX.

Or provide 5v to pin 7 thru a 1K resistor and ground pin 7 to 
TX.  The resistor limits current when the line is grounded;
See:  http://www.kl7uw.com/TX-INHIBIT.htm

pin 7 needs to be HI=5v or LO=grounded.  Leaving pin 7 unterminated 
(floating) will result in erratic operation.

73, Ed - KL7UW
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From: Brian Moran <brianmo at yahoo.com>
To: Elecraft Reflector <elecraft at mailman.qth.net>
Subject: [Elecraft] TX_INH input (pin 7 on ACC) behavior


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)

What makes this a little more difficult to debug is that after the 
line has been measured with the meter, causing it to work, TX_INH 
sometimes continues to work (High=inhibit, LO=tx). Until I power 
cycle the radio.
Anyone who has done this before have any insight?thanks,-Brian N9ADG


73, Ed - KL7UW
   http://www.kl7uw.com
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