[Elecraft] K3 Key-down on power-up

[Don Wilhelm]

Dave and folks,

That behavior is a natural consequence of an unpowered RS-232 signal 
line.  The voltage on the RTS (or DTR) input to the K3 will be zero or 
close to it when the computer is not powered. If you check the R-232 
signal levels you will find that the region around zero volts (+3v to 
-3v) is "undefined" - the system is designed that way to include noise 
immunity - the RS-232 receiver output might be either ON or OFF when its 
input is at zero volts - it is not predictable and the real state 
depends on the behavior of the particular LT1039ACN driver/receiver chip 
in the K3.  Couple that with the fact that the RS-232 'no signal' or 
'Space' state must be between +3 and +15 volts to assure an OFF 
condition at the output of the RS-232 receiver - there is just no way to 
avoid the 'problem'.

As a result, if your K3 is set for PTT-KEY from the RTS or DTR lines, it 
would be prudent to power the computer on before the K3 so these input 
signals are at a defined state (hopefully 'space').  Even unplugging the 
cable will not help because that will also result in zero volts at the 
input.

73,
Don W3FPR

Dave Hachadorian wrote:
> I've got my K3 set up for cw on Field Day with a laptop usb port driving 
> a usb to serial converter. Rig control and cw keying are done over the 
> serial cable, using the K3's built-in ability to send cw from pin 4(?) 
> of the rs-232 cable.
> 
> Everything works fine, but I got quite a scare when I turned on the K3 
> and it came up keyed-down 120 watts into no antenna.  It seems that if 
> the laptop is not running the logging program, rs-232 pin 4 can very 
> well be in a key-down state. I don't see any way to prevent this event 
> from happening again, except to unplug the rs-232 cable, turn the rig 
> on, make sure vox is off, turn off the K3, plug in the rs-232 cable, and 
> then turn the K3 back on again.
> 
> Once it happens to you, you get gunshy about turning the rig on with 
> rs-232 connected. Maybe there could be some sort of inhibit to prevent 
> this from happening? Or maybe someone can suggest an easier way to 
> prevent this from happening. On my old TS-850, there was an actual 
> switch to disable vox, and I always disabled vox before turning on the rig.
> 
> My permanent setup back home uses LPT keying, so I can leave the 
> computer key jack unplugged.
> 
> Thanks.
> 
> Dave Hachadorian, K6LL
> Big Bear Lake, CA
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