[Elecraft] Weird issue K3/KPA-500

[W0MU Mike Fatchett]

Thanks Don.

Mike W0MU

J6/W0MU November 21 - December 1 2011 CQ WW DX CW
W0MU-1 CC Cluster w0mu.net


On 10/11/2011 11:27 PM, Don Wilhelm wrote:
> Mike,
>
> Your band decoder is apparently loading down the K3 band data lines to
> the point where they cannot be unambiguously decoded by the KPA500.
> Yes, less than 3 volts on the band data lines is in the "noise region"
> for digital logic.
>
> You may have to add isolation diodes and pullup resistors in your band
> decoder to solve the problem.
>
> It is true that the K3 has pullup resistors, but they can only supply so
> much current. Apparently, your band decoder requires more current than
> the K3 pullup resistors can provide.
>
> The solution is to add a diode at the band decoder Band 0-3 lines.  The
> cathode should point toward the K3.  Then on the anode side of the
> diodes, add a pullup resistor to whatever voltage is required by your
> band decoder.
>
> The early K3s "did it right" in providing open drain outputs for the
> band data lines, relying on the attached device to provide the pullup
> resistors.  BUT, many devices do not provide those pullup resistors -
> many radios have the pullups at the driver end rather than following the
> normal rules for logic circuits which place the pullups at the
> receiver.  SO, the K3 added pullup resistors at the drivers.
> NEXT, enter the KPA500, which now needed to provide protection for
> itself from band decoders which DO provide pullup resistors (but to a
> voltage higher than 5 volts), so the KPA500 added isolation diodes and
> its own pullup resistors to the band data lines.  That then requires
> that the high level for the band data lines be a diode drop level above
> the normal high logic level.  No problem if only the K3 and KPA500 are
> connected, but if a band decoder that takes current in the high level
> state is connected, the K3 will not supply enough current from its
> pullup resistors to bring the high logic level to an unambiguous (to the
> logic devices) high level.  In other words, the band decoder you have
> chosen is sucking too much current from the K3 when the band decode
> lines are in a high logic level state.  The addition of diodes on the 4
> band data lines (cathodes pointing toward the K3) is the only solution,
> and then you will have to add pullup resistors on the anode side of the
> diodes to assure (to the band decoder) that the high levels are properly
> detected.
>
> Bottom line - the K3 original design "did it right", but caved in to
> popular demand because many band decoders did not "do it right".  Then
> the KPA500 had to contend with devices that "did it right", but at
> higher voltages than the KPA500 could tolerate - so we are back to the
> original "question" of where the proper place for the band data pullup
> resistors should be.  I am sorry to say that it is quite a mixed bag of
> confusion since one band decoder does it this way, and another does it
> that way.  Right now, the only sensible solution is to add the
> protection diodes and pullup resistors to each band decoder that is
> attached to the K3 and KPA500 combination.
>
> 73,
> Don W3FPR
>
>
>
> On 10/11/2011 11:08 PM, W0MU Mike Fatchett wrote:
>> I am attempting to hook up an Array Solutions DBS-2 Band decoder on the
>> K3.  When I have it connected using a Y cable the KPA-500 no longer
>> follows the bands it seems to get confused.  I have the KIO rev B board
>> which I believe should have the pull up resistors but I am seeing 2.3 to
>> 2.5 volts on the Band out pins.  I think this should be between 3-5
>> volts.  Are there KIO rev B boards that might not have been upgraded?
>> SN on this K3 is 2745.
>>
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