[Elecraft] Weird issue K3/KPA-500

[Don Wilhelm]

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.
>