[Elecraft] Problems with band decoder

[Don Wilhelm]

Instead of the band data lines being loaded down so the logic "1" value 
is not reached, it is more likely that the RS-232 lines are not being 
pulled to the logic "0" level because of pullup resistors in the band 
decoder receiver inputs.

If the band decoder that you are using has pullup resistors, this may 
indeed be the case.

The first K3s did not have pullup resistors in the K3 - the band decode 
outputs were just FET open drain.  That meant that a band decoder was 
expected to have the pullup resistors on the decoder end, but many do not.
Elecraft then added pullup resistors to the K3 outputs rather than 
requiring that customers modify their band decoders.  That is not the 
best solution for logic drivers and receivers (the pullup resistors 
should properly be at the receiver end, not at the driver end), but it 
was 'necessary' in ham circles where some devices expect the drivers to 
have the pullup resistors.

Enter the KPA500 - the KPA500 went to extreme measures by adding diodes 
to the band data lines to prevent damage (and confusion) for the band 
data lines from devices that did add pullup resistors at the band data 
receiving end and those pullups were to voltages higher than +5 volts.

So you may be faced with a situation where your band data decoder *also* 
needs diodes to isolate the voltage of the pullup resistors at the 
receiver from the K3/K3S band data lines.

Adding a series diode in your band decoder with the cathode pointing to 
the K3 band data lines should cure the problem.

73,
Don W3FPR

On 7/12/2016 8:42 PM, Randy Farmer wrote:
> You don't say, but I presume the band decoder is operating 
> correctly.Since the band change function of the KPA500 works, this 
> says that the serial data bus between the amp and the radio is working 
> fine. The band decoder may be loading down the 4 band data lines to 
> the point they don't make the logic "1" voltage the KPA500 needs. It's 
> probably a major pain, but try measuring the voltages on the 4 band 
> data lines with a DVM with everything connected and report back the 
> logic "1" (most positive) voltage you see on the data lines as you 
> cycle through the bands. The KPA500 should still change bands properly 
> if you give it a short shot of RF, but the situation should really be 
> corrected.
>
> 73...
> Randy, W8FN
>
> On 7/12/2016 11:07 AM, Jim Spaulding wrote:
>> I am using a Y cable from the K3 Aux port to a KPA500 and a WX0B band 
>> decoder for a six pack antenna switch.   When the Band Decoder is 
>> added, the KPA500 does not automatically follow band changes from the 
>> K3 although band changes on the KPA500 flow to the K3.  Has anyone 
>> else had this problem and is there an easy fix?