[Elecraft] K3S interface problem with KPA500/KAT500

Tue Apr 29 14:42:56 EDT 2025

Take a good look at the KPAK3AUX cable table on page 45 of the KPA500 rev E1 manual. There are several pins that are not connected between the K3 and the KPA. There are also some that should not be connected, such as pin 8, theelecraft at mailman.qth.net power on/off pin. This pin on the KPA500 is not compatible with the one on the K3/K3S, and they should not be connected. There are a few other pins that likewise should not be connected. On the E850463 cable, these pins have been cut at the factory. I believe there is also a cable that may be supplied that has all 15 pins connected straight-through. Don't use this cable!

The Table shows which pins need to be connected; with the exception of pin 15, which is optional, and really not needed with the K3, all these pins should be in place and connected. Also be careful - it is very easy to ben the pins on the HD-15 connector. Doing so will be cause for a lot of four-letter words and pulling of hair. They can be bent back if you are ***really*** careful, but probably a replacement cable will be needed.

The K3S manual, rev A1, describes the Aux signals in a table on page 20, with further signal descriptions in the text of that manual. The AUX IO pins that the KPA500 uses are described in the table on page 44 of the KPA500 rev e1 manual. You really should take a good look at the manuals in this area to understand what is carried on that cable and what may be important here. The BAND signals are on the pins as follows:
Band0 - pin 13, Band1 - pin 3, Band2 - pin 9, and Band3 - pin 14, with GND connections on pins 5 and 12. The output levels of the BAND signals are described on page 21 of the K3S manual.

After you go over these manuals, I am sure you will have more questions. At this point we need to make sure that the K3S is properly sending the BAND signals - it should be. The fact that the KPA is not reacting to them indicates a disconnect between the two units. That usually occurs in the cable or the cable’s connectors, but your situation seems different. If you can determine that the K3S Band signals are indeed being output, then we will have more puzzlement. ;-). The Band signals have pull-ups on them when the K3S is powered, so you should see these signals change from 0V (ground) to +5V as the K3S changes bands.

And, no we do not document the Auxbus protocol. It works, which is pretty much all that is important. It is carried on pin 2 of the Aux cable. We know that is working since the KPA is able to communicate with the K3S. It is the other way that is the problem. The only signaling from the K3 to the KPA is the band data on the Band pins.

73,
Jack, W6FB

> On Apr 29, 2025, at 12:13 PM, Terry Conboy <n6ry at arrl.net> wrote:
> 
> Jack,
> 
> Thanks so much for your thoughtful reply.
> 
> RADIO=K3 is set in the KPA 500.
> 
> I connected the K3S directly to the KPA500 with one of the E850463 (KPAK3AUX) cables.  The K3S still follows the KPA500, but not the other way.  I tried the other cable in its place with the same result.  Status messages from the KPA500 do appear in the K3S display.
> 
> I’m not sure what you mean about the "cut pins" on the cables.  I’m just using the stock Elecraft cables without the E850462 keying line interrupter.
> 
> Your suspicion about a problem with the K3S Aux I/O port seems well founded.  Do you know if there is any documentation about how the port signaling operates?  There are the four BCD lines (pins 3, 9, 13, & 14) and one from the K3S labeled AUXBUS (pin 2), but not much info about how the bus knows what device gets to “talk” on the bus.  I didn’t see anything on the Elecraft site, but it could be included in another manual.
> 
> Thanks & 73,
> Terry N6RY
> 
>> On Apr 28, 2025, at 10:58 AM, Jack Brindle <jackbrindle at me.com> wrote:
>> 
>> Make sure that you have RADIO = K3 in the KPA500 menu. I suspect you do since you indicate the K3 is following the KPA500 when you press one of the band buttons on the KPA. In the KPA, RADIO must be set to either K3 or BCD for it to follow the K3 BAND signals. If it is set to K3, then the KPA will send information to the K3 when something happens. 
>> 
>> The KAT listens for communications from the K3 on the Aux I/O data bas, and should be changing its selector bins based on the frequency the K3 sends it.
>> 
>> This all makes me suspect that something is wrong with the K3’s AUX I/O port. You can (carefully) look at the voltages on the AUX cables to make sure the BAND signals are changing as they should. If not, then the issue is in the K3. 
>> 
>> My suggestion is to check things out one at a time. Connect the KPA500 directly to the K3 using the proper cable (this one should have pins cut at one end). You should be able to get that going without transmitting - when the K3 changes bands, the KPA should follow. This is completely due to changes in the K3’s BAND signals. Once that works, then add the KAT500 to the system. Remember, the KAT connects between the K3 and the KPA so that it can disable the PTT going to the KPA when the KAT needs to tune.
>> 
>> These may seem like very simple steps, but they should isolate the problem and get us closer to a working system.
>> Let me know how your tests go.
>> 
>> 73,
>> Jack, W6FB
>> 
>> 
>>> On Apr 28, 2025, at 12:20 PM, Terry Conboy <n6ry at arrl.net> wrote:
>>> 
>>> I bought the 500 pair last fall and connected them to my K3S (2017 vintage) with the official Elecraft cables (which I've checked for correct pin connections and no shorts).  All three units are at current software versions.  The issue I have is that if I change bands on the K3S, nothing happens with the tuner or amp.  However, when I change bands on the KPA500, the K3S follows as expected.
>>> 
>>> I have the internal tuner in the K3S (disabled when using the 500 pair).  I set the CONFIG: KAT3 for KAT500Y.
>>> 
>>> Of course, the tuner and amp detect the TX output and switch quickly, but I’d like to have the tuner follow the K3S for WSPR receive.
>>> 
>>> I’ve corresponded with the customer service crew at Elecraft, but no help so far.  Anyone have any ideas about Config settings or anything else to check?
>>> 
>>> Thanks,
>>> Terry N6RY
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> 