[Elecraft] KPA-500 Question

[Joe Subich, W4TV]

> If the band decoder devices have their own pullup resistors
> (especially those who pullup to +12 volts), the K3 and/or the band
> decoder may have problems because of differences in the supply
> voltage to the pullups, and the cure is to add series diodes (with
> the cathode end toward the K3) to the inputs of the band data
> decoder.

Series diodes to devices with +12V pull-ups will not solve the issue
of "mixed" pull-up voltages.  The K3 will still see the higher of
the pull-up voltages (+5 in the K3, +5 in the KPA-500, +12 or +13.8
in the "foreign" band decoder).  The real issue occurs when the
pull up resistor in the "foreign" device is too small and the K3
can not sink the resulting current - or if the "foreign" decoder
uses low power CMOS inputs which requires a low state voltage
below 0.3V!

> The KRC2 used with the K3 avoids this situation since it uses the
> AUXBUS signal to determine which band the K3 is on.

microHAM Station Master also avoids this situation since it uses
the K3/P3 CAT bus (and provides an alternate CAT interface) for
its band data.

73,

    ... Joe Subich, W4TV
        microHAM America, LLC.
        http://www.microHAM-USA.com
        http://groups.yahoo.com/group/microHAM


On 7/10/2011 3:19 PM, Don Wilhelm wrote:
>    Fred and all,
>
> As Dick's post stated, the K3 band data outputs can be paralleled with
> the KPA500 lines.
> The KPA500 has internal protection diodes to isolate it from external
> band decoders that might damage the KPA500 band data input circuits.
>
> The only "catch" is that currently the K3 has internal pullup resistors
> to 5 volts on the band data outputs.  If the band decoder devices have
> their own pullup resistors (especially those who pullup to +12 volts),
> the K3 and/or the band decoder may have problems because of differences
> in the supply voltage to the pullups, and the cure is to add series
> diodes (with the cathode end toward the K3) to the inputs of the band
> data decoder.  The diodes prevent current from the external decoder's
> pullup resistors from flowing into the K3.
>
> The KRC2 used with the K3 avoids this situation since it uses the AUXBUS
> signal to determine which band the K3 is on.
>
> 73,
> Don W3FPR
>
> On 7/10/2011 2:25 PM, Fred Maas wrote:
>> Thanks for reply.  That is what I thought.
>>
>> I am not familiar with the various outboard devices to which you refer (as
>> have never needed them).  I am not suggesting that the KPA500 needs
>> additional antenna outputs though that is one way to do it.  That would
>> constritute an internal antenna switch.  The KPA500 represents many upgrades
>> from my 1980's amplifier so I am surprised that my 80's amp has a capability
>> that the KPA500 doesn't.
>>
>> The band data information from the K3 goes to the KPA500 to tell it what
>> band it is to be on.  Are you saying this can be paralleled safely and also
>> instruct another device?  I would think such might not be compatible and a
>> risky thing to do.
>>
>> In my set up (with an old Yaesu FL-7000) the band data information from the
>> K3 goes to the FL7000 to tell it which band to be on.   The FL7000 then
>> associates one of four antenna positions with a band, this is changeable at
>> any time and then remembered, and outputs relay control to a remote antenna
>> switch.  If the remote switch is not in use, the "antenna" which is
>> associated by band is irrelevant.  I am surprised that the KPA500 does not
>> have this capability standard and built in with the outboard remote antenna
>> relay being an option.  Without this, band changing is not, in fact,
>> automatic.
>>
>> 73,  Fred
>>
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