[Elecraft] KPA-500 Question

[Don Wilhelm]

  Joe is correct that the diodes will not prevent the K3 drivers from 
seeing the higher voltages from an external band decoder, and if the 
voltage is higher than the 5 volt level that the K3 resistors are tied 
too, current will flow through the two pullup resistors.  The real cure 
is to remove the pullup resistors from the K3 (that can be done, the 
driver chip is still open drain).

The K3 driver is not likely to be damaged - the driver IC is a 
TPIC6C595, and its data sheet says the voltage can be as high as 33 
volts and the open drains can sink as much as 100 ma. continuously 
(although I would suggest a safety factor of up to 50%).

To restate - the problem is current through the two pullup resistors 
(which are powered from two different power sources, and can best be 
cured by removing the pullup resistors from the K3.

Elecraft added the pullup resistors some time ago so users would not 
have to add pullup resistors to those band decoders that did not have 
them, but since then we face this situation with those band decoders 
that do have pullup resistors installed.

73,
Don W3FPR

On 7/10/2011 3:47 PM, Joe Subich, W4TV wrote:
>
>
>> 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
>