[Elecraft] [K3] Band data problem

[Joe Subich, W4TV]

> Paralleling the 2.2K resistors with 820ohms brings the combination
> to 600ohms which in series with the 220ohms results in 820ohms to +5V
> which when driving the RCS12 inputs yields 3V remaining on the band
> data lines. This provides a sufficient level for a "1" with
> reasonable noise margin.

I would be careful when decreasing the value of the pull-up resistors
in the K3.  The "Band Data" outputs are open drain - decreasing the
pull-ups increases the current the outputs must sink at logic low on
top of any current sourced by the external devices.  It might be better
to buffer the RCS12 using something like the 74ACT244 (at less than
$0.25) to drive the 2N3904s.

73,

    ... Joe, W4TV


On 12/2/2012 8:53 AM, Jim Miller wrote:
> A follow up to the K-Line and RCS12 integration. The problem was that the
> RCS12 was excessively loading the band data lines. The fix was additional
> drive on the pullups on those lines inside the K3. I used 820 ohms in
> parallel with each of the internal 2.2K pullups and all is now working well.
>
> Details:
> The RCS12 band inputs drive a 2N3904 common emitter buffer using a 1K base
> drive resistor. This puts one Vbe drop plus 1K directly on the K3 Band Data
> lines. These lines have a 2.2K pullup and a series 220 protection resistor
> for a total of 2.4K to +5v. A quick bit of math shows that when this is
> loaded with the RCS12 the resultant voltage on the band data lines falls
> below 2V. Although the RCS12 is happy, anything else expecting to read the
> band data lines will show zeros. The KPA500 will show 5.3Mhz (60m)
> indication as a result.
>
> Paralleling the 2.2K resistors with 820ohms brings the combination to
> 600ohms which in series with the 220ohms results in 820ohms to +5V which
> when driving the RCS12 inputs yields 3V remaining on the band data lines.
> This provides a sufficient level for a "1" with reasonable noise margin.
>
> The page 4 of the appnote on the Elecraft site
> http://www.elecraft.com/K3/mods/K3ALCMod_Rev_B.pdf shows a convenient way
> to implement this mod.
>
> 73
>
> jim ab3cv
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