I didn't want yet another piece of clutter in my already box and
cable littered shack so I chose to mod the K3.
In all probability an inverting buffer or possibly a pair of low drive,
dual optoisolators could replace the 2N3904 transistors directly. I
suspect the transistors are simply buffering and inverting the inputs
to a PIC of some kind and are there to protect against 12V inputs.
Alternatives are to add pull-ups (2K2 Ohm) in the RCS-12 (but that goes
to the same issue of added dissipation on the K3 drivers) or replace
the base resistors with 2K2 (the transistors should have sufficient
gain to work with 1 mA instead of 1.5mA base drive. Even adding 1K-1K5
in each of the band data lines (connector at the RCS-12) might work as
a "no modification/no external box" fix. However, since I don't have
an RCS-12 I have no way of trying it.
73,
... Joe, W4TV
On 12/2/2012 11:28 AM, Jim Miller wrote:
Hi Joe
There's certainly more than one way to deal with this. External buffer
chips are an inexpensive solution. I didn't want yet another piece of
clutter in my already box and cable littered shack so I chose to mod the K3.
The K3 driver chips are not an issue at all. Any additional external pullup
would add to their load but would also provide the desired additional
pullup drive and make it possible to increase the value from 820ohms to
perhaps 1K. The KAT500/KPA500 add a fraction of a milliamp in total so they
are not an issue. They are the only devices other than the RCS12 on the
band outputs in my case.
The only other consideration is that the increased internal pullup drive
(lowered resistance) puts more load on the 5v regulator sourcing current to
those pullups. Again not an issue.
73
jim ab3cv
On Sun, Dec 2, 2012 at 9:41 AM, Joe Subich, W4TV <[email protected]> wrote:
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.
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