http://elecraft.85.s1.nabble.com/Pull-up-Question-tp2441352p2443435.html
Let's see ... that would apply to the KRC2 in the "parallel input
mode" along with the W9XT BCD-10 and the Yaesu Quadra for starters.
have internal pull-up resistors.
... Joe, W4TV
> -----Original Message-----
> From:
[hidden email]
> [mailto:
[hidden email]] On Behalf Of Don Wilhelm
> Sent: Saturday, March 07, 2009 2:56 PM
> To: Roger
> Cc: elecraft
> Subject: Re: [Elecraft] [K3] Pull-up question
>
>
> Roger,
>
> That is where the situation gets 'sticky'. It all depends on
> what the
> circuits at the end away from the K3 have implemented. You have two
> pullup resistors on each line and 2 separate voltage sources.
>
> 1) If the voltage to the pullup resistors in the internal
> device do not
> match the voltage to the pullup resistors in the K3, there will be a
> continuous flow of current through both pullups on any one
> line when the
> drivers are at their open, non-conducting state). If the
> voltages are
> close to the same value, this may not cause a problem, but if
> they are
> quite different (say one is 12 volts and the other is 5 volts), that
> could create a big problem.
> 2) The drivers in the K3 (when at the low level) must carry
> the current
> provided by both pullup resistors. Whether that is a problem or not
> depends on the value of the pullup resistors and the current
> capability
> of the drivers.
>
> In my view of a good design, open collector (or open drain)
> drivers are
> the only thing to use. The terminating device can determine what
> voltage is needed for reliable switching at its input and that device
> controls the high level by providing its own pullup resistors.
> Secondly, the pullup values can be sized according to the high level
> input current required by that device and can control its immunity to
> noise by controlling its input conditions. All is under the
> control of
> the external device designer. Unfortunately, much of the ham gear
> available uses bare, open inputs and depends on the driving device to
> provide the voltage and levels it needs at its inputs.
>
> 73,
> Don W3FPR
>
> 73,
> Don W3FPR
>
> Roger wrote:
> > Mike Fatchett,
> > OK, but what happens when you use
> them with devices that already work without them,e.g. Array
> Solutions 6 Pack and early band decoders?
> > Roger K8RS
> >
> >
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