Posted by
Jack Brindle on
Mar 08, 2009; 6:13am
URL: http://elecraft.85.s1.nabble.com/Pull-up-Question-tp2441352p2443607.html
To add a bit more to Don's last point, there is a situation that few
folks (especially engineers) realize with respect to interconnect
drive systems. With todays low-power microcontrollers and digital
systems, it is very easy to accidentally power another device from the
driving device. In fact it is a pretty big problem in high-
availability (usually telco) systems where one board is supposed to be
powered off, but is being powered through it's I/O pins from another
board. The two solutions to this are very similar, basically doing as
Don suggests, using open-collector (or open-drain) drivers with
pullups on the receiving board. This eliminates any chance of
accidental power being applied to the receiving system. The second
solution is to use opto-isolators to isolate the two boards, but this
is really the same as #1 in that the receiving board provides the power.
In this case the decoder could actually be powered by the K3 through
its pullups, although they are limited by the high resistance of the
pullups. In radios with true TTL drive there is much more power
available to power a band decoder through its IO lines. We mitigate
this in the KRC2 by providing series limit resistors which effectively
limit the problem. It still exists, but at a much lower probability.
So what is the real problem if this happens? The device will not be
properly powered, so not all circuits will work. Worse, it will try to
function, attempting to provide drive to other circuits even though
not enough current is available. In the end, the poor microcontroller
tends to toast itself.
This is actually a very interesting area of study for systems that has
emerged in the past several years, and as I noted is of great
importance in the telco / high availability systems arena.
-Jack Brindle, W6FB
=======================================================================
On Mar 7, 2009, at 11:56 AM, Don Wilhelm wrote:
> 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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