[Elecraft] [K3] Pull-up question

[Don Wilhelm]

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
>   
>