[Elecraft] [K3] Pull-up question

[Joe Subich, W4TV]

> 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.

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. 
On the other hand the TopTen Systems BD-Y, the original microHAM 
Band Decoder, the Yaesu FL-7000, and Tokyo High Power HL-1.5Kfx, 
have internal pull-up resistors. 

73, 

   ... Joe, W4TV 
 



> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] 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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