K3 FSK External Pullup

[kk7uq]

Is an External Pullup resistor needed on the FSK (ACC pin 1) input line?
One of our Navigator Interface users reported that if the FSK line is also
tied to an external pullup resistor, that it  caused his K3 to refuse to
stay powered up when the cable was connected.  He subsequently removed the
pullup and the system operated normall.

The pullup resistor is a 4.7K through a diode to +5V.  Our original cables
for the K3 were wired with this pullup connected, and no problems have been
reported until now.  We will remove the pullup connection from our drawing
since it is not needed, and all future cables will not have this connection.

Has there been a change in the K3 that would cause this problem?  If so, if
earler users upgrade their units to lastest specs, will they also have this
problem?

Clint
KK7UQ



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[Don Wilhelm-4]

Clint,

I looked at the manual, and the K3 FSK input has an internal pullup
resistor, so none is required externally.
That is unless the logic signal you are using to drive the FSK input has
some unique requirements (like needing more saturation current than the
K3 pullup resistor provides.  A normal TTL level open collector or open
drain driver should operate that line just fine.

On the K3, it is the *outputs* that need external pullup resistors - the
K3 must provide pullup resistors for its *inputs* so the K3 does not do
'funny things' when those inputs are left open.  A floating logic input
is subject to noise and erratic action - and that is true for any
device, not only the K3.

73,
Don W3FPR


Clint Hurd wrote:

> Is an External Pullup resistor needed on the FSK (ACC pin 1) input line?
> One of our Navigator Interface users reported that if the FSK line is also
> tied to an external pullup resistor, that it  caused his K3 to refuse to
> stay powered up when the cable was connected.  He subsequently removed the
> pullup and the system operated normall.
>
> The pullup resistor is a 4.7K through a diode to +5V.  Our original cables
> for the K3 were wired with this pullup connected, and no problems have been
> reported until now.  We will remove the pullup connection from our drawing
> since it is not needed, and all future cables will not have this connection.
>
> Has there been a change in the K3 that would cause this problem?  If so, if
> earler users upgrade their units to lastest specs, will they also have this
> problem?
>
> Clint
> KK7UQ
>
>
>
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