[Elecraft] K3 - FSK hookup to ACC connector

[David Woolley (E.L)]

N2TK wrote:

> For FSK, the manual states that pin #1 (FSK IN) in ACC jack has a pull-up
> resistor to 5V. So I assume the standard NPN interface from a COM port will
> work fine for that.

Standard COM ports are *not* open collector.  They have to actively 
drive in both directions.  Any input to which you connect them needs to 
tolerate up to 12 volts open circuit voltage, in either polarity, with a 
300 ohm source impedance.  This may well require protection diodes to 
prevent the substrate diode on the receiver being forward biassed.  It 
might also require clamping the positive excursions to less than 5 volts.

I'd suggest the simplest approach is to use a resistor and an 
opto-isolator, with a reverse diode across the isolator input, to avoid 
reverse breakdown of the LED.





-- 
David Woolley
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