[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.

It has been pointed out to me, off list, that the standard interface 
here might not actually be referring to the standard interface of a COM 
port, but to a commonly used interface circuit between the COM port and 
the transceiver key input, such as given in 
<http://www.aa5au.com/gettingstarted/rtty_start6.htm> in relation to the 
MMTTY software.

If that was what you did mean, the circuit I was referred to is an open 
collector NPN buffer with a 1K base resistor.  I was going to say that 
that circuit would work, but the more I think about it, the less I like 
it.  The absolute maximum emitter base voltage for this sort of 
transistor is typically 5 or 6 volts, and this circuit will try to 
exceed it by a good 4 to 5 volts.  I guess the junctions zeners and the 
resistor limits the current to levels that don't cause damage, but it is 
still operating out of its safe operating area.  That can be fixed by 
putting a diode across the base and emitter.  (Alternatively, as a 
result of production spreads, many specimens of the transistor may 
actually have break down voltages that are high enough not to cause a 
problem.)

Also, 1K is far too low a value.  To comply with RS232, it needs to be 
at least 4K, and assuming the pullup is the internal pullup in a PIC and 
and using a 2N2222 as an example, the current gain will be at least 35, 
which means that a resistor of several 100K will saturate the device 
when driven from an in spec serial port (820K for a standard value, 
maybe 470K, for some safety margin).

I still prefer the opto-isolator approach, because it is intrinsically 
safer and gives very good RF isolation.  With a (Darlington) 
opto-isolator, interface the K2 PTT, I used 5.1K resistors, but such 
isolators only have current transfer ratios of around unity.



-- 
David Woolley
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