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
"The Elecraft list is a forum for the discussion of topics related to
Elecraft products and more general topics related ham radio"
List Guidelines <http://www.elecraft.com/elecraft_list_guidelines.htm>
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