[1000mp] PTT Interface

Wed, 25 Dec 2002 11:48:57 -0500

I played with an issue like this a few years ago.  Here is how I approached
the problem.

First determine what the rig requires.  Use a resistor substitution box or
bunch of resistors to determine at what point the rig keys properly.
Measure this voltage.  This is the voltage that the Opto or transistor will
need to pull down to.

Now check the specs of the interface box.  Will the Opto pull down to this
level?  Sometimes its just a matter of driving the opto a little harder.
Check the spec to see what the maximum current is that the opto will take.
Size the input resistor to deliver this level of current.

When building my own opto interfaces, I try to pick the opto to deliver a
very low final pull-down voltage.

Jeff W4DD

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Carl Kratzer
Sent: Wednesday, December 25, 2002 7:47 AM
To: [email protected]
Subject: [1000mp] PTT Interface

Has anybody used an optoisolator circuit to activate the PTT line on the
rear
of the radio, using either the RCA jack or the PTT line in the "packet"
connector?  I tried to interconnect a Rigblaster NOMIC sound card interface
to
the rear panel "packet" connector on my Mark V Field.  After considerable
experimentation, I determined that, while the rigblaster would work fine on
the front-panel mike connector (whose PTT line is approximately +6 volts) as
well as the CW jack, it would not activate the PTT line on the rear of the
radio, which is +13.6 volts.  I am reasonably certain that I have eliminated
miswiring of the cable as the possible cause.  Apparently, the optoisolator
circuit in the rigblaster NOMIC is not compatible with the higher voltage
PTT
line on the rear of the Mark V Field.  The circuit used in the NOMIC is a
4N33
optoisolator driven by RTS or DTR line from PC serial port through diode and
680 ohm resistor.  The output side of the 4N33 has a floating base lead,
with
the emitter and collector leads used to key the radio PTT.

Any suggestion for a simple mod to the circuit to allow compatibility with
the
rear PTT/packet jacks?  Thanks in advance.

Carl, K3RV.

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