[aprssig] Mic-E and HTs

Robert Bruninga bruninga at usna.edu
Tue Dec 13 08:01:14 CST 2005

>>> "Scott Miller" <scott at opentrac.org> 12/12/05 4:22 PM >>>
>Right, I follow all of that, but it doesn't address 
>the issue of most handhelds ... which key PTT 
>through the audio line. 

Works the same way, but in addition to the diode
bypass from the PTT input to the PIC to the PTT
output from the pic, you just add a capacitor too.
Ill draw it...

See http://www.ew.usna.edu/~bruninga/mic-ePTT.GIF

de WB4APR, Bob

----- Original Message ----- 
From: "Robert Bruninga" <bruninga at usna.edu>
To: <aprs at kd4rdb.com>; <aprssig at lists.tapr.org>; <scott at opentrac.org>
Sent: Monday, December 12, 2005 1:15 PM
Subject: RE: [aprssig] Mic-E and HTs

Mic-E PTT mode of operation:

> First it has to sense what the status of PTT is so that it
> can know when you release PTT so it can inject the packet.

In PTT mode of operation, the Mic-E processor has to
have a PTT input and a PTT output.  The microphone
PTT switch has to be connected to the Mic-E PTT input
and then the Mic-E processor has to then detect this
and then output its own PTT onward to the rig.

This is so that the Mic-E processor can sense the
release of PTT by the operator, but *still keep PTT
asserted* on its output until after the packet
burst.   There cannot be any drop of PTT to the
rig between release of the operator and end of
the packet, or all the advantage of the PTT mode
is lost because the TX will begin to shut down,
the PLL will begin to switch to receive, and the
receiver squelch of the other guy will begin to act.
And this would then require an additional TXD
delay to allow all that to happen.  Thus, turning
the 150 ms Mic-E PTT mode burst into a full
sized 500 mS packet.

So, the Mic-E input PTT and PTT output are separate.
Now, it is a good idea to connect a diode from the
PTT input circuit to the PTT output circiut so that
the microphone and radio will work normally even if
the Mic-E circuit is turned off or the Mic-E processor
fails.  COnnect this diiode with its cathode on the
Mic-E input and its anode on the Mic-E output.

Hope that helps
Bob, Wb4APR

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