[aprssig] Transmit now pin

Wes Johnston, AI4PX wes at ai4px.com
Wed Feb 23 09:20:17 EST 2011

I'm cross posting this to a few aprs groups....

One of our locals is complaining about the muting effect of his tracker.
His radio is being desensed by the aprs tx on the 2nd radio.  So I'm
thinking of alternatives for him.

1)Use the tracker as a mic encoder that transmits on the rising edge of the
transmit now pin.  The packet is squirted into the voice rig and picked out
at the repeater site.  I think both the OT2 and the TT can hook up in
parallel with the mic jack and monitor the PTT and reassert it.

2)Use the tracker on a separate radio but run the PTT line from the mic to
the "tx now" pin on the tracker.  This way when he unkeys the tracker will
tx a position off the 2nd radio in the car.  Maybe an opto-isolator to keep
things playing nicely.

The issue with either of these solutions is that the trackers need to be
able to transmit at a very much reduced rate on their own when that TX now
pin hasn't been activated.   I think it's possible to program a tracker (OT2
or TT) to transmit at a 20 minute interval and also use the TX now pins....
but in an ideal setting, I'd like the TX now to only tx every 2 or 3
minutes, not every time the person keys up.  In other words, I'd like my
position sent when I start talking but not every time, and I'd like my
position sent periodically even when I'm not talking.  Of course the tracker
needs to be respectful of silence on the repeater output freq.

I'd say the issue of being able to TX automatically even when in mic-e mode
is pretty important.  If the only time my tracker transmits is when I'm
acutally using the voice rig, that is a deal breaker and defeats the purpose
of knowing where a car is...

Another issue that comes to mind is that of TXDelay.  If the transmission is
triggered by the TX now pin, we can assume that the radio is already on air
and doesn't need a long TXd (maybe 50ms??), but if the transmission is
triggered by a timer, the txd needs to be longer (maybe 150ms?).  In the
end, I guess this is trivial, but it'd be nice.

Anyone fooled with any of this?  I'll probably set up a tracker to test all
of this, but if anyone's done it I'd love to hear about it.

"Language shapes the way we think, and determines what we can think about."
-- B. L. Whorf
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