RRC 1258 Support in English > Hardware, Cabling, Installations

FT7800: getting PTT signal to control side

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pe1hzg:
Hi,

Perhaps to inspire others:
I am preparing for the annual Jamboree-on-the-air in october where Scouts make radio contacts to other Scouts via amateur radio. JOTA is big in the Netherlands with typically 175 stations active, and the 2m FM frequencies very busy (the 145.500 calling frequency being congested so much that you can't call CQ w/o having a QSY frequency ready, because someone else will call CQ while you try to go QSY).
Obviously a lot of new hamradio operators with perhaps sub-optimal operating practice, but in my group we have half a dozen of scouts with hamradio licenses with another four currently going through the paces of getting their license. A lot of energy and enthusiasm ongoing.

I have trouble because one can't really run two 2m stations on the same location because transmitters will block and I have enough scouts that want to make radio contacts that running two stations is desirable. So, I'm planning to run an experiment with a pair of RRC1258's to remote an FT7800 where the head of the radio is at Scouting but the radio unit itself is in my shack, connecting to my antenna.

Long introduction, and there is a catch: we normally operate using headphones to control the amount of local QRM in the JOTA radio shack. We find that Scouts are much better able to participate if the audio quality of the radio connection is good, and headsets are preferred above a room with speakers shouting.
However, when using headsets with a group, it is very nice to have a "monitor feature" so when someone is speaking while transmitting, other scouts will hear what is being transmitted to the other group.

The FT7800 does not provide this monitor feature. It was not hard to find points in the RRC1258 for microphone audio and speaker amplifier input to add a monitor feature, but the microphone is enabled all the time (at least in the points I found) and there is no PTT signal in the RRC1258 control unit.
I asked Mike and he confirms - the PTT signal is (only) in the serial data stream between head and radio and there is no logic signal in the control RRC to conditionally feed audio from mike to speaker, only when the radio is transmitting. Unfortunately, Mike agreed with my finding. What to do?

Pondering on the problem a bit, I think I now found a solution. The radio RRC can have one its outputs on the I/O connector configured for "PTT". And it is possible to use the I/O feature of the RRC pair to move a logic signal from the radio-RRC to the control-RRC.

In my lab, I now tried this:

* On the I/O connector of the radio RRC, I installed a "loopback plug". The loopback plug has pin 6 (OUT1, open collector) and pin 2 (IN2) connector together, and the junction connected via a 10K pullup resistor to pin 7 (8V out)
Update: looking at the RRC schematics, probably don't need the pullup, just loop 2 and 6 togther
* On the radio RRC, I configured OUT1 to be used as PTT signal. The PTT signal is now transferred via IN2/OUT2 to the control RRC
* On the control RRC, I connected pin 3 (OUT2, open collector) to a pull-up resistor to pin 7 (8V out)
* On the control RRC, I configured OUT2 to use I/O, so it is tracking the IN2 input from the radio RRC
Voila. I now have a logic signal that activates if the PTT line is active. Now what is left is to feed the microphone signal via a 4053 switch to the input of the audio amplifier.

But I though that bringing the PTT signal to the radio RRC this way was unusual and worth reporting about.

Geert Jan

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