I have an IC729 and a R72 rx in the shack here, and I wanted to try some experiments with using separate aerials for receive & transmit. Getting the two pieces of equipment to "talk" to each other using the CI-V interface was no problem, but try as I might I could not see a way to prevent a master-master situation i.e. turning either tuning knob caused the other rig to follow.
What I wanted was a master-slave set-up; in my case the IC729 to control the R72 but NOT the opposite. This would mean that having started a QSO I could use the receiver for split operation, or as a kind of RIT.
Initial thoughts consisted of having a switch to break the CI-V line as and when transceive was not needed, but this wasn't very elegant and would involve lots of switching on and off. Then I had a great idea. What common electronic device lets signals pass one way but not the other? Correct! The good old humble diode!
A quick lashup proved that my theory was right. The 0.7 volt drop does not effect operation of the data signal in the desired direction, which is a little surprising as the signal is only a 2 volt signal (unterminated) to begin with. Maybe the fanout is reduced with the diode fitted, but I have been unable to test this.
In the end I made up a lead with the diode in series with the coax "inner", inside one of the jack plug covers Don't bother trying to work out which way round to solder it: if control is the wrong way reverse the lead. It's probably worth marking the lead so that it isn't used for audio work - might cause a few strange effects!