Connecting the TX Inhibit Circuit to the K3S

In order for the TX Inhibit circuit which I built into the Sequencer (See HERE) to work with the Elecraft K3S, I needed to wire a couple of connections from the Sequencer to the K3S ACC jack and change some Menu Settings on the K3S. Since I already have other accessories connected to that jack, I purchased a DB15HD "Y" cable as well as a DB-15 Male Breakout Board.  The "Y" cable was connected to the K3S ACC jack, my other accessories were connected to one branch of the "Y" and the other branch was connected to the breakout board.  You can see the breakout board in the photo below with a 2-conductor shielded cable which connects Pin 7 (TX Inhibit line) and Pin 13 (BANDØ line) to the Sequencer.

DB-15 Male Breakout Board

The K3S has several Band Outputs which will produce either a Ø or a 1 condition with Ø being when the line is pulled to ground (Ø V) and the 1 being when the line is floating at 5 V.  These Ø and 1 outputs can be used to control many things.  In order for these Band Outputs to be used to control the TX Inhibit circuit, some settings in the K3S need to be adjusted.  First,

CONFIG:KIO3B is set to TRN.  This sets the BANDØ-3 outputs to reflect the parameters of the CONFIG:XVn ADR menu entry.  There is a chart in the manual that shows which conditions will be produced by different Transverter Addresses.  I chose the TRN setting for the CONFIG:KIO3B menu item instead of HF-TRN because using the latter would give me a 1 output on various HF bands in addition to 2-M.  Using the TRN setting, I only get a 1 output on BANDØ when the K3S is tuned to the 2-M band.  That is where I need to enable the TX Inhibit line.

As I'm using the Internal 2-M Transverter in the K3S, it is delineated as Transverter 1 (TRN1) and therefore, I set CONFIG:XV1 ADR to Int. trn1.  This causes the radio to output a 1 on BANDØ when the K3S' VFO A is tuned to 2-M.  With the above settings, I will ONLY see a 1 on the BANDØ output line when the radio's A VFO is tuned to 2-M and not on any other bands.

To make sure I really understood all this, I connected the lines from the Breakout Board to the sequencer with clip leads and a volt-meter.   When I would change bands I could see the 5VDC output vary according to where the CONFIG:KIO3B menu item was set.  Watching the output on BANDØ allowed me to also test the TRN and the HF-TRN settings as well.  Once I had that figured out, I used a clip lead to put the sequencer into transmit and could see that my circuit (KL7UW's circuit) was functioning properly.

The Sequencer contains a 5 VDC power supply which I built from a circuit I found by KL7UW and that voltage is used to control the K3S' TX Inhibit line.  From the K3S manual: "Pin 7 of the ACC connector can be configured as a transmit inhibit input by setting CONFIG:TX INH to LO=Inh (or HI=Inh). Holding pin 7 low (or high) will then prevent transmit."  Putting the TX INH setting to HI = Inh will tell the K3S to NOT go into transmit if there is a 5 V signal on that line.  The KL7UW circuit connects to Pin 13 (BANDØ) to determine when the K3S is tuned to 2-M.  If that line is HI (5 V) then the circuit turns on a transistor which then pulls in a relay.  That relay connects the 5 VDC power supply I built in the Sequencer to Pin 7 (TX Inhibit) to prevent the K3S from transmitting.

To make this work, I had to set the CONFIG:TX INH menu setting in the K3S to HI = Inh so that a HI (5 VDC) voltage on Pin 7 will prevent the K3S from transmitting.  When there is a HI (5 VDC) voltage on Pin 7 of the K3S, the front panel of the K3S indicates this by flashing the TX indicator.

The whole purpose of this TX Inhibit circuit configuration is to make sure that the K3S will NOT transmit any signal until the T/R Relay has completely switched to the XMIT position.  Stage 4 of the Sequencer connects internally within the Sequencer box to the KL7UW circuit so that it shorts out the 5 VDC supply that is connected to Pin 7.  That places a LOW (0 V) on Pin 7 and the K3S can now transmit.  But this ONLY takes place after power is removed from the LNA and the CX-600NL relay, the T/R relay is de-energized to place it on the TX position, and the Amplifier PTT circuit is closed.  ONLY then can the K3S produce a signal.  This is a safety circuit to keep the system from accidentally transmitting when it shouldn't. 

One more piece of the puzzle is now complete.  I am down to needing to install one connector on the TX feedline (7/8-inch Heliax), finish the amplifier power supply, and a couple of other minor items and I'll be ready to transmit!!!