Several years ago I saved a PDF document regarding the tuner interface. 
I just used the new JAWS OCR document feature and copied the text below. 
There were two graphic waveform diagrams that I removed from the text. 
The very last paragraph talks about adding a 1K resistor between the TT 
and TS lines. These are the center two pins of the 6 pin connector.
For what it's worth, below is the  document.
Rob  K6DQ

Technical Description of the Kenwood Tuner Interface
By AA4P B
Kenwood HF transceivers such as the TS-50 have a 6-pin tuner interface 
connector. If a tuner is connected when the radio is first turned on, it 
senses the type of tuner attached and configures the interface protocol 
accordingly. If no tuner is attached then the radio's tune function is 
disabled. There are two bi-directional TTL level (0 and +5 Volts) lines 
that provide the communications between the radio and the tuner. These 
are called TS (Tune Start) and TT (Tune Terminate). These lines are 
pulled up to +5 Volts by resistors in the radio. Either the radio or the 
tuner can pull the lines to ground with an open-collector driver.
The Kenwood AT-50 tuner uses a serial data communications with commands 
sent from the radio to the tuner on the TS line and commands from the 
tuner to the radio on the TT line. Kenwood has been unwilling to release 
the exact protocol and commands. One would assume that there would be a 
tune command from the radio, a bypass command from the radio, and a 
match-found command from the tuner. It might be possible to 
reverse-engineer the commands by monitoring the ascii data flow on the 
TS and TT lines while operating an AT-50.
The Kenwood At-300 uses a bi-directional hand-shaking protocol on the TS 
and TT lines. During power-up, the radio momentarily pulls the TS line 
low and looks for the tuner to respond by immediately pulling the TS 
line low. If this occurs then the firmware in the radio is initialized 
in the AT-300 mode and the following timing diagrams are followed.

Waveform diagrams omitted here.
--------------------------------------------------------------------------------

The two traces in each of the three examples represent the voltage 
present on the TS and TT lines (zero or +5 Volts). The indicator to the 
right of the trace indicates whether the radio or the tuner is 
controlling the line. Most of this information was taken from the 
Kenwood AT-300 Maintenance Manual.

Normal Tuning
In the first timing diagram, the operator has momentarily pressed the 
tune command and the radio is signaling the tuner to begin by pulling 
the TS line low (zero volts). The tuner responds by pulling the TT line 
low a short time later. The radio responds to the low TT line by 
outputting a 10 Watt CW tune signal. As soon as the tuner finds an 
appropriate match for the antenna, it releases the TT line back high (+5 
Volts). The radio responds by terminating the CW tune signal and then 
releases the TS line back high. The tuning cycle is now complete.
Failed or Canceled Tuning

The second timing diagram shows what happens if the operator cancels a 
tune in progress by pushing the tune button or the radio times out after 
15 seconds of tune signal because the tuner is unable to find an 
appropriate match. To begin, the operator momentarily presses the tune 
button and the radio pulls the TS line low. A short time later the tuner 
responds by pulling the TT line low and the radio transmits the CW tune 
signal. At that point the radio starts a 15 second timer. If time 
expires or the operator presses the tune button again before the tuner 
has found a match then the radio releases the TS line back high. The 
tuner then releases the TT line back high and places itself in bypass 
(antenna connected directly to the radio).

Bypass Request
The third timing diagram shows what happens when the operator presses 
and holds the tune button for 2 seconds to place the tuner in bypass. 
This is the one case where the radio controls the TT line and the tuner 
controls the IS line (they are both bi-directional, open-collector 
controlled lines). The radio pulls the TT line low to signal the tuner 
to enter the bypass mode. The tuner acknowledges by pulling the TS line 
low. The radio then releases the TS line. The tuner goes into bypass and 
releases the TT line. No tune signal is transmitted by the radio.
Hardware Connections

The Kenwood tuner interface utilizes a 6-pin connector as shown below. 
Note that this view is of the rear of the connector on the end of the 
cable that plugs into the radio. Information on the JST connector and 
distributors can be found at http://www.jst.com

Waveform diagrams omitted here.
--------------------------------------------------------------------------------

A Quick and Dirty Tune Function
The tune button on the Kenwood radios can be made to function by simply 
placing a 1K, 1/4-Watt resistor between the TS and TT lines. Momentarily 
pressing the tune button will cause the radio to transmit the 10-Watt CW 
tune signal until the operator presses the tune button a second time to 
cancel the tune. Please note that the resistor must be in place when the 
radio is first turned on in order to fool it into thinking there is a 
tuner attached. This simple method can be used to provide a manually 
controlled tune function. The easiest way to fully implement the full 
Kenwood tuner protocol, including bypass, will be to develop an 
interface with a PIC microcontroller.

End of document