This is not answer to the question but another way of achievieving the
result:

I am assuming that the Kenwood I/F is similar to the Icom. Icom is also
selling a quite expensive I/F. I did not want to buy the I/F and I was not
in the build-my-own mood either.

My solution: I used a USB to serial converter and removed the level
converter (it was not a MAX232). One of the Belkin USB-to-serial adapter I
already had is bulky enough to allow me to do the surgery very easily. The
plastic shell opens w/o problems. Since they are many uses for a serial
adapter w/ "TTL" levels, I wired the input of the level converter to the
9-PIN connector.

This adapter is used to configure an IC706 MKII. My laptop has no serial
line so I had to use a USB converter anyway. The cable from the DB9 to the
IC706 combines RX and TX as described earlier.

Good luck.
"pinpassion" wrote in message
oups.com...
Hi Gang,

I am trying to work a circuit that I can use to
program a VHF receiver. I'm going to use the Maxim
MAX232 IC to do the TTL to RS-232 voltage conversion,
and this is very common. The trick I have to accomplish
is to take the TXD and RXD (transmitted data/received data)
on the TTL side of the chip, and combine it into 1 line
going to the radio.

MAX 232 Chip Combiner
-------------- |-------|
| | | |
| TXD |--------------------| |
| | | out|----------RADIO Data Line
| | | |
| RXD |--------------------| |
| | |-------|
--------------


Most interfaces are full duplex and they don't split up
the TXD and RXD signals. On this particular Kenwood radio,
they do. There is still data beng sent to the radio, and
data received from the radio. It is just using one line as
the pipe for both data lines. I was thinking two transistors
and a blocking diode. Anybody have any ideas? Kenwood sells
a cable that does this, but it's expensive and I am trying to
find a homebrew solution. All I need is the combiner circuit.
I already have the schematic for the MAX 232 circuit. This is
5 volts TTL. The radio is a Kenwood TK-690H.

Thanks in advance for any help.

Mike