Programming a Kenwood TK-690 VHF Transceiver

[pinpassion]

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