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

I'm assuming you have no control line on your side to set the dataflow
direction, but that the radio enables its transmitter when required,
and sets it to HiZ when receiving. It's possible that some obscure
control character(s) or other RS232 line sets data direction, but let's
assume the arrangement is simple (does your manual say anything about
this?).

Simply tying the RXD line directly to the radio data line, and the TXD
line to that with a 3K or so resistor ought to work. Anything you send
will be echoed on your RXD line if you use this scheme, but under our
simplicity assumption it is likely the Kenwood cable does this as well.

Note that I'm assuming RXD is the input to the MAX 232 and TXD is the
output. RS232 nomenclature is ambiguous.

Regards,
Glenn Dixon AC7ZN

Oopsie, didn't read your post carefully enough--the radio uses TTL
levels so the MAX 232 is near the radio? I assumed the data line was
RS232 levels (dumb me--you said TTL twice). Lets see....well something
around 3K may still work, though the data line would have to be
short...

Glenn

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

In article .com,
says...

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

snippety

Are you 100% certain that this particular radio even uses TTL levels on the
programming side?

I say this only because I work with TK-790's frequently, through my day job, and they
do not require any sort of RS232 to TTL conversion. They're programmed through a custom cable
straight from the computer's serial port to the microphone connector.

Perhaps you should dig into the 690's service manual before expending much effort?

Happy hunting.

Hi,

Thanks everybody for the help and suggestions.

I have heard through other users of the Kenwood 90 series
radios that the factory cable has the MAX232 chip within
one of the connectors, as well as a data combiner circuit
that sends/receives information over 1 data line through the
radios microphone connector. The radio deals with 5 volt TTL
signals and not the 12 volt rs-232 voltages. If you open your
factory cable, you will probably see these circuits and can
verify what I have heard.

Mike