On a sunny day (Sat, 2 Jun 2018 20:41:13 +0000 (UTC)) it happened Martin
Gregorie  wrote in :

>On Sat, 19 May 2018 19:07:45 +0000, Martin Gregorie wrote:
>
>Well, after some other distractions plus an uncharacteristically slow
>delivery of some bits from Rapid (Scopebanana plug cable, additional
>3.5mm plugs&sockets, I lashed up a terminator box for the AXE027 cable
>with Rx-Tx shorting switch and 4mm plugs for instrument connection.
>
>Then I used ptlrc running on my RPi 1+ B to run some tests. Everything
>was as expected - including adapter's output, which is just over 4v
>according to my 'scope, a fairly old Hameg 203s. Making ptlrc send its
>LICENSE file gave several seconds of output - enough to make reading the
>scope easy (it wasn't visible from my keyboard).
>
>No word from the Dittel agents, so next job is to make up a D-9 to 3.5mm
>stereo jack cable, test, and check the levels from my pFranc serial
>adapter before trying it on the Dittel radio.


If all else fails, you could buy a MAX232 chip (about 2 UKP / 3 Euro IIRC),
and build you own logic level to RS232 interface.
You will need 3 1uF/16V electrolytic capacitors too:
 http://panteltje.com/pub/logic_level_to_rs232_with_max232.jpg
Your logic level 3.5 mm plug tx goes to pin 11, and comes
out as real RS232 TXD on pin 14 to the D connector.
The real RS232 RXD from the D connector comes in on pin 13,
and goes to your 3.5mm connector's rx.
'n.c' stands for 'not connected' pin.

The MAX 232 has a build in voltage converter that creates about +10V and -10V
for the correct output swing.
Be careful  buying MAX232 from ebay, I got a bunch of bad ones from there.
The ones I got from conrad are OK.

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