So today I got new RSR232 cable info ... which disagrees seriously from the manual (page 15).
Now the 3.5 mm jack base is the signal (rather than the tip), there is no RS232 ground connection, and 3.5 mm jack tip goes to a new terminal, neither of which the Rx and Tx wires go to .
wtf
Edit: tested and this works. The user manual is BS.
The Sparkfun RS232 - TTL level shifter (with or without the female DB9 connector) has only circuit board connections to three pins of 9: Tx, Rx, and Ground.
Currently, this only works on a RS232 to USB on a laptop serial monitor but at least I can see the data enough to work out decoding it now.
The revised DB9 wiring utilizes a neat voltage level manipulation circuit but seems odd without a real ground, and so the TTL shifter needs re-wiring, and there a bazillion suggestions on the Internet for tying DB9 pins one way and another with zener diodes to sort through ...
The 3.5 mm jack has the base tied to the cable jacket foil, but will be carrying 5V at the jack tip, unreferenced to the receiving side ground except at the power supplies.
Edit2:
In case someone else runs into this, since it is unusual, the Reed Datalogger uses a DTR asserted signal from the receiving end, normally supplied by a PC DB9 serial port. This is typically a +9 to +12 V positive signal with some very weak current carrying capacity (about 35 mA). This DTR voltage goes up the RS232 adapter cable to the Reed datalogger, where (inside) it connects to the base of a signal transistor. The high logic RS232 signal then comes back out of the datalogger, (using this voltage supply, through the internal transistor), and goes back down the cable to the Tx pin of the adapter cable, and ultimately to the Rx pin of the (RS232) receiving end. (Negative voltage signals are also sent down this same output signal wire, as well as a zero V (neutral) idle signal.)
It seems that this DTR voltage can be high enough, even at +5V, to operate (most of) the RS232 to TTL level shifter/inverter devices typically available. However, a resistor (and possibly a diode) should be installed in series to prevent trouble if wired up wrong somewhere along the way. So, in theory, the DTR line can be supplied (with caution) by the +5V Arduino voltage output, and (with more caution) possibly by an Arduino +5V digital output pin set to high logic. The Tx signal from the datalogger connects to the receiving RS232 Rx pin as usual. The ground wire is not connected to anything on the datalogger end. [This is an output only system, and it is unclear if the DTR signal is being used by the data logger for its intended purpose also. However, it seems technically impossible for the datalogger to send any signal without the voltage supplied by the receiver end DTR +voltage signal.]
(Not all devices connected this way can necessarily use the low +5V at DTR, and may require more voltage, up to +12V, in order to output the RS232 signals properly, especially over any great cable lengths.)
It is still unclear if the RS232 DTR logic low (negative voltage!) is required in this case, but it seems not.
Apparently some anemometers output two wire RS232 using this system.
There are some devices that use the RS232 RTS logic low (negative voltage, as low as -12V) combined with the RS232 DTR logic high (positive voltage) to make a large voltage differential (up to 24V) to supply some circuits.
EDIT3:
Finally the RS232 data comes out other than zeros and a few random other bits. I found a manual for a device using the same base circuit board which has an extra connection.
See B&W image below. Compare this to the WRONG page 15 version of the REED manual. (Ignore some data byte info that are for an anemometer only).
Without the additional 2.2k resistor, basically only zeros (00) are sent to the serial plug.
.