On a sunny day (Sat, 19 May 2018 13:39:39 +0000 (UTC)) it happened Martin
Gregorie  wrote in :

>If needed, I have another USB adapter but its an older pfranc unit and I
>don't know what its serial signal levels are either. USB-serial adapters
>are obviously running off 5v, but do they normally provide 12v or TTL
>levels on their RS232 pins?

If it has a D connector then it very likely will output RS232 compatible
signals.
I have both, (with D connector) and a with logic level output pins.

Note that it is not only that voltage level, the ones with logic level outputs
are INVERTED,
so high level is low level etc. compared to real RS232 (D connector).
The ones with logic level output will interface directly with a microprocessor,
the D connecetor RS232 ones need a MAX232 chip or equivalent to interface with
a microprocessor.
MAX232 was the standard interface chip in the old days,
it generates its own + and - higher voltage from 5V. and inverts the signal
back,

You need a real one with D connector for that thing you want to interface.

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