[GreenKeys] Low-level and High-level TTY conversion

[Nick England]

Hi gang - I've gotten email from a volunteer submarine restorer who
has what he says are 28KSR and 28RO with low-level signaling (+/- 6vdc
polar). They want to use these units with the ham gear in their radio
room.

Is there a ham TU compatible with Low-level signalling ? Would just an
RS-232 interface work or are they all unipolar? Would he have to build
high-low level converters?
Thanks in advance for your help.
Nick
www.navy-radio.com