[GreenKeys] Mark/space reversed ?

[Jim Haynes]

Well back in the 1950s hams and commercial and military mostly made
the space frequency the lower of the two on RF.  But at audio it was
the higher of the two, for reasons I never understood.  There was the
saying LSMFT - Low Space Makes Fine Teletype (copped from a Lucky Strike
cigarette slogan).  But there were a few stations even then that used
reversed shift.  Since then there has been some military stuff that
operated with space high - perhaps as a consequence of some military
gear being upper-sideband-only and keeping the space high convention
for the audio signal.  And in general companies today use whatever
shift polarity they please.

In the same vein, the RS-232 modem interface standard calls for negative
voltage for mark and positive for space, while MIL-STD-188 calls for
the reverse.  Except that recent changes to the MIL standard allow
for RS-232 polarity as an option.

I think all that proves is that if there are two different ways to
do something, both of them will get used.