I've only mc1489 chips to play with here, on=1.25, off=1.0,
and mc1489A, on=1.95, off=0.8. Both those would be ok with
my bc337 pulldown ttl output. I'm pretty sure my old printer
required on=+3, off=-3. As for various pcs here they probably
have m/b from mid 90's to 2007 and I've no idea as to what
rs232 levels they switch at. Point is it's much easier to
add a 1488 to my circuit to eliminate rs232 threshold as
potential problem than to shutdown the web/mail/ntp server,
(now in pool), to do any experiments.
I also agree that "wild goose" chase may apply to possibility
that thresholds are relevant. However using ttl level direct
to lpt would have saved me the trouble of adding the 1488 chip
just to make sure.
Indication there is a problem, either in the interface or
the 60kHz MSF receiver is that reach doesn't remain at 377
when room is unheated.
There's also another problem I have to resolve and that is
ntpd possibly not keeping up with temperature changes, giving
a square wave variation of about +/- 1ms in offset, lately
during very cold spell, with main period of about 24 hr.
Otherwise with lesser temperature variation, the offsets ramp
up/down to around +/- 300us corresponding roughly to heating
system switching on/off. That's going to require experimenting
on a spare system with addition of heater to crystal, or if I
bring one of old 486dx back into life, possibly by swapping
jumpers to use an external clock source.