W3FPR asked:
Are you asking about spikes on the TTL level side or the RS-232 level side?
If it is on the TTL side, and the RS-232 side at the same time period has a
voltage beyond the 'region of uncertainty' between +3 volts and -3 volts,
one can only blame the receiving device, but I do believe the problem goes
further than that.
In the proper RS-232 world, the minimum levels (+/- 3 volts) are the minimum
that can/should be expected at the receiver end, while the driver voltage
can go anywhere greater than those values (up to +/-25 volts) and still meet
the spec. The designer of the driving circuits have to make a decision
about how much cable they want to drive and what the safety margin they want
to use above the minimum. Some laptops are limited on the power supply end
of things and may chose to drive the positive signal with only 5 volts but
will usually drive the negative with something like -12 or -15 volts. They
lose some noise immunity on the positive side by doing that, but since in
RS-232 signalling the negative voltage is the logical 1 (or ON signal), they
can usually get away with it unless the noise source puts extra negative
going spikes on the cable.
I would tend to believe that something has failed in Tom's laptop rather
than thinking that it is a design flaw - yes, most laptops cannot drive the
200 ft. cables that used to be common in the RS-232 world, but that world
was usually not driven by laptop serial ports anyway. In a typical home or
small office setup, the 5 volt positive level still provides more tha a volt
of noise imunity on the positive signalling level, and since the negative
signalling level is driven to a -12 volt level, there is plenty of margin
for noise on the level that will register a logical 1 at the receiving end.
The spikes have nothing to do with N0SS's laptop - sorry for the
misunderstanding. Tom's follow-up post reminded me of having
looked at this a while ago.
I hope N0SS has a dickie serial port & it isn't indicative of ThinkPads
in general (hard to get real serial ports on modern laptops & hope one
day to find another one with two & all the other goodies one day - have
some things to do that really can't be done any other way... will be
keenly watching for any final post from Tom).
The spikes I think are where the chirp (low level sound of cricket in
both receive & transmit audio - numerous reports here of it on receive)
when a K2 is being talked to. Incoming data on KIO causes huge
spikes with 'scope probe in the vicinity of level far greater than spikes
seen when the K2 is sending data.
Haven't tried breaking out that line & shielding it, or doing something
to clean up the power to the interface chip - not sure why 232>TTL
would cause this & TTL>232 wouldn't.
73, VR2BrettGraham
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