On Tue, 27 Jul 2004, Bill Clawson wrote:
> If I remember the RS-232 spec correctly, the input
> range goes from +/- 3V (min) to +/- 12V (max).
The upper limit actually is 25 (!), not 12. RS-232 transmitters are now
supposed to send at most 15, but one very old rev of the spec allowed up
to 25, so receivers have to tolerate that.
Most modern transmitters send +/-12 (if they have on-board +/-12V power
voltages), +/-9 (if the transmitter chip has to generate its own from a +5
supply, which it does by doubling it, less minor losses), or +/-5.5
(doubling from a +3.3V supply, less losses).
> The level comparator in the typical RS-232 receiver is
> running at about 1 or 1.5 V, so you can get away with quite alot...
Right, most receivers set their threshold somewhere around +1V so that an
unconnected or grounded input looks negative (which is "idle" for data
lines and "off" for control lines).
> The drives/receivers are inverters, so
> you might check to see if your signal is inverted...
I'd be surprised to see the RS-232 voltages inverted, since they're quite
well standardized. Serial links using TTL/CMOS voltages are anyone's
guess, but the output of a real RS-232 transmitter is pretty consistent
because everyone tests for interoperability.
> > Works great from my laptop
> > and the build server, but not from the PC/104...
> > The only difference between serial signals from my
> > laptop and the PC/104 is the PC/104 is running +/-9V
> > the laptop +/-5V. The build server runs +/-12V.
Dave, could it be that what you've got on the IMU is *not* an RS-232 port,
but an RS-422 port? Such mismatches can cause very strange behavior.
Similarly, check continuity of the ground wire in the cable. A broken
ground can do bizarre things.
Thirdly -- apologies if you're already answered this, I didn't save all of
the earlier mail -- could this be DCE-vs-DTE confusion? (That is, do you
need to add [or subtract] a null modem?)
Henry Spencer
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